8
The soil microbial community is driven by solar power. The primary energy supply is photosynthetically fixed carbon which enters the soil ecosystem primarily as plant biomass and exudates. Hence, it is logical that a site of maximized biological activity is the portion of the ecosystem encompassed by the soil and root interface – the rhizosphere. The provision of energy to the microbial community by root exudates, dead roots, and sloughed cells results in intense microbial metabolic activity plus an enhancement of microbial interactions – e.g. competition, symbiosis, amensalism, and predation – over levels occurring in nonrhizosphere soil. Furthermore, since plants benefit from the mineralization capacity of the soil microbial community and the improved soil structure derived from the enhanced microbial activity, the rhizosphere microbial community represents that portion of the soil ecosystem with maximal effect on the aboveground community.
During the growing season, the rhizosphere is an expanding focus of biological energy. New microbial habitats are produced continually through root growth. Thus, scientific interest in elucidation of rhizosphere processes is derived from the desire to understand the basic biological activities occurring therein as well as from the fact that rhizosphere interactions provide a model for successful modification of native microbial community dynamics (e.g. root inoculation with symbiotic nitrogen‐fixing populations) as well as for study of the potential uses of genetically modified microbes. Opportunities for use of genetically modified microorganisms in the rhizosphere and adjacent soils include population manipulation to reduce problems with plant pathogenic microbes as well as to achieve soil renovation or other bioremediation objectives.
Therefore, this analysis of rhizosphere/mycorrhizosphere properties is presented with the overall objective of elucidation of the properties of the microbial processes occurring within the rhizosphere and those factors which make it an unique portion of the soil ecosystem. Traits of the total rhizosphere will be examined along with the microbe–plant interactions affecting plant growth and microbial community development. This analysis will be followed by an evaluation of mycorrhizal symbioses and the impact of these associations on properties of the rhizosphere.
This analysis of rhizosphere/mycorrhizosphere properties is presented with the overall objective of elucidation of the properties of the microbes and the soil itself that control overall ecosystem function and sustainability. Such research requires accurate determination of rates and location of the biological processes. As you consider the limitations to quantification of the microbial community in comparison to plant growth, the questions of how the reaction rate changes with distance from the root and what controls the magnitude and rate of the biological reactions become more complex. That is, “Is the soil sampled at 0.5 cm from the root or is the soil collected at 1.0 or 2.0 cm more representative to the soil system itself?”
To provide a clear understanding of soil sample collection limitations, the research presented in Chapter 8 represents classical interpretation of experimental field data. The more modern methods are presented in chapters of the text based on modern methods such as DNA analyses, statistical variations in soil sampling and management, etc. (for example, the nitrogen cycle presentations). Grouping of the research data into that developed with more classical procedures (Chapter 8) and a separate group that includes the more current analytical methods (DNA analyses) will hopefully allow for better understanding of soil biological processes. Note that the References section for this chapter is heavily weighted in what could be said to be the classical class whereas the more current DNA‐based studies are presented in large part in the newer publications reported throughout the chapter.
8.1 The Rhizosphere
By definition, the rhizosphere is that portion of the soil under the direct influence of the roots of higher plants whereas the rhizoplane encompasses the root surface and its adhering soil. As shown in Figure 8.1, the soil microbes in the vicinity of the growing roots are stimulated by the provision of a surface upon which to grow as well as by the nutrients contained in leachates, sloughed cells, and decaying roots. Microbial colonies develop unevenly along the root surface, with maximal populations occurring where the exudates are lost from the root. Therefore, occurrence of microcolonies is a good indicator of carbon and energy source leakage from the root.
Characteristics that define the rhizosphere are as follows.
· The properties of the plant are the predominant controllers of the ecosystem rather than those of the soil. This situation results from the fact that the plant is supplying the nutrient and energy source, plus space for the microbes to grow, as well as altering the gaseous regime of the soil.
Figure 8.1 Schematic diagram of the physical relationship of primary components of the rhizosphere (not drawn to scale).
· The microorganisms prevailing in the rhizosphere originate in the soil and are characteristic of the soil microbial community. But because of the variation in the nature of root exudates between plant species, the specific organisms comprising the rhizosphere microbial community do vary with plant species and soil type (e.g. Grayston et al. 1998; Latour et al. 1996; Maloney et al. 1997).
· Because of the combination of increased microbial respiration occurring in the root zone compared to the adjacent nonrhizosphere soil and the root respiration, the gas regime around the root differs from that of the surrounding soil. Carbon dioxide levels are generally elevated and molecular oxygen tensions are reduced in comparison with the gaseous content of nonrhizosphere‐impacted soil.
· The influx of carbon and energy in the form of root exudates selects for microbes capable of using the nutrients most efficiently – the fast‐growing microbes. In comparison, nonrhizosphere microbial communities typically exist in a “feast and famine” state. Nutrients provided to nonrhizosphere populations primarily result from the influx of nutrients contained in infiltrated water (a typical source in a nonmanaged ecosystem is fixed carbon mobilized by the decay of surface litter) or are carried into the site through movement of animal life (including the nutrients contained in the animal body when the organism expires as well as organic substances translocated by the activity of ants, termites, and earthworms). It is tempting to consider the rhizosphere community to be continuously bathed in nutrients. This concept may be of value in developing an overall model of the community, but it is far from the situation at the microsite. As is discussed in greater detail later in this chapter, quantities of root exudates as well as their chemical composition vary along the root length, change throughout the growing season, and are dependent upon root density and plant–plant interactions.
· Since many of the products of root metabolism are acids (including carbon dioxide which forms a weak acid when dissolved in soil water), soil minerals are solubolized in the rhizosphere (e.g. Bolan et al. 1997; Hinsinger 1997; Krishnamurti et al. 1997). Thus, osmotic impacts on microbial growth are accentuated in the rhizosphere, mineral nutrient availability is modified, and clay mineral transformation may occur. For example, linkage of potassium availability and loss of potassium ion from mica flakes plus transformation of phylogopite into vermiculite in inoculated pine root (Pinus sylvestris L.) rhizosphere soils were shown by Leyval and Verthelin (1991).
8.1.1 The Microbial Community
The rhizosphere effect or the degree of stimulation of an activity of population by the energy input of the plant root has classically been described by evaluation of the ratio of activity per unit weight of rhizosphere soil to the activity per unit weight of nonrhizosphere soil (the R/S value). Values greater than 1 indicate selective stimulation in the rhizosphere, equivalence of the two activities suggests no rhizosphere effect, whereas a R/S ratio of less than 1 reveals inhibition of the activity in the rhizosphere.
Rhizosphere bacterial populations: Bacterial populations are stimulated by root exudates. Populations greater than 109 g−1 rhizosphere soil are commonly detected. Selective stimulation of rapidly growing, gram‐negative, rod‐shaped bacteria (Pseudomonas sp., Flavobacterium sp., and Alcaligenes sp. especially and occasionally Agrobacterium sp.) is observed. These bacterial species are found as microcolonies covering approximately 4–10% of the root surface. Competition, the primary selective, intermicrobial interaction, is based on growth rate, metabolic versatility, and growth factor requirements as follows.
· Growth rate: The conditions of the rhizosphere favor those bacteria which have short generation times. Clearly, organisms that convert the carbon and energy contained in the root exudates to biomass efficiently and rapidly will achieve greater population densities and occupy more of the space than the less vigorous and efficient members of the community.
· Nutrients: A variety of fixed carbon compounds are available to the microorganism within the rhizosphere. Stress of competition may result in exhaustion of a particular growth substrate, whereas abundant supplies of others remain. Thus, successful bacterial populations are those capable of using a variety of biochemical growth substrates. Data suggest that, in general, the organisms in the rhizosphere are not carbon limited (e.g. Cheng 1996), but supplies of specific carbon sources may become more limiting due to intense competition.
· Growth factors: Root exudates commonly contain a variety of amino acids and vitamins which may be used as microbial nutrients or growth factors. Comparison of the growth factor‐synthesizing capacities of rhizosphere bacteria suggests that some advantage is conferred on organisms requiring amino acids and no other growth factors. Interestingly, the proportion of bacteria in the rhizosphere which require complex nutritional growth factors declines, but their actual numbers increases. This somewhat confusing observation results from the fact that the number of these organisms in the ecosystem increases but the total population increases at an even faster rate. Therefore, although there are more growth factor‐requiring bacteria in the rhizosphere than are found in nonrhizosphere soil, they make up a smaller portion of the total population.
Two other factors affecting microbial growth in the rhizosphere are surface area available for colonial development and antibiotic production. Since the simple addition of nutrients to soil can also stimulate the same group of bacteria as are stimulated in the rhizosphere, nutrient supply has to be a primary selective factor in the rhizosphere but, as nutrient needs are met, the point of competition may be shifted to factors impacted by the minute distances between individual cells and physical obstructions to growth, e.g. surface area and toxicant production. Competition for surface area affects population development primarily at the microsite where cell–cell interactions are intensified. As indicated in Chapter 7, antibiotic production appears to play a role in ammensal interactions in the rhizosphere. With the close proximity of bacteria, the intense competitions occurring, and the positive impact of inoculation of plant roots with antibiotic‐producing microbial strains, antibiotics could be of major importance in this ecosystem, but absolute elucidation of antibiotic production and importance in the root ecosystem has yet to be developed.
Fungi in the rhizosphere: Quantification of the rhizosphere effect on soil fungal populations is equivocal at best. This difficulty is derived, to a large degree, from inability to separate active from inactive fungal propagules. Fungal population densities may be quantified through enumeration of colonial development on defined media or mycelial strands may be counted directly in soil samples. Colonies growing on defined media may result from outgrowth of spores contained in the soil sample or from mycelial strands. Direct observation of hyphal strands in soil provides a measure of total fungal biomass, but not species or genus identity. Colonies growing on the defined media can readily be classified to genus and species, but they may have developed from a quiescent spore serendipitously existing in the environs of the root or from an actively respiring fungal colony. Thus, data collected for the evaluation of rhizosphere effect on fungal populations are difficult to interpret. Total fungal counts appear not to be impacted by association with the root, but some individual populations may be stimulated. Plant roots do produce compounds which stimulate outgrowth of fungal resting structures and hyphal development. Similarly, zoospores of phycomycetes are chemotactically attracted to the growing root tip. These observations suggest a positive rhizosphere effect on fungal populations.
Other rhizosphere inhabitants: Organisms whose populations tend not to react to influxes of readily decomposable organic matter are usually not affected by root growth. This grouping includes actinomycetes, protozoa, and algal populations. The actinomycetes generally derive their energy supply from decomposition of less readily decomposable soil organic matter components whereas algal populations use solar energy. Protozoan populations are limited by the distribution and density of prey populations. Because of the high population density of prey required to support increases in protozoan cell numbers and the limited precision of the methods used to quantify protozoa in soil, a slight increase of protozoa in rhizosphere soil could occur and escape detection.
8.1.2 Sampling Rhizosphere Soil
The basic definition of the rhizosphere (that portion of the soil profile under the direct influence of the plant root) provides an excellent conceptual resolution of the rhizosphere and nonrhizosphere domains but, from a practical viewpoint, it provides little upon which to base a soil sampling procedure. Questions which may arise in preparation of rhizosphere soil samples pertain to the distance from the root that the plant effect extends, the degree of the impact (for indeed, we realize that there are minute changes in soil properties at considerable distance from the growing root resulting from the presence of the root), and how to avoid diluting true rhizosphere soil with nonrhizosphere soil.
Even if the effect in question will extend hypothetically only 1 cm from the root surface, that region of impact of the root exudates is not uniform along the length of the root. Greater intensity of interaction of the root with the soil community is found at the zone of active growth. Therefore, defining the rhizosphere as extending 1 cm from the root surface in all directions for the full extent of the root length will still necessarily result in dilution of more active soil with less active soil. Since in reality the capacity to answer these questions involving the definition of the exact portion of the soil mass impacted by the root is nonexistent, most studies of the rhizosphere have involved an arbitrary definition of the soil mass to be collected for field soils, in particular, and a variety of means of physical separation of roots and soil in laboratory and greenhouse studies.
The arbitrary nature of collection of rhizosphere soils in field plots is demonstrated by an examination of common soil sampling techniques. The more time‐consuming method is to dig a soil pit around a plant and aseptically remove soil within a predetermined distance of the plant root – commonly a 1 cm distance. For example, Ruark et al. (1991) compared the interaction of acid rain and ozone treatments on hydrogen, aluminum, calcium, and magnesium ion concentrations in root‐adjacent (rhizosphere) and bulk (nonrhizosphere) soils for loblolly pine (Pinus taeda L.) seedlings growing in the field by this type of method. The procedure was adequate to demonstrate gross differences in anion concentration due to atmosphere problems and plant growth. Alternatively, the plant may be carefully removed from the soil. Nonadhering soil is allowed to fall to the ground whereas soil adhering to the root system, defined to be the rhizosphere soil, is gently shaken into a collection container. Soil in direct contact with the root that cannot be gently shaken from the root surface, but must be washed from the root to be collected, is arbitrarily defined as rhizoplane soil. The limitation associated with either of these procedures is that root‐impacted and unaffected soils are inevitably mixed at unknown ratios. Thus, rhizosphere effects are at least underestimated and potentially may be totally overlooked should sufficient dilution with unaffected soil occur.
Techniques similar to those used to separate nonrhizosphere and rhizosphere soils in the field may be applied to microcosm studies incubated in the laboratory or greenhouse. For example, see Norton and Firestone (1991) where soil was sampled at small intervals from coarse, and fine young roots of Ponderosa pine (Pinus ponderosa Laws.) seedlings for analysis of the metabolic status of the bacterial and fungal communities. Alternatively, rhizosphere and nonrhizosphere soils may be physically isolated in the microcosm (e.g. Hartel et al. 1989; Martens 1982). Tate et al. (1991) isolated pitch pine (Pinus rigida Miller) seedling rhizosphere soils from nonrhizosphere soils with a 1 μm mesh fabric. Soluble exudates could pass through the membrane, but root tissue was retained. With either procedure, limitations of dilution of plant‐impacted soil by nonplant‐affected soil compromise data interpretation as was indicated for field soil sampling procedures.
It is reasonable to assume that results from the study of bulked soils underestimate the true stimulatory (or even inhibitory) effect of the plant root on soil microorganisms. This leveling of the data is further accentuated when the distribution of the active microbes along the root is considered (recall that root exudates are not released continuously along the root, hence microbial colonies are found in “hot spots” in the rhizosphere). Thus, the bulked rhizosphere soil sample is not only diluted by nonrhizosphere soil but it is also a composite of microsites containing highly active microbial communities plus microsites in which a more reduced stimulation or inhibition soil microbial activity has occurred.
8.1.3 Plant Contributions to the Rhizosphere Ecosystem
The role of the plant in the rhizosphere is to provide a surface for microcolony development and to produce the fixed carbon compounds used by soil microbes for carbon, energy, and as growth factors. Major questions regarding this process are associated with quantification of the quantities of photosynthate entering the rhizosphere, identification of the organic components of root exudates, and elucidation of the variation in these parameters throughout the growing season and between plant species. A related concern in native systems and with dual cropping systems is determination of the impact of the interaction of root systems from different plant species on the quantities and composition of root exudates.
As a result of the observation that in native soils, photosynthate produced by higher plants (as compared to more limited quantities of carbon fixed by algal activity) generally provides the primary energy source for the soil microbial community, there has been a long‐standing interest in quantifying the proportion of carbon fixed by the plant released into the rhizosphere community and the ultimate deposition of the fixed carbon (i.e. incorporation into humic substances, microbial biomass, or oxidation to carbon dioxide by the soil biota). Microbiologists see this carbon/energy source emanating from the plant root structure as the major driving force for the soil ecosystem. In contrast, plant biologists may regard production of root exudates as both a benefit to the plant and a potential growth limitation. Benefits are accrued from the stimulation of soil microbial populations whose functions encourage plant development (e.g. through nitrogen mineralization, reduction of plant pathogenic interactions, and synthesis of growth‐controlling plant hormones such as indolacetic acid and giberellic acid). Negative implications are associated with the potential for excessive loss of photosynthate and therefore carbon and energy available for plant biomass production.
A variety of means of quantifying and tracing the movement of root exudates have been developed. Logically, it would seem that a useful procedure for assessing the nature and quantity of fixed carbon products exuded from growing roots would be to quantify the biochemicals accumulated in the growth medium of plants grown under axenic conditions in liquid culture either in a greenhouse or growth chamber culture (e.g. see study conducted by Griffin et al. 1977). The primary difficulty encountered with such studies is that the microbial community has a stimulatory effect on root exudate production and may alter the nature of the biochemicals contained in root exudates (Barber and Martin 1977; Klein et al. 1988). Barber and Martin (1977) found that between 5% and 10% of the photosynthetically fixed carbon in wheat and barley plants was lost from roots grown under axenic conditions whereas 12–18% of photosynthate was lost from roots growing in nonsterile soils. Thus, realistic studies of root exudation should be conducted in situations reflecting native soil conditions, as closely as possible.
Utilization of radio‐labeled carbon with plants growing in soils appears to provide an improved estimate of photosynthate incorporation into soil fractions. For this, root exudates are evaluated in greenhouse or growth chamber experiments where photosynthate can be labeled with 14C‐labeled carbon dioxide. Aboveground and belowground portions of the plant must be physically separated so that access of the soil microbial community to 14CO2‐ or 14C‐labeled photosynthetically produced organic compounds can only occur through the root tissue. Movement of the 14C‐labeled photosynthate into the rhizosphere is determined by quantifying the 14CO2 evolved from rhizosphere soil and the incorporation of 14C into humic substances (e.g. using sodium hydroxide fractionation of soil organic matter) and microbial biomass (e.g. with the chloroform fumigation technique). An alternative to using 14C‐labeled substrates for quantifying root exudate production was exploited by Barber and Lynch (1977) who utilized the relationship between the quantity of nutrients available for microbial growth and the mass of cells synthesized (i.e. growth yields). By calibrating rhizosphere microbial biomass produced per unit of growth substrate, a growth yield constant was developed. This constant was used to convert the quantities of microbial biomass produced in the presence of roots growing in liquid culture in growth chambers into mass of photosynthate available for their growth.
A wide range of values for root exudate production are reported in the literature. Nutrients available to rhizosphere microbes vary with plant species, plant developmental stage, and soil temperature, as well as species distribution of the plant community. Newman (1985) concluded that soluble root exudate ranges from 1 to 10 g per 100 g dry roots. Values found in the literature vary depending upon the inclusiveness of the data collected. Some reports of plant carbon incorporated into soil fractions include total plant carbon retained in soil organic fractions whereas in other studies, root exudate production is evaluated exclusive of carbon contributions by root cell death and root death. Martin (1977) recorded a range of rhizodeposition of 14.3–44.4% of photosynthetically fixed carbon (for wheat plants) whereas Davenport and Thomas (1988) found 10% of 14CO2 fixed by corn (Zea mays L.) in belowground plant components compared to 40% for bromegrass (Bromus inermis Leyss). This differential separation of fixed carbon between aboveground and belowground plant biomass resulted in a rhizodeposition rate in the bromegrass twice that of the corn. During the last 31 days of the 55‐day period from germination to seed set of blue grama (Bouteloua gracilis [H.B.K.] Lag.), 33% of the fixed carbon was found in the root biomass, 23% in the root‐derived organic matter contained in the soil, and 22% was released as carbon dioxide (Dormaar and Sauerbeck 1983). These values contrast to early studies such as that by Martin (1977) where 0.8–1.3% of total organic carbon in root‐free soils had been derived from plant photosynthate.
The rhizosphere microbial community can increase the rhizodeposition rate of photosynthetically fixed carbon. Microbes significantly increased root exudation of Agropyron cristatum and Agropyron smithii but had no effect on B. gracilis (Biondini et al. 1988). For the former grass strains, root exudates were reduced 40% and 83% respectively from native soil values when grown in axenic culture. The root biomass of B. gracilis was increased by the presence of an active rhizosphere community (Klein et al. 1988). This microbial stimulation of root exudation may result directly from synthesis of plant hormones by the microbes that are stimulatory to exudate production. An indirect stimulation of exudation could result from microbial utilization of the exudates, thereby preventing their accumulation. This would increase outward diffusion of exudate components from the root.
Merckx et al. (1986) compared the release of photosynthate by maize and wheat roots growing in a growth chamber. After six weeks growth, 1.5% and 2.0% of the corn and wheat photosynthate, respectively, was found as soil organic carbon residues. Photosynthetically fixed carbon dioxide was also detected as soil microbial biomass and as carbon dioxide respired by the soil microbial community. For the wheat plants, approximately 20% of the fixed carbon was detected in each of these carbon pools. Distribution of photosynthetically fixed carbon within these various pools can vary with soil temperature (Meharg and Killham 1989). Increasing the soil temperature from 5 °C to 25 °C increased root‐soil respiration from 5.7% to 24.15%. The proportion of the photosynthate retained in the plant root and soil was greater at 5 °C and 25 °C, with a minimum at 15 °C. The total fixed carbon released as root exudates by A. cristatum, A. smithii, and B. gracilis was estimated to be 8%, 17%, and 15% of photosynthetically fixed carbon.
Microbial growth in the rhizosphere can also be affected indirectly by the quantities of plant nutrients available for plant biomass production. Merckx et al. (1987) grew maize plants at high and low nutrient levels. Plant biomass production was limited at the low nutrient levels after 35 days growth. In soils with the lower nutrient level, 2% of the total 14CO2 fixed by the plant was retained in the soil at all harvest times (38, 35, and 42 days) whereas with the nongrowth‐limiting nutrient levels, 4% of the plant photosynthate was retained in the soil. Maximal soil incorporation was found in the lower nutrient soils at 35 days growth whereas the quantities retained in the high nutrient soils increased throughout the study period. Microbial biomass contained between 28% and 41% of the total soil 14C in the lower nutrient soil whereas in the higher nutrient soil, this value ranged from 20% to 30% of soil residual photosynthetically fixed carbon.
When evaluating plant contributions to soil microbial energy transformation, it is important to consider annual turnover of the root biomass itself. This fixed carbon contribution to microbial nutrient cycling may be considerable. For example, Whitford et al. (1988) found that in the Chihuahuan Desert, 85–90% of the mass of herbaceous annual roots and 40% of the mass of woody shrub roots was decomposed annually. These rates are comparable to or greater than similar processes reported for mesic ecosystems. For comparison, Dahlman and Kucera (1965) found an annual increment in the surface 25 cm of soil (the A horizon) of a prairie grass land in central Missouri of 429 g m−2. This represented about 25% of the total root biomass in this horizon, suggesting an annual root biomass turnover rate of about 25%. For comparison, in a study of live root decomposition in four forests (a mixed deciduous forest in Virginia, a mixed deciduous forest and a Pinus resinosa plantation in Massachusetts, and an Acer saccharum‐Quercus borealis forest in Wisconsin), losses of between 20% and 60% of their initial mass over a 4‐year period were detected for each forest (McClaugherty et al. 1984).
Other methods used to trace movement of carbon fixed by the plants through the soil microbial and organic matter pools include pulse‐chase experiments with 14CO2 (e.g. Rattray et al. 1995) and experiments assessing 13C abundance in soils converted from growth of C‐3 vegetation to C‐4 plants (e.g. Qian and Doran 1996). With the former studies (pulse‐chase), plants are incubated in a 14CO2‐enriched atmosphere. After an appropriate incubation period in the enriched atmosphere, the 14C‐enriched air is replaced with normal ambient air. The change in the 14C enrichment of plant tissue and soil organic components is quantified with time. Initially, the bulk of the radioactivity is detected in plant tissue. After the labeled carbon dioxide is removed from the atmosphere, no further increase in labeled plant tissue occurs. Then, movement of the labeled carbon into soil components is quantified. These studies provide an indication of the amount of plant carbon lost in root exudates and the time course of the process. The use of the stable, heavy isotope of carbon (13C) is valuable in situations where plant communities can be changed (switched from C‐3 to C‐4 plants) and minimal alteration of soil structural properties is desired. In this case, the increased incorporation of 13C into biomass of the C‐4 plants provides the tracer for carbon movement in the soil system.
The composition of root exudate is a primary parameter in selecting for individual species active in the rhizosphere community and the diversity of that community. The composition of root exudates varies with growth condition of the plant and its developmental stage. The variety of compounds which may be contained in root exudates is indicated by a list of candidates compiled by Alexander (1977). These soluble, organic carbon sources accumulated by plants grown aseptically include the following.
· Amino acids: all naturally occurring amino acids.
· Organic acids: acetic, butyric, citric, fumaric, glycolic, lactic, malic, oxalic, propionic, succinic, tartaric, and valeric acids.
· Pentoses and hexoses: arabinose, deoxyribose, fructose, galactose, glucose, maltose, mannose, raffinose, rhamnose, ribose, sucrose, and xylose.
· Pyrimidines and puridines: adenine, cytidine, guanine and uridine
· Vitamins: p‐aminobenzoate, biotin, choline, inositol, nicotinic acid, pantothenate, pyridoxine, and thiamine.
· Enzymes: amylase, invertase, phosphate, and protease
This smorgasbord of readily catabolized organic substances is supplemented by all components contained in sloughed root cells and decaying root biomass. As was indicated above, as much as one‐fifth to one‐third of the fine root biomass may turn over annually. The selective stimulation of microbial populations by the root exudates is readily demonstrated by the alteration of metabolic diversity (see Chapter 2 for a description of methods for determining the soil community property) compared to nonrhizosphere soil (e.g. see Garland 1996a, 1996b for use with whole soils and Frey et al. 1997 for study of soil isolates). The method has been useful in assessing the effect of changes in root exudates due to elevated atmospheric carbon dioxide levels on rhizosphere community dynamics (e.g. Hodge et al. 1998; Rillig et al. 1997).
The activity of the rhizosphere population is determined not only by the composition of root exudate, sloughed cells, and decaying root tissue but also by the total quantities of fixed carbon produced, as was suggested above. From considering the relationship of these three fixed carbon pools to total plant productivity, it is clear that rhizosphere activity is dependent upon plant nutrient status and reaction of the plant to environmental stresses. Examples of data reflective of this microbe–plant growth state interaction are provided by studies of ryegrass (Lolium perenne) under a variety of fertilization strategies (Turner and Newman 1984; Turner et al. 1985), and in studies of grassland management (Ingram et al. 1985). The ryegrass studies demonstrated increased root exudation under phosphorus deprivation. Evaluation of range plants showed an interaction between herbage removal and irrigation, with irrigation causing significant declines in general rhizosphere activity which was mitigated by herbage removal. Active fungal hyphae were decreased by both treatments whereas overall microbial activity was stimulated by herbage removal, suggesting a shift in the relationship of active fungal and bacterial populations in the stressed plant rhizosphere.
A further question regarding the rhizosphere effect relates to the longevity of the plant impact on the soil community following demise of the plant. Death of the plant results in a decline in the rhizosphere community. Little, if any, microbiological effect is carried over in the soil until the next invasion by root tissue. Although there is an impact of adjacent plants on the nature of the rhizosphere community, preceding plant species existent in the ecosystem have little effect on rhizosphere community composition of succeeding plants. The new vegetation determines, to a large degree, its own rhizosphere community composition. Exceptions to this statement of no carryover of rhizosphere effects relate to soil properties which are altered by the presence of root surface or root modulating organisms. For example, fixed nitrogen accumulated in a soil environment due to the symbiotic association of Rhizobium sp. and legumes may provide a nitrogen nutrient source for succeeding plant communities. Similarly, physical or chemical modifications of the soil mineral environment necessarily persist.
8.1.4 Benefits to Plants Resulting from Rhizosphere Populations
Development of an active rhizosphere community has a variety of indirect and direct impacts on plant biomass production. Many of these benefits are proposed since it is difficult to quantify minute modifications of the plant root environment and their effect on overall plant development, but logic combined with sound data suggest that an active, dynamic rhizosphere microbial population is beneficial, if not essential, for optimal plant productivity. Rhizosphere property modifications beneficial to plant growth and development range from the more difficult to prove production of growth‐promoting substances to the clearly documented gains of symbiotic root associations involved in nitrogen fixation.
A wide variety of rhizosphere microorganisms have been demonstrated to produce plant growth hormones, including indolacetic acid (IAA), gibberellins, and cytokinins (e.g. see Nietko and Frankenberg 1989; Tien et al. 1979). These substances could be proposed to stimulate root tissue development, thereby increasing the capacity of the root system to provide nutrients and water required for aboveground biomass function.
A more indirect, but clearly beneficial, result of stimulation of soil community by root invasion is the contribution of rhizosphere microbial communities to development of a stable soil structure conducive to plant community development (i.e. improved soil aggregation) (e.g. Amellal et al. 1998; Andraede et al. 1998) (see Chapter 1 and Tate 1987 for a further discussion of microbial involvement in soil structural development). The rhizosphere community produces polysaccharide material such as capsules and slimes which cement soil mineral particulates into microaggregates. Furthermore, soil structural improvements are gained from fungal mycelial production as well as by association of soil particles with root tissue. This improvement in soil structure through increased soil aggregation results in improvements in soil aeration, water infiltration, and root penetration.
Perhaps the primary benefits of an active rhizosphere community to the plant community result from mineralization of plant biomass. Organic nitrogen, phosphorus, and sulfur compounds are oxidized, thereby liberating ammonium, phosphorus, and sulfate. Thus, the nonbioavailable organic forms of these essential nutrients are converted to plant‐available mineral forms.
Two root‐based symbiotic associations are instrumental in producing a thriving, stable plant community, nitrogen fixation associations (Rhizobium–legume and actinorrhizal associations) (see Chapter 13) and mycorrhizal symbiosis (see Section 8.2). These interactions increase fixed nitrogen resources and facilitate nutrient transfer to the higher plants.
Additionally, plant growth stimulation may result from solubilization of inorganic plant nutrients by production of organic or inorganic acids by rhizosphere organisms. Microbially produced organic acids may solubolize essential minerals simply through acidification of rhizosphere soil or by chelation (e.g. siderophore production) of the metal. The capacity of organic acids excreted by soil bacteria to solubolize soil minerals is exemplified by studies of 2‐ketogluconic acid production by gram‐negative bacteria growing in glucose liquid media (Duff et al. 1963). Additionally, the acidification of the rhizosphere environs through metabolic production of hydrogen ions alters the pH sufficiently to mobilize soil minerals (e.g. Gillespie and Pope 1990a, 1990b).
Solubolization of a variety of metals contained in natural and synthetic insoluble salts and minerals has been demonstrated. Siderophores and other metal chelators are synthesized by a variety of bacteria, including common soil organisms (e.g. Carrillo et al. 1992; Fekte et al. 1983; Hoefte et al. 1991; Jurkevitch et al. 1992; Kloepper et al. 1980a; Meyer et al. 1989; Pesmark et al. 1990) as well as mycorrhizal fungal associates (e.g. Cline et al. 1982; Federspiel et al. 1991). A number of examples of stimulation of plant biomass production by these microbially synthesized metal chelators have been published (e.g. Bar‐Ness et al. 1991; Crowley et al. 1991; Derylo and Skorupska 1992; Kloepper et al. 1980b). Bossier and Verstraete (1986) have presented a model in which the rhizosphere is the most impacted soil microsite for siderophore synthesis.
8.1.5 Plant Pathogens in the Rhizosphere
A common maxim in plant pathology is that a soil‐borne pathogen is more destructive to a plant growing in sterile medium than it would be to a plant developing in native soil where an active rhizosphere community exists. This observation suggests that the rhizosphere microbial community directly or indirectly inhibits invasion of the plant tissue by the pathogen. This disease reduction could result from direct competition between the pathogen and the root microbes, antagonism of the pathogen by root microbes, or alteration of root exudate diffusion into the soil environment in a manner that interferes with chemotaxis of the pathogen to the root. (See van Loon et al. 1998 for a review of this topic.)
Root‐inhabiting microbes and plant pathogens may compete for space, nutrients, or even binding sites on the root surface. Space and nutrient competition could result in failure of the pathogen to develop critical population densities for disease initiation whereas competition for specific binding sites would reduce the capability of the plant pathogen to initiate the infection process.
As indicated in Chapter 7, the rhizosphere is one soil site wherein antibiotic production may confer survival advantage upon microbial populations. Both antibiotics and siderophores produced by a variety of Pseudomonas strains have been shown to reduce or preclude infection of wheat roots by take‐all disease (Hamdan et al. 1991; Kloepper et al. 1980b; Thomashow et al. 1990; Thomashow and Weller 1990). Other suggested interactions of antibiotic microbes in biocontrol of root disease are associated with Rhizoctonia infections (de Freitas and Germida 1991) and onion white rot (Utkhede and Rahe 1980). Similarly, rhizobacteria may also be used to suppress growth of sensitive plants. For example, Pseudomonas spp. have been isolated from roots of winter wheat (Triticum aestivum L.) and downy brome (Bromus tectorum L.) which suppress growth of the downy brome weed.
An additional role of rhizosphere microbes in reducing root disease incidence is interfering with chemotactic attraction of the pathogen to root receptor sites. Chemotactic interactions are more difficult to document in the field but are readily deduced to be of importance in plant disease production. A variety of compounds listed above as components of root exudates may serve as attractants for plant pathogens. Growth of root inhabitants (including mycorrhizal fungi) necessarily reduces both the quantity and diversity of organic compounds diffusing from the root, thereby diminishing the probability of encounter of a plant pathogen.
8.1.6 Manipulation of Rhizosphere Populations
Modification of the composition of any soil community structure through inoculation with exogenous microbes requires a thorough knowledge of microbial interactions as well as “a little bit of luck” if a reasonably predictable outcome of the procedure is desired. Historically, intervention into rhizosphere community dynamics has been a premier example of the microbiologist’s expertise – some might say utility. Biological nitrogen fixation has been encouraged through inoculation of legumes with Rhizobium sp. to encourage symbiotic nitrogen fixation for many decades (see Chapter 13) Long before the microbiology of the symbiosis was understood, legumes have been exploited in crop rotations for their capacity to increase soil fertility. (Historically, in many agricultural systems, fallow cropping with legumes has been used to maintain soil quality. With these cropping systems, the effect of the legume was maximized by not harvesting the crop. Rather, the crop was plowed under to maximize the quantify of fixed nitrogen and organic matter retained in the system.) The long‐established procedure of legume inoculation has been supplemented with methods to supply fungal inocula to stimulate mycorrhizal development plus utilization of a variety of plant growth‐stimulating bacteria, including the free‐living nitrogen‐fixing bacteria of the genus Azospirillum (see examples listed in Table 8.1).
Considering that the potential for modification of soil microbial community structure through inoculation with alien strains has long been viewed with pessimism, the question of why root inoculation is frequently successful must be examined. Although as many failures in inoculation for population alteration are reported as successes, a cursory review of the literature could lead to the conclusion that the only limitations to the procedure are selection of an appropriate microbial strain for inoculation, development of a carrier to sustain the microbial population, and selection of a delivery system. Indeed, a variety of strains, carriers, and inoculation procedures have been developed; but unanswered basic science questions still remain. Considerations that must be examined include why some inoculation studies are successful while others are not, and why some microbial strains are established in the community more readily than others. Complete understanding of these queries is not available, but some basic information exists to suggest means of improving the probability of successful intervention in rhizosphere community dynamics.
Table 8.1 Examples of reports of modification root communities by amendment with alien microbial populations
|
Microbe |
Recipient |
References |
|
Rhizobium spp. |
Legume seeds |
See Chapter 13 |
|
Frankia spp. |
Higher plants |
See Chapter 13 |
|
Mycorrhizal fungi |
Higher plants |
See text |
|
Pseudomonas putida |
Wheat |
Vrany et al. (1981) |
|
Bacillus polymyxa |
Lodgepole pine |
Holl and Chanway (1992) |
|
Pseudomonas fluorescens |
Wheat |
Parke et al. (1986) |
|
Azospirllium spp. |
Spring and winter wheat |
Harris et al. (1989) |
|
Bacillus sp. |
Spring wheat |
Chanway et al. (1988) |
|
Azospirillum brasiliense |
Wheat |
Bashan et al. (1987) |
|
Pseudomonas fluorescens |
Potato |
Bahme et al. (1988) |
|
Pseudomonas serrata |
Wheat and peas |
Astrom and Gerhardson (1988) |
A central maxim in soil ecology is that if a niche exists in an ecosystem for a microbe to function, it will be occupied by a member of the existing soil community. Additionally, displacement of an established, functional member of the community is difficult. Yet, such displacement is frequently desirable in that an improved function of the ecosystem may be achieved by establishment of a more efficient or active microbial strain.
Inoculation of the rhizosphere provides a good example of when this displacement or augmented activity is possible. The first consideration in inoculation of a soil site with an exogenously developed microbial strain relates to the occurrence of a niche in which the microbe has the capacity to function. Nascent root tissue can be a provider of both a habitat and a niche. The external structure of the root is a surface supportive of colonial development whereas a variety of functional opportunities are derived from the invasion of the root tip into the soil (e.g. decomposition of components of root exudates, nitrogen mineralization, nitrogen fixation) which at least initially have not been implemented by existing populations. Thus, the production of new root tissue overcomes the primary barrier to microbial community composition modification. Remaining obstacles are delivery of an appropriate inoculum to the habitat and the capacity of the alien organism to compete with indigenous populations.
Clearly, inoculation of established root systems is difficult and results in a low probability of development of the inoculated populations. This problem of inoculation of established roots has been overcome by drenching of root systems (e.g. Parke et al. 1986) or it has been avoided entirely by inoculation of seeds before planting. Typically, seeds may be drenched with liquid inocula or pelleted solid carriers of the microbial inoculum prior to planting (see Chapter 13 for discussion of inoculation with rhizobia and Bashan 1986 for use with Azospirillum sp.). The assumption underlying this procedure is that the growing root will carry the inoculum with it as the root mass expands. Considering that rhizobial inocula are at least partially established in the field using these methods, seed inoculation may be of less than perfect efficiency, but it still occurs. Azospirillum brasilense is also mixed throughout the soil and from plant to plant by growth of wheat roots (Bashan and Levanony 1987, 1989). Alternatives to root drenches and soil inoculation include banding of the microbial inoculum material into the soil adjacent to the expanding root mass. This procedure is underlain by the assumptions that the roots will grow through the band of inoculum, the inoculum will survive sufficiently long in the soil for this encounter to occur, and that the inoculant will compete successfully with indigenous soil microbes for habitable surfaces on the root.
Competition of alien organisms with indigenous soil microbial populations is probably the least understood aspect of any soil community inoculation procedure. The added organisms must be capable of overcoming the normal soil community defenses to invasion as well as coping with soil physical and chemical barriers. Many of the growth requirements for the microbe are met by the provision of nutrients and growth surface by the root; hence dealing with biological competitors becomes the primary difficulty to establishment.
Development of microbial strains which would be useful for field inoculation is generally initiated by isolation of an organism from the indigenous soil population which is amenable for selection of genetic variants. This isolate may then be modified either through selection of natural genetic variants (mutants) or by genetic engineering procedures where appropriate genes are inserted into the genome. In either case, the objective is to develop an organism which is more efficient in a given process (e.g. nitrogen fixation) or which possesses an improved metabolic capacity (e.g. ability to decompose an environmental contaminant).
By starting with a native member of the soil or root community, it may be assumed that most of the necessary traits for successful competition with the indigenous population preexisted in the parent strain. The question then becomes, “Did the genetic manipulation or laboratory culture reduce or eliminate this competitive capacity?” The query is frequently answered negatively. For example, legume nodules are commonly formed by indigenous rhizobial strains in spite of the inoculation of seeds with more efficient varieties. Development of Rhizobium strains with improved nitrogen fixation capabilities has become a fine art yet the art is commonly lost when the efficient strains do not enter the site of expression of their enhanced ability – the root nodule. Unfortunately, laboratory‐selected rhizobia are not always the best competitors for nodule occupancy (e.g. Klubek et al. 1988; Materon and Hagedorn 1982; Moawad and Bohlool 1984; Robert and Schmidt 1983; Smith and Wollum II 1989; van Rensburg and Strijdom 1985). Variability in nodule occupancy by alien rhizobial strains is exemplified by data from van Rensburg and Strijdom (1985) where nodule occupancy by inoculant strains ranged from 17.7% to 100%, with the highest values associated with legumes growing in soils with limited native rhizobial populations of appropriate specificity for the legume. As discussed in Chapter 13, successful occupancy of the nodule by the inoculant depends upon its capacity to deal with the general rhizosphere population as well as possession of appropriate biochemical traits to efficiently interact with the root to form an effective nodule.
Thus, the rhizosphere microbial community structure has been manipulated through inoculation of the plant with alien organisms to improve plant development (i.e. in most cases, improve crop yields). Great potential exists for further exploitation of these skills to alter the microbial community to accelerate plant community development and reduce soil pollutant loads. Plant growth improvement may be achieved by establishment of members of the rhizosphere community with the capacity to decompose plant toxicants or improve nutrient availability or augment soil granulation whereas the rhizosphere community may also be utilized to couple the nutrient generation ability of higher plants (e.g. fixed carbon supplies) with the ability of a laboratory‐generated rhizosphere inoculant bacterial species to degrade a troublesome pollutant (perhaps, even cometabolically) (see Chapter 16).
8.2 Mycorrhizal Associations
Fungus–root or mycorrhizal associations have been known to exist since descriptions by Frank in the early 1880s (cited in Allen 1991). These associations have been extensively described The description, or definition, provided for these fungal and higher plant symbioses, to a large degree, is linked to the definer's bias toward the importance of the plant or the microbe to ecosystem function. This is best seen by comparing statements by Marx and Allen. Marx (1972) describes mycorrhizal associations as follows.
The infection of feeder roots of most flowering plants by symbiotic fungi and the transformation of these roots into unique morphological structures called mycorrhizae (fungus‐roots) undoubtedly constitutes one of nature's most widespread, persistent, and interesting examples of parasitism. Most plants of economic importance to man are actually dual organisms – part plant and part symbiotic root‐inhabiting fungi.
This plant‐oriented view stresses the utilization or perhaps even the “theft” of plant photosynthate by the controlled parasites in this symbiotic association. In contrast, Allen (1991) described the fungal–plant interaction from a more neutral or microbially oriented aspect.
A mycorrhizae is a mutualistic symbiosis between plant and fungus localized in a root or root‐like structure in which energy moves primarily from plant to fungus and inorganic resources move from fungus to plant.
In this definition, the mutualistic benefits of the interaction are stressed.
From an environmental or soil community consideration, the fungal–plant associations in mycorrhizal symbioses can be viewed on a gradient of increasing association of the plant and soil community (Table 8.2). At the most extreme position lie the general soil microbial populations. These organisms are involved in mineralization of native soil organic matter, soil‐incorporated organic components (plant and animal debris), microbial biomass, and soluble organic carbon components contained in infiltrated water. This community functions totally at the mercy of the nutrient supply which may range from nearly total deprivation (i.e. a desert soil) to absolute luxury (e.g. soil receiving nutrient‐laden industrial effluent). Of more direct relevance to the discussion at hand is the fact that the nutrients produced by organic compound mineralization by these general soil microbes may not be directly available to the aboveground community. Clearly, nutrient transfer to higher plants is dependent upon proximity of the microbial colony to active roots. In this case, a locational displacement as small as a few millimeters could determine the rate of mineral nutrient recovery by the plant.
An enhancement of ecosystem three‐dimensional associations is derived from the transition from general soil populations to rhizosphere‐linked communities. Rhizosphere communities catalyze all the processes described above for nonrhizosphere soil but their value to aboveground biomass sustainability is augmented by improved efficiency of transfer of mineral nutrients to the plant community.
Table 8.2 Comparison of properties of nonrhizosphere, rhizosphere, and mycorrhizosphere soils
|
Nonrhizosphere |
Rhizosphere |
Mycorrhizosphere |
|
|
|
Note that nonrhizosphere soil metabolic activity is not affected directly by rhizosphere interactions, but mycorrhizal fungi may enter this soil region and catalyze biogeochemical processes.
The ultimate association of soil microbes with growing plants is symbiotic fungal–root associations. This mycorrhizal symbiosis could be said to bridge these community types – nonrhizosphere, rhizosphere, and fungus–root. Although there is an intimate association of the fungus with plant root cells (with some types of mycorrhizae, actual penetration of the root cell itself occurs), the fungal biomass extends well beyond the rhizosphere into nonroot‐impacted soil. Thus nutrients may be accumulated from nonrhizosphere soil by the fungus, and transported through the rhizosphere community into the plant root. Inversely, the photosynthate from the plant supports growth and development of the entire hyphal network. Furthermore, this mycelial network may directly link roots of plants of the same or different species in that the same fungal body may infect a number of individual plants.
The physical association of mycorrhizal fungi with plant roots has been extensively described and serves as the primary basis for classification of mycorrhizae. These associations may be classed as ectomycorrhizae, endomycorrhizae, or ectendomycorrhizae. In this grouping of mycorrhizae, the commonly encountered vesicular‐arbuscular mycorrhizae are considered endomycorrhizae. A number of reviews of the biology of mycorrhizae have been published (e.g. Allen 1991; Mukerjii et al. 1991). Thus, only basic descriptions of these classes of mycorrhizae will be presented here.
Ectomycorrhizal fungi penetrate intracellularly and partially replace the middle lamellae between cortical cells of feeder roots. These fungi form a dense mycelial net around and between the plant cells termed a Hartig net. Ectomycorrhizal associations are also characterized by a dense, generally continuous hyphal network over the feeder root surface called a fungal mantle. This fungal mantle varies from one to two hyphal diameters to as many as 30 or 40 depending upon the fungal associate, the host, and the environmental conditions. Examples of plants forming ectomycorrhizal associations are species in the families Pinaceae, Salicaceae, Betulaceae, and Fugaceae. Most ectomycorrhizal fungi are basidomycetes (primarily of the families Amanitaceae, Boletaceae, Cortinariaceae, Russulaceae, Tricholomataceae, Rhizopogonaceae, and Sclerodermataceae).
Endomycorrhizae are distinguished by the fact that the fungus penetrates the cortical cells of feeder roots and may form large vesicles and arbuscles (hence the term vesicular‐arbuscular mycorrhizae). These fungi do not form dense fungal mantles but do develop a loose, intermittent arrangement of mycelium on the root surface. Endomycorrhizae are formed by most agronomic, horticultural, and ornamental crops, as well as some forest tree species that do not form ectomycorrhizae. The fungal species are phycomyces – many of which are in the genus Endogone.
As with many biological classification systems, many organisms form associations that are not easily classified into any of the predominant groupings. For mycorrhizal associations, the ectendomycorrhizae constitute this grouping. These mycorrhizae resemble ectomycorrhizae in forming a Hartig net and a fungal mantle. A resemblance with the endomycorrhizae is associated with their penetration of cortical cells. This grouping is the least studied and the nature of the fungal symbionts has not been totally elucidated.
Although mycorrhizae provide intriguing subjects for biological and physiological analysis, the emphasis for this study is to delineate (i) the role of mycorrhizal symbioses in ecosystem function, (ii) their importance in nutrient cycling in soil–plant interactions, and (iii) their utility in bioremediation plans.
8.2.1 Mycorrhizae in the Soil Community
A variety of elegant studies of soil enzymes and respiratory activity in relationship to soil physical and chemical properties and biological interactions (both belowground and aboveground) have been conducted. Some of this research has been directed at quantifying fungal and bacterial contributions to these activities through incorporation of population specific antibiotics (e.g. cycloheximide to inhibit fungal activity and streptomycin to control bacterial activity) into the laboratory‐incubated soil–plant samples but evaluation of the role of mycorrhizal fungi in these soil functions is rarely considered. Standard methods for soil collection rarely preserve the soil–root–fungal associations. Hence, even direct counts of fungal mycelia in the soil would rarely, if ever, be linked to plant roots. This omission of a consideration of these important trophic interactions occurs in spite of the knowledge that mycorrhizal fungi constitute a major portion of soil biomass and that these fungi extend throughout the soil profile, far beyond the regions classified as rhizosphere soil. An example of the association of soil enzymatic activity with root and fungal development was provided by Spalding et al. (1975). In their study, cellulase, invertase, polygalacturonase, and peroxidase activities were found to increase in soil under lycopodium fairy rings, suggesting a mycorrhizal fungal association with organic matter metabolism.
The potential for mycorrhizal contribution to total soil metabolic activities can be appreciated by examining examples of the quantities of biomass contained in these structures. Fogel and Hunt (1983), in a study of a young, second‐growth Douglas fir (Pseudotsuga menziesii [Mirb.] Franco) stand, found that mycorrhizae constituted 6% of the total tree standing crop. Furthermore, they found that roots and mycorrhizae contained larger reserves of nitrogen, phosphorus, potassium, and magnesium than did the forest floor or soil fungi. Fine roots and mycorrhizae contributed between 84% and 78% of the total tree organic matter to the soil. This study suggests that although the mycorrhizal fungal biomass only constitutes a small portion of the standing crop, it may account for 50% of the organic matter throughput in the P. menziesii stand. Read (1984) suggests that mycorrhizal fungal biomass may be the largest microbial biomass component of many forest soils. Allen (1991) states that “mycorrhizal fungi may be the single largest consumer group of net primary production in many, if not most, terrestrial biomes.” Vesicular‐arbuscular fungal hyphae have been quantified at densities of up to 38 m cm−3 (Allen and Allen 1986). It must be remembered that although a portion of the fungal biomass is intimately associated with the root, the fungal net extends throughout the soil surface. (See Allen 1991 for a review of this topic.)
From these observations, a number of benefits of mycorrhizal development to the total soil microbial community can be delineated. Foremost among the contributions to the soil community is the capacity to cycle nutrients, i.e. mineralize accumulated biomass. Although most research efforts have been directed at quantifying mineral nutrient transfer from fungal biomass or soil to plant tissue, it is reasonable to assume that a portion of the metabolic products of this fungal activity will be released to the soil microbial community. For example, mycorrhizal fungi have been shown to mineralize soil organic phosphate through synthesis of phosphatase (e.g. Dighton 1983 and Pasqualini et al. 1992) and to solubolize mineral phosphorus through the acidification of the soil habitat through carbon dioxide production (see Knight et al. 1989).
Indirect benefits to the soil community by mycorrhizal fungal growth result from a generalized improvement of soil structure. Fungal hyphal development increases the association of soil particulates into aggregates (see Tisdall and Oades 1982 and for a review of this topic, Tate 1987). This improved soil structure has a direct impact on the indigenous microbial community through improved aeration and moisture infiltration and an indirect effect via stimulation of plant root growth. Clearly, any augmentation of root development increases the quantities of fixed carbon reaching the soil microbial community.
8.2.2 Symbiont Benefits from Mycorrhizal Development
Mycorrhizal fungi are clearly instrumental in augmenting plant nutrient availability in nutrient‐stressed ecosystems. A wide range of data (at times contradictory data) has been collected demonstrating nutritional benefits to plant communities in stressed communities through mycorrhizal symbiotic associations. Most research has shown improved nutrient transfer to the plant tissue through augmentation of the adsorbing surface of roots by extension of the fungal mycelium into nonrhizosphere soil. Gains in phosphate, nitrogen, and water transfers are most commonly reported. The expansive literature documenting these benefits has been reviewed by Allen (1991), Bagyarak (1991), Gupta (1991), and Mukerjii et al. (1991).
Perhaps in a more intriguing vein, mycorrhizal fungi may not only enhance soil–plant transfer of nutrients, but may also be instrumental in movement of nutrients between plants (e.g. Camel et al. 1991, Eason et al. 1991, Hamel et al. 1991, Read et al. 1989). Read et al. (1989) demonstrated through the use of 14CO2 that carbon moves freely between plants connected by mycorrhizal mycelium. This plant–plant bridging by fungal hyphae occurs between host plants of the same or differing species. Similarly, Chiariello et al. (1982) found that 32P‐phosphate sprayed on leaves of Plantago erecta in grassland was transferred to the shoots of about 20% of close neighbors. Vesicular‐arbuscular mycorrhizae were noted to connect the root systems of neighbors of different species and considered to have mediated the nutrient transfer. Camel et al. (1991) found that the hyphal front may advance at a rate of 2.3 cm week−1 in soil sand mixtures and that bridges of at least 90 mm may be formed. These workers suggest a competitive advantage of this bridging in plant groupings where mycorrhizal inoculum is limited. Plants developing in a less desirable portion of the ecosystem (e.g. a shaded site) may gain benefits of the photosynthetic activity of the less stressed members of the community as well as gains from nutrient production of the mycorrhizal function through an interconnected root system.
A further stimulus to plant biomass productivity is associated with the capacity of mycorrhizal associations to reduce or prevent plant disease development (see Jalali and Jalili 1991 for a review of this topic). The association of the fungus with plant disease‐susceptible root tissue may reduce pathogen vulnerability through modification of exudate composition or concentration, stimulation of plant disease‐protective response through its own infection of the root tissue, inhibit competing microbial populations directly through synthesis of antibiotics, and limit access of the plant pathogen to root tissue by physically occupying the root surface.
Enhancement of fungal biomass by the symbiotic association with root tissue is generally attributed to augmented availability of photosynthate in the form of simple sugars for fungal energy and carbon needs and reduced competition with the general rhizosphere microflora. Kucey and Paul (1982) found that within mycorrhizal, nodulated faba beans (Vicia faba L.) about 4% of the carbon fixed by the host was transferred to the fungal associate. (For a more detailed examination of this topic, see Allen 1991.) A competitive advantage for the fungal associate in dealing with soil microbial populations in general and more specifically rhizosphere microbes is derived from its close physical association with the host root. Intrusion into the interlameller space between cortical cells (as well as cell penetration by the endomycorrhizae) and intimate contact with the root surface facilitate nutrient recovery from the host tissue.
Of interest from the viewpoint of impact of the host plant on the behavior of mycorrhizal fungi in soil is the potential for selective stimulation of spore outgrowth. Published results regarding selective stimulation of fungal spore outgrowth by plant root products are mixed yet a real microsite stimulation most probably does occur. Bechard and Piche (1989) found that in their study of carrots and the vesicular‐arbuscular mycorrhizal fungus Gigaspora margarita, root volatiles provided little stimulation of spore germination and that the exudates alone had no effect. Considerable stimulation was detected from the synergistic interaction between the volatile and exudate root products. From their study of the effect of root exudates of Trifolium repans on Glomus fasiculatus germination, Elias and Safir (1987) conclude that the primary stimulation of spore outgrowth results from exudates from phosphorus‐deficient clover seedlings when compared to the effect of exudates from phosphorus‐sufficient seedlings, suggesting that it is the quality of the exudates which affect the spore germination rates. Further studies (Nair et al. 1991) suggest that two isoflavonoids (7‐hydroxy‐4’‐methoxy isoflavone and 5,7,‐dihydroxy,4’‐methoxy isoflavone) may be signal molecules in vesicular‐arbuscular mycorrhizal symbioses. Other workers (see Bowen 1969) conclude that root exudates had no effect on spore outgrowth. It may be deduced from evaluation of these disparate observations that mycorrhizal fungal spores are sensitive to root exudates and that variability in composition (quality) of these plant products with host nutritional status greatly affects this interaction.
A further observation of the infection process in mycorrhizal development is a potential interaction of plant growth‐promoting bacteria (e.g. Bowen and Theodorou 1979; Garbaue and Bowen 1989; Meyer and Linderman 1986a,b). Bowen and Theodorou (1979) found that a variety of bacteria could depress, have no effect on, or stimulate mycorrhizal development in the rhizoplane of Pinus radiata, suggesting the necessity of fungal compatibility with the rhizosphere bacterial populations. Garbaue and Bowen (1989) actually found that “helper” microorganisms exist in the rhizosphere that improved the efficiency of ectomycorrhizal infection of Pi. radiata D. Don growing in a sandy podzol. Meyer and Linderman (1986a) found that a combination of plant growth‐stimulating bacteria and mycorrhizal development of subterranean clover (Trifolium subterraneum L.) resulted in considerably greater plant biomass synthesis than with either microbial component taken singly.
8.2.3 Environmental Considerations
The statement by Allen (1991) that “Mycorrhizae represent one of the least understood, most widespread, and most important biological symbioses on Earth” is no more greatly appreciated than when application of this fungal–plant interaction to ecosystem problems is considered. Enlightenment about successful exploitation of mycorrhizal associations is gained by a consideration of the current applications of mycorrhizal interaction management (development of functional plant communities on disturbed sites – land reclamation, and inoculation of nursery stock for field planting) and an interesting but as yet little explored option (development of genetically engineered fungal associates for protection of host plants from chemical contaminants in their soil).
Although commonly considered from the viewpoint of industrially polluted soil, reclamation management to improve soil quality is more frequently related to “tired” agricultural soil systems (i.e. those cultivated in some cases for several centuries by methods designed to maximize crop yields at times at the sacrifice of maintaining soil quality). Traditionally, this soil management includes procedures which encourage a soil structure conducive to aboveground plant community development (see Tate 1987 for a review of this topic) and establishment of stable populations of organisms involved in biogeochemical cycling (see Tate 1985). Due to their ability to improve longevity and productivity of aboveground plant communities, mycorrhizal associations are a critical component in soil reclamation management.
Plant community gains from management of mycorrhizal associations for soil quality improvement are derived from both soil structural enhancements resulting from fungal contributions to soil aggregate formation and the improved availability of essential plant nutrients. Rapid and somewhat long‐lasting benefits accrue from amendment of degraded soil with a variety of organic matter sources and fertilizers (e.g. Fresquez et al. 1990; Stroo and Jencks 1985; Tester 1990; Wong and Ho 1991), but for minimization of anthropogenic intervention, a stable soil microbe–plant interactive community must be developed. As indicated above, mycorrhizal associations are best developed under stressed conditions, especially phosphate stress. Examples of additional limitations common to degraded soils which encourage mycorrhizal symbioses development include fertilizer application practice and cropping system (Ellis et al. 1992), soil erosion (i.e. loss of fungal propagules with surface soil losses; Day et al. 1987), vegetation (Newman et al. 1981), and soil organic matter (Harvey et al. 1981). Hence, for rapid, and enduring development of a reclaimed ecosystem, site management should include inclusion of fungal propagules in the reclamation target soil, implementation of plant nutrient management procedures which will not prevent mycorrhizal development, and where possible, utilization of inoculated transplants for establishment of aboveground plant communities.
Questions associated with inculation of seedlings with mycorrhizal fungal propagules relate not to the benefit of such associations to plant survival and productivity – these have long been assumed to be major pluses – but rather to optimization of inoculation processes and selection of the best fungal strains for the symbioses. For example, Marx et al. (1978) documented stimulation of loblolly pine seedlings by a variety of ectomycorrhizal fungi whereas more recently Theodorou and Bowen (1987) evaluated the selectivity of basidiospore germination and selection by pine seedlings and development of an effective method for inoculum conservation in the absence of host plants. Duponnois and Garbaye (1991) investigated the utilization of mycorrhization helper bacteria in establishing ectomycorrhizal fungal associations in forest nurseries. The unquestionable utility of such inoculation is exemplified by studies of Lindemann et al. (1984), where mycorrhizal interactions among other soil microbial activities were evaluated in mine spoil reclamation, and of Wilson et al. (1991) where mycorrhizae were utilized to reclaim portions of a dewatered flue‐gas desulfurization sludge pond.
Future considerations of the utility of exploiting mycorrhizal associations for soil reclamation may also be based on extension of genetic engineering procedures to mycorrhizal fungi and modification of the carbon substrate metabolism capacity of these organisms. As was discussed above, mycorrhizal fungi extend from the growing root into the surrounding soil. The soil particle‐associated mycorrhizal fungal hyphae catabolize soil organic carbon components. Clearly, potential exists to develop mycorrhizal fungi with the capacity to decompose pollutants that may occur in the vicinity of the roots of plants selected for soil reclamation (e.g. pine trees and associated shrubs) or even crop plants. The objective for the first situation would be to develop fungal associates that provide a benefit to the plant beyond those generally associated with mycorrhizal interactions, i.e. removal of potential plant toxicants. In the case of the crop plant, the fungi could serve as a “first line of defense” per se, thereby preventing toxicants which may contaminate food plants from encountering and being accumulated in sensitive root tissue.
8.3 The Mycorrhizosphere
Traditionally, rhizosphere activity and mycorrhizal symbioses have been studied as if they represented separate ecosystem components but the preceding discussion of these ecosystem phenomena leads at least to the suspicion that they occur concurrently on many plant roots rather than as distinct entities. In all soil systems, a growing plant will develop some sort of microbial–plant interaction termed a rhizosphere. As our understanding of mycorrhizal symbioses expands, it is becoming more obvious that the vast majority of higher plants are mycorrhizal. Thus, we cannot conclude that all rhizospheres are mycorrhizospheres, but it is also evident that most of the data collected from field studies of rhizosphere populations must have resulted from evaluation of mycorrhizospheres – with the experimenter being ignorant of the manifestation of fungal mycelium in association with the roots, or not possessing the requisite training to differentiate mycorrhizal root structures. This conceptual oversight is gradually being corrected.
In evaluating the quantities of exudates and their chemical substituents, quantification of the intensity of mycorrhizal association is necessary. With symbiotic fungal–plant associations, essentially the entire root surface may be covered by several layers of fungal mycelium (ectomycorrhizal associations) or a major portion of the root surface may be free of obvious fungal hyphal development with the fungus deriving its nutrient through intracellular penetration (endomycorrhizal associations). In either case, the heterotrophic fungi consume root exudates plus the organic carbon contained in sloughed cells, thereby altering both the quality and quantity of root exudates. Some components of the exudate may be decreased or totally depleted.
A further complexity of carbon cycling in the more commonly occurring root soil complex, the mycorrhizosphere, is that one reservoir of fixed carbon in the soil is overlooked. Plant carbon is converted into the mycorrhizal fungal biomass. This carbon pool extends far beyond the rhizosphere, thus serving as a vehicle to translocate carbon from the plant to the soil community in general, following death of the fungal symbiont (see Fogel 1988 for further discussion of this topic). Therefore, it must be observed that the total nature of the microbial populations in the region of the root is altered by the presence of the fungal symbiont. Direct and indirect impacts of this interaction between the fungus and root exudates on the rhizosphere and rhizoplane microbial populations include determination of the active species and alteration of the total microbial biomass produced (e.g. Meyer and Linderman 1986b), as well as the control of plant pathogen interactions. These microbial consequences of fungal symbiont and rhizosphere bacteria interactions reflect back to the plant in that they may result in improved plant growth (Azcon 1989; Will and Silvia 1990)
These limited evaluations of the impact of both rhizosphere populations and mycorrhizal associations with plant growth emphasize the need to expand future root community studies to include all levels of trophic interactions – be they the result of symbiotic fungi, nitrogen fixation symbioses (Rhizobium–legume or actinorrhizal) or general rhizosphere bacterial, fungal, protozoan, or nematode population growth. Each of these entities withdraws nutrients from the common source – plant photosynthate – either directly or indirectly, produce biomass, and alter the productivity or lack thereof of their neighbors. Ultimately, total ecosystem dynamics are controlled by the complex interactions of each member of the soil community.
8.4 Conclusion
The rhizosphere is most certainly an oasis of biological activity within the soil ecosystem. This ecosystem is represented by a diversity of microbial populations, the complete range of plant–microbe and microbe–microbe interactions, and the relative inclusiveness of all essential biogeochemical processes for total ecosystem development. An overall view of the processes occurring in the rhizosphere/mycorrhizosphere may be summarized by three words – interactions, productivity, and protection. That is, the rhizosphere is populated by a vast variety of life forms each of which is impacted both positively and negatively by all other creatures existing in the system; the productivity of the microbial community as well as the aboveground community is interlocked with the viability and stability of the microbial community (i.e. neither the plant community or the microbial components can function optimally singly); and lastly, the microbial community confers a resistance to the plant component of the system by competing with and in many cases destroying potentially harmful plant pathogens.
References
1. Alexander, M. (1977). Introduction to Soil Microbiology. New York: Wiley.
2. Allen, M.F. (1991). The Ecology of Mycorrhizae. New York: Cambridge University Press.
3. Allen, E.B. and Allen, M.F. (1986). Water relations of xeric grasses in the field: interactions of mycorrhizae and competition. New Phytol. 104: 559–571.
4. Amellal, N., Burtin, G., Bartoli, F., and Heulin, T. (1998). Colonization of wheat roots by an exopolysaccharide‐producing Pantoea agglomerans strain and its effect on rhizosphere soil aggregation. Appl. Environ. Microbiol. 64: 3740–3747.
5. Andraede, G., Mihara, K.L., Linderman, R.G., and Bethlenfalvay, G.J. (1998). Soil aggregation status and rhizobacteria in the mycorrhizosphere. Plant Soil 202: 89–96.
6. Astrom, B. and Gerhardson, B. (1988). Differential reactions of wheat and pea genotypes to root inoculation with growth‐affecting rhizosphere bacteria. Plant Soil 109: 263–270.
7. Azcon, R. (1989). Selective interaction between free‐living rhizosphere bacteria and vesicular‐arbuscular mycorrhizal fungi. Soil Biol. Biochem. 21: 639–344.
8. Bagyaraj, D.J. (1991). Ecology of vesicular‐arbuscular mycorrhizae. In: Handbook of Applied Mycology. Volume 1. Soil and Plants (eds. D.K. Arora, B. Rai, K.G. Mukerji and G.R. Knudsen), 3–34. New York: Marcel Dekker.
9. Bahme, J.B., Schroth, M.N., van Gundy, S.D. et al. (1988). Effect of inocula delivery systems on rhizobacterial colonization of underground organs of potato. Phytopathology 78: 534–542.
10. Barber, D.A. and Lynch, J.M. (1977). Microbial growth in the rhizosphere. Soil Biol. Biochem. 9: 305–308.
11. Barber, D.A. and Martin, J.K. (1977). The release of organic substances by cereal roots into soil. New Phytol. 76: 69–80.
12. Bar‐Ness, E., Chen, Y., Hadar, Y. et al. (1991). Siderophores of Pseudomonas putida as an iron source for dicot and monocot plants. Plant Soil 130: 231–241.
13. Bashan, Y. (1986). Alginate beads as synthetic inoculant carriers for slow release of bacteria that affect plant growth. Appl. Environ. Microbiol. 51: 1089–1098.
14. Bashan, Y. and Levanony, H. (1987). Horizontal and vertical movement of Azospirillum brasilense CD in the soil and along the rhizosphere of whet and weeds in controlled and field environments. J. Gen. Microbiol. 133: 3473–4380.
15. Bashan, Y. and Levanony, H. (1989). Wheat root tips as a vector for passive vertical transfer of Azospirillum brasilense CD. J. Gen. Microbiol. 135: 2899–2908.
16. Bashan, Y., Levanony, H., and Ziv‐Vecht, O. (1987). The fate of field‐inoculated Azospirillum brasilense CD in wheat rhizosphere during the growing season. Can. J. Microbiol. 33: 1074–1079.
17. Bechard, G. and Piche, Y. (1989). Fungal growth stimulation by CO2 and root exudates in vesicular‐arbuscular mycorrhizal symbiosis. Appl. Environ. Microbiol. 55: 2320–2325.
18. Biondini, M., Klein, D.A., and Redente, E.F. (1988). Carbon and nitrogen losses through root exudation by Agropyron cristatum, A. Smithii, and Bouteloua gracilis. Soil Biol. Biochem. 20: 477–482.
19. Bolan, N.S., Elliott, J., Gregg, P.E.H., and Weil, S. (1997). Enhanced dissolution of phosphate rocks in the rhizosphere. Biol. Fertil. Soils 24: 169–174.
20. Bossier, P. and Verstraete, W. (1986). Ecology of Arthrobacter JG‐9‐detectable hydoxamate siderophores in soils. Soil Biol. Biochem. 18: 487–492.
21. Bowen, G.D. (1969). Nutrient status effects on loss of amides and amino acids from pine roots. Plant Soil 30: 139–142.
22. Bowen, G.D. and Theodorou, G. (1979). Interactions between bacteria and ectomycorrhizal fungi. Soil Biol. Biochem. 11: 119–126.
23. Camel, S.B., Reyes‐Solis, M.G., Ferrera‐Cerrato, R. et al. (1991). Growth of vesicular‐arbuscular mycorrhizal mycelium through bulk soil. Soil Sci. Soc. Am. J. 55: 389–393.
24. Carrillo, G.C., del Rosario, G., and Vazquez, M. (1992). Comparative study of siderophore‐like activity of Rhizobium phaseoli and Rhizobium fluorescens. J. Plant Nutr. 15: 579–590.
25. Chanway, C.P., Nelson, L.M., and Holl, F.B. (1988). Cultivar‐specific growth promotion of spring wheat by coexistent Bacillus spp. Can. J. Microbiol. 34: 925–929.
26. Cheng, W.X. (1996). Is available carbon limiting microbial respiration in the rhizosphere. Soil Biol. Biochem. 28: 1283–1288.
27. Chiariello, N., Hickman, J.C., and Mooney, H.A. (1982). Endomycorrhizal role for interspecific transfer of phosphorus in a community of annual plants. Science 217: 941–943.
28. Cline, G.R., Powell, P.E., Szaniszlo, P.J., and Reid, C.P.P. (1982). Comparison of the abilities of hydroxamic, synthetic, and other natural organic acids to chelate iron and other ions in nutrient solution. Soil Sci. Soc. Am. J. 46: 1158–1164.
29. Crowley, D.E., Want, Y.C., Reid, C.P.P., and Szaniszlo, P.J. (1991). Mechanisms of iron acquisition from siderophores by microorganisms and plants. Plant Soil 130: 179–198.
30. Dahlman, R.C. and Kucera, C.L. (1965). Root productivity and turnover in native prairie. Ecology 46: 84–89.
31. Davenport, J.R. and Thomas, R.L. (1988). Carbon partitioning and rhizodeposition in corn and bromegrass. Can. J. Soil Sci. 68: 693–701.
32. Day, L.D., Sylvia, D.M., and Collins, M.E. (1987). Interactions among vesicular‐arbuscular mycorrhizae, soil, and landscape position. Soil Sci. Soc. Am. J. 51: 636–639.
33. De Freitas, J.R. and Germida, J.J. (1991). Pseudomonas cepacia and Pseudomonas putida as winter wheat inoculants for biocontrol of Rhizoctonia solani. Can. J. Microbiol. 37: 780–784.
34. Derylo, M. and Skorupska, A. (1992). Rhizobial siderophore as an iron source for clover. Physiol. Plant. 85: 549–553.
35. Dighton, J. (1983). Phosphatase production by mycorrhizal fungi. Plant and Soil 71: 455–462.
36. Dormaar, J.R. and Sauerbeck, D.R. (1983). Seasonal effects of photoassimilated carbon‐14 in the root system of blue grama and associated soil organic matter. Soil Biol. Biochem. 15: 475–479.
37. Duff, R.B., Webley, D.M., and Scott, R.O. (1963). Solubolization of minerals and related minerals by 2‐ketogluconic‐acid producing bacteria. Soil Sci. 95: 105–114.
38. Duponnois, R. and Garbaye, J. (1991). Effect of dual inoculation of douglas fir with the ectomycorrhizal fungus Laccaria laccata and mycorrhization helper bacteria (MHB) in two bare‐root forest nurseries. Plant Soil 138: 169–176.
39. Eason, W.R., Newman, E.I., and Chuba, P.N. (1991). Specificity of interplant cycling of phosphorus: the role of mycorrhizas. Plant Soil 137: 267–274.
40. Elias, K.S. and Safir, G.R. (1987). Hyphal elongation of Glomus fasciculatus in response to root exudates. Appl. Environ. Microbiol. 53: 1928–1933.
41. Ellis, J.R., Roder, W., and Mason, S.C. (1992). Grain‐sorghum‐soybean rotation and fertilization influence on vesicular‐arbuscular mycorrhizal fungi. Soil Sci. Soc. Am. J. 56: 789–794.
42. Fallik, E., Okon, Y., and Fisher, M. (1988). Growth response of maize roots to Azospirillum inoculation: effect of soil organic matter content, number of rhizosphere bacteria and timing of inoculation. Soil Biol. Biochem. 10: 45–50.
43. Federspiel, A., Schuler, R., and Haselwandter, K. (1991). Effect of pH, l‐ornithine and l‐proline on the hydroxamate siderophore production by Hymenoscyphus ericae, a typical ericoid mycorrhizal fungus. Plant Soil 130: 259–261.
44. Fekte, F.A., Spence, J.T., and Emery, T. (1983). Siderophores produced by nitrogen‐fixing Azotobacter vinelandii OP in iron‐limited continuous culture. Appl. Environ. Microbiol. 46: 1297–1300.
45. Fogel, R. (1988). Interactions among soil biota in coniferous ecosystems. Agric. Ecosyst. Environ. 24: 68–85.
46. Fogel, R. and Hunt, G. (1983). Contribution of mycorrhizae and soil fungi to nutrient cycling in a Douglas‐fir ecosystem. Can. J. For. Res. 13: 219–232.
47. Fresquez, P.R., Francis, R.E., and Dennis, G.L. (1990). Sewage sludge effects on soil and plant quality in a degraded, semiarid grassland. J. Environ. Qual. 19: 324–329.
48. Frey, P., Freyklett, P., Garbaye, J. et al. (1997). Metabolic and genotypic fingerpriting of fluorescent pseudomonads associated with the douglas fir Laccaria bicolor mycorrhizosphere. Appl. Environ. Microbiol. 63: 1852–1860.
49. Garbaue, J. and Bowen, G.D. (1989). Stimulation of ectomycorrhizal infection of Pinus radiata by some microorganisms associated with the mantle of ectomycorrhizas. New Phytol. 112: 383–388.
50. Garland, J.L. (1996a). Analytical approaches to the characterization of samples of microbial communities using patterns of potential C source utilization. Soil Biol. Biochem. 28: 213–221.
51. Garland, J.L. (1996b). Patterns of potential C source utilization by rhizosphere communities. Soil Biol. Biochem. 28: 223–230.
52. Gillespie, A.R. and Pope, P.E. (1990a). Rhizosphere acidification increases phosphorus recovery of black locust: I. Induced acidification and soil response. Soil Sci. Soc. Am. J. 54: 533–537.
53. Gillespie, A.R. and Pope, P.E. (1990b). Rhizosphere acidification increases phosphorus recovery of black locust: II. Model predictions and measured recovery. Soil Sci. Soc. Am. J. 54: 538–541.
54. Grayston, S.J., Want, S.Q., Campbell, C.D., and Edwards, A.C. (1998). Selective influence of plant species on microbial diversity in the rhizosphere. Soil Biol. Biochem. 30: 369–378.
55. Griffin, G.J., Hale, M.G., and Shay, F.J. (1977). Nature and quantity of sloughed organic matter produced by roots and of axenic peanut plants. Soil Biol. Biochem. 8: 29–32.
56. Gupta, R.K. (1991). Drought response in fungi and mycorrhizal plants. In: Handbook of Applied Mycology. Volume 1. Soil and Plants (eds. D.K. Arora, B. Rai, K.G. Mukerji and G.R. Knudsen), 55–75. New York: Marcel Dekker.
57. Hamdan, H., Weller, D.M., and Thomashow, L.S. (1991). Relative importance of fluorescent siderophores and other factors in biological control of Gaeumannomyces graminis var. tritici by Pseudomonas fluorescens 2‐79 and M4‐80R. Appl. Environ. Microbiol. 57: 3270–3277.
58. Hamel, C., Nesser, C., Barrantes‐Cartín, U., and Smith, D.L. (1991). Endomycorrhizal fungal species mediate 15N transfer from soybean to maize in non‐fumigated soil. Plant Soil 138: 41–47.
59. Harris, J.M., Lucas, J.A., Davey, M.R. et al. (1989). Establishment of Azospirillum inoculant in the rhizosphere of winter wheat. Soil Biol. Biochem. 21: 59–64.
60. Hartel, P.G., Billingsley, J.W., and Williamson, J.W. (1989). Styrofoam cup‐membrane assembly for studying microorganism‐root interactions. Appl. Environ. Microbiol. 55: 1291–1294.
61. Harvey, A.E., Jurgensen, M.F., and Larsen, M.J. (1981). Organic reserves: importance to ectomycorrhizae in forest soils of Western Montana. For. Sci. 27: 442–445.
62. Hinsinger, P. (1997). Dissolution of phosphate rock in the rhizosphere of five plant species grown in an acid, P‐fixing mineral substrate. Geoderma 75: 231–249.
63. Hodge, A., Paterson, E., Grayston, S.J. et al. (1998). Characterisation and microbial utilisation of exudate material from the rhizosphere of Lolium perenne grown under CO2 enrichment. Soil Biol. Biochem. 30: 1033–1043.
64. Hoefte, M., Seong, K.Y., Jurkevitch, E., and Verstraete, W. (1991). Pyoverdin production by the plant growth beneficial Pseudomonas strain 7NSK2: ecological significance in soil. Plant Soil 130: 249–257.
65. Holl, F.B. and Chanway, C.P. (1992). Rhizosphere colonization and seedling growth promotion of lodgepole pine by Bacillus polymyxa. Can. J. Microbiol. 38: 303–308.
66. Ingram, R.R., Klein, D.A., and Trlica, M.J. (1985). Responses of microbial components of the rhizosphere to plant management strategies in a semiarid rangeland. Plant Soil 85: 65–76.
67. Jalali, B.L. and Jalili, I. (1991). Mycorrhizae in plant disease control. In: Handbook of Applied Mycology. Volume 1. Soil and Plants (eds. D.K. Arora, B. Rai, K.G. Mukerji and G.R. Knudsen), 131–154. New York: Marcel Dekker.
68. Jurkevitch, E., Haear, Y., and Chen, Y. (1992). Differential siderophore utilization and iron uptake by soil and rhizosphere bacteria. Appl. Environ. Microbiol. 58: 119–124.
69. Klein, D.A., Frederick, B.A., Biondini, M., and Trlica, M.J. (1988). Rhizosphere microorganism effects on soluble amino acids, sugars and organic acids in the root zone of Agropyron cristatum, A. Smithii and Bouteloua gracilis. Plant Soil 110: 19–25.
70. Kloepper, J.W., Leong, J., Teintze, M., and Schroth, M.N. (1980a). Pseudomonas siderophores: a mechanism explaining disease‐suppressive soils. Curr. Microbiol. 4: 317–320.
71. Kloepper, J.W., Leong, J., Teintze, M., and Schroth, M.N. (1980b). Enhanced plant growth by siderophores produced by plant growth‐promoting rhizobacteria. Nature 286: 885–886.
72. Klubek, B.P., Hendrickson, L.L., Zablotowicz, R.M. et al. (1988). Competitiveness of selected Bradyrhizobium japonicum strains in Midwestern USA soils. Soil Sci. Soc. Am. J. 52: 662–666.
73. Knight, W.G., Allen, M.F., Jurinak, J.J., and Dudley, L.M. (1989). Elevated carbon dioxide and solution phosphorus in soil with vesicular‐arbuscular mycorrhizal western wheatgrass. Soil Sci. Soc. Am. J. 53: 1075–1082.
74. Krishnamurti, G.S.R., Cieslinski, G., Huang, P.M., and Vanrees, K.C.J. (1997). Kinetics of cadmium release from soils as influenced by organic acids – implication in cadmium availability. J. Environ. Qual. 26: 271–277.
75. Kucey, R.M.N. and Paul, E.A. (1982). Carbon flow, photosynthesis, and N2 fixation in mycorrhizal and nodulated faba beans (Vicia faba L.). Soil Biol. Biochem. 14: 407–412.
76. Latour, X., Corberand, T.S., Laguerre, G. et al. (1996). The composition of fluorescent pseudomonad populations associated with roots is influenced by plant and soil type. Appl. Environ. Microbiol. 62: 2449–2456.
77. Leyval, C. and Verthelin, J. (1991). Weathering of a mica by roots and rhizospheric microorganisms of pine. Soil Sci. Soc. Am. J. 55: 1009–1016.
78. Lindemann, W.C., Lindsey, D.L., and Fresquez, P.R. (1984). Amendment of mine spoil to increase the number and activity of microorganisms. Soil Sci. Soc. Am. J. 48: 574–578.
79. Maloney, P.E., Vanbruggen, A.H.C., and Hu, S. (1997). Bacterial community structure in relation to the carbon environments in lettuce and tomato rhizospheres and in bulk soils. Microb. Ecol. 34: 109–117.
80. Martens, R. (1982). Apparatus to study the quantitative relationships between root exudates and microbial populations in the rhizosphere. Soil Biol. Biochem. 14: 315–317.
81. Martin, J.K. (1977). Factors influencing the loss of organic carbon from wheat roots. Soil Biol. Biochem. 9: 1–7.
82. Marx, D.H. (1972). Ectomycorrhizae as biological deterrents to pathogenic root infection. Annu. Rev. Phytopathol. 10: 429–454.
83. Marx, D.W., Morris, W.G., and Meral, J.G. (1978). Growth and ectomycorrhizal development of loblolly pine seedlings on fumigated and nonfumigated nursery soils infested with different fungal symbionts. For. Sci. 24: 193–203.
84. Materon, L.A. and Hagedorn, C. (1982). Competitiveness of Rhizobium trifolii strains associated with red clover (Trifolium pratense L.) in Mississippi soils. Appl. Environ. Microbiol. 44: 1096–1101.
85. McClaugherty, C.A., Aber, J.D., and Melillo, J.M. (1984). Decomposition dynamics of fine roots in forested ecosystem. Oikos 42: 378–386.
86. Meharg, A.A. and Killham, K. (1989). Distribution of assimilated carbon within the plant and rhizosphere of Lolium perenne: influence of temperature. Soil Biol. Biochem. 21: 487–490.
87. Merckx, R., van Ginkel, J.A., Sinnaeve, J., and Cremers, A. (1986). Plant induced changes in the rhizosphere of maize and wheat. I. Production and turnover of root‐derived material in the rhizosphere of maize and wheat. Plant Soil 96: 85–94.
88. Merckx, R., Dijkstra, A., den Hertog, A., and van Veen, J.A. (1987). Production of root‐derived material and associated microbial growth in soil at different nutrient levels. Biol. Fertil. Soils 5: 126–132.
89. Meyer, J.R. and Linderman, R.G. (1986a). Response of subterranean clover to dual inoculation with vesicular‐arbuscular mycorrhizal fungi and a plant growth‐promoting bacterium. Soil Biol. Biochem. 18: 185–190.
90. Meyer, J.R. and Linderman, R.G. (1986b). Selective influence on populations of rhizosphere or rhizoplane bacteria and actinomycetes by mycorrhizas formed by Glomus fasiculatum. Soil Biol. Biochem. 18: 191–196.
91. Meyer, J.‐M., Hohnadel, D., and Hallé, F. (1989). Cepabactin from Pseudomonas cepacia, a new type of siderophore. J. Gen. Microbiol. 135: 1479–1487.
92. Moawad, H. and Bohlool, B.B. (1984). Competition among Rhizobium spp. for nodulation of Leucaena leucocephala in two tropical soils. Appl. Environ. Microbiol. 48: 5–9.
93. Mukerjii, S., Mukerji, K.G., and Arora, D.K. (1991). Ectomycorrhizae. In: Handbook of Applied Mycology. Volume 1. Soil and Plants (eds. D.K. Arora, B. Rai, K.G. Mukerji and G.R. Knudsen), 187–215. New York: Marcel Dekker.
94. Nair, M.G., Safir, G.R., and Siqueira, J.O. (1991). Isolation and identification of vesicular‐arbuscular mycorrhiza‐stimulatory compounds from clover (Trifolium repens) roots. Appl. Environ. Microbiol. 57: 434–439.
95. Newman, E.I. (1985). The rhizosphere: carbon sources and microbial populations. In: Ecological Interactions in Soil: Plants, Microbes, and Animals (ed. A.E. Fitter), 107–121. Boston: Blackwell.
96. Newman, E.I., Heap, A.J., and Lawley, R.A. (1981). Abundance of mycorrhizas and root surface microorganisms of Plantago lancelata in relation to soil and vegetation. A multivariate approach. New Phytol. 89: 95–108.
97. Nietko, K.F. and Frankenberg, W.T. Jr. (1989). Biosynthesis of cytokinins in soil. Soil Sci. Soc. Am. J. 53: 735–740.
98. Norton, J.M. and Firestone, M. (1991). Metabolic status of bacteria and fungi in the rhizosphere of ponderosa pine seedlings. Appl. Environ. Microbiol. 57: 1161–1167.
99. Parke, J.L., Moen, R., Rovira, A.D., and Bowen, G.D. (1986). Soil water flow affects the rhizosphere distribution of seed‐borne biological control agent. Soil Biol. Biochem. 18: 583–588.
100. Pasqualini, S., Panara, F., and Antonielli, M. (1992). Acid phosphatase activity in Pinus pinea–Tuber albidum ectomycorrhizal association. Can. J. Bot. 70: 1377–1383.
101. Pesmark, M., Frejd, T., and Mattiasson, B. (1990). Purification, characterization, and structure of pseudobactin 589 A, a siderophore from a plant growth promoting Pseudomonas. Biochemistry 29: 7348–7356.
102. Qian, J.H. and Doran, J.W. (1996). Available carbon released from crop roots during growth as determined by carbon‐13 natural abundance. Soil Sci. Soc. Am. J. 60: 828–831.
103. Rattray, E.A.S., Paterson, E., and Killham, K. (1995). Characterisation of the dynamics of C‐partitioning within Lolium perenne and to the rhizosphere microibal biomass using C‐14 pulse chase. Biol. Fertil. Soils 19: 280–286.
104. Read, D.J. (1984). The structure and function of the vegetative mycelium of mycorrhizal roots. In: The Ecology and Physiology of the Fungal Mycelium (eds. D.H. Jennings and A.D.M. Rayner), 215–240. New York: Cambridge University Press.
105. Read, D.J., Francis, R., and Finlay, R.D. (1989). Mycorrhizal mycelia and nutrient cycling in plant communities. In: Ecological Interactions in Soil: Plants, Microbe, and Animals (ed. A.E. Fitter), 193–217. Boston: Blackwell.
106. van Rensburg, H.J. and Strijdom, B.W. (1985). Effectiveness of Rhizobium strains used in inoculants after their introduction into soil. Appl. Environ. Microbiol. 49: 127–131.
107. Rillig, M.C., Scow, K.M., Klironomos, J.N., and Allen, M.F. (1997). Microbial carbon‐substrate utilization in the rhizosphere of Gutierrezia sarothrae grown in elevated atmospheric carbon dioxide. Soil Biol. Biochem. 29: 1387–1394.
108. Robert, F.M. and Schmidt, E.L. (1983). Population changes and persistence of Rhizobium phaseoli in soil and rhizospheres. Appl. Environ. Microbiol. 45: 550–556.
109. Ruark, G.A., Thornton, F.C., Tiarks, A.E. et al. (1991). Exposing loblolly pine seedlings to acid precipitation and ozone: effects on soil rhizosphere chemistry. J. Environ. Qual. 20: 828–832.
110. Smith, G.B. and Wollum, A.G. II (1989). Nodulation of Glycine max by six Bradyrhizobium japonicum strains with different competitive abilities. Appl. Environ. Microbiol. 55: 1957–1962.
111. Spalding, B., Duxbury, J.M., and Stone, E.L. (1975). Lycopodium fairy rings: effect on soil respiration and enzymatic activities. Soil Sci. Soc. Am. Proc. 39: 65–70.
112. Stroo, H.F. and Jencks, E.M. (1985). Effect of sewage sludge on microbial activity in an old, abandoned minesoil. J. Environ. Qual. 14: 301–304.
113. Tate, R.L. III (1985). Microorganisms, ecosystem disturbance and soil formation processes. In: Soil Reclamation Processes. Microbiological Analyses and Application (eds. R.L. Tate III and D.A. Klein), 1–33. New York: Marcel Dekker.
114. Tate, R.L. III (1987). Soil Organic Matter. Biological and Ecological Effects. New York: Wiley.
115. Tate, R.L. III, Parmelee, R.W., Ehrenfeld, J.G., and O'Reilly, L. (1991). Enzymatic and microbial interactions in response to pitch pine root growth. Soil Sci. Soc. Am. J. 55: 998–1440.
116. Tester, C.F. (1990). Organic amendment effects on physical and chemical properties of a sandy soil. Soil Sci. Soc. Am. J. 54: 827–831.
117. Theodorou, C. and Bowen, G.D. (1987). Germination of basidiospores of mycorrhizal fungi in the rhizosphere of Pinus radiata D. Don. New Phytol. 106: 217–224.
118. Thomashow, L.S. and Weller, D.M. (1990). Role of antibiotics and siderophores in biocontrol of take‐all disease of wheat. Plant Soil 129: 93–99.
119. Thomashow, L.S., Weller, D.M., Bonsall, R.B., and Pierson, L.S. III (1990). Production of the antibiotic phenazine‐1‐carboxylic acid by flurosecent Pseudomonas species in the rhizosphere of wheat. Appl. Environ. Microbiol. 56: 908–912.
120. Tien, T.M., Gaskins, M.H., and Hubbell, D.H. (1979). Plant growth substances produced by Azospirillum brasilense and their effect on the growth of pearl millet (Pennisetum americanum L.). Appl. Environ. Microbiol. 37: 1016–1024.
121. Tisdall, J.M. and Oades, J.M. (1982). Organic matter and water‐stable aggregates in soils. J. Soil Sci. 33: 141–163.
122. Turner, S.M. and Newman, E.I. (1984). Growth of bacteria on roots of grasses: influence of mineral nutrient supply and interactions between species. J. Gen. Microbiol. 130: 505–512.
123. Turner, S.M., Newman, E.I., and Campbell, R. (1985). Microbial population of ryegrass (Lolium perenne) root surfaces: influence of nitrogen and phosphorus supply. Soil Biol. Biochem. 17: 711–716.
124. Utkhede, R.S. and Rahe, J.E. (1980). Biological control of onion white rot. Soil Biol. Biochem. 12: 101–104.
125. Van Loon, L.C., Bakker, P.A.H.M., and Pieterse, C.M.J. (1998). Systemic resistance induced by rhizosphere bacteria. Annu. Rev. Phytopathol. 36: 453–483.
126. Vrany, J., Vancura, V., and Stanek, M. (1981). Control of microorganisms in the rhizosphere of wheat by inoculation of seeds with Pseudomonas putida and by foliar application of urea. Folia Microbiol. 26: 45–51.
127. Whitford, W.G., Stinnett, K., and Anderson, J. (1988). Decomposition of roots in a Chihuahuan desert ecosystem. Oecologia 75: 8–11.
128. Will, M.E. and Silvia, D.M. (1990). Interaction of rhizosphere bacteria, fertilizer, and vesicular‐arbuscular mycorrhizal fungi with sea oats. Appl. Environ. Microbiol. 56: 2073–2079.
129. Wilson, G.W.T., Hetrick, B.A.D., and Schwab, A.P. (1991). Reclamation effects on mycorrhizae and productive capacity of flue gas desulfurization sludge. J. Environ. Qual. 20: 777–783.
130. Wong, J.W.C. and Ho, G.E. (1991). Effects of gypsum and sewage sludge amendment on physical properties of fine bauxite refining residue. Soil Sci. 152: 326–332.