In the preceding pages, the multifaceted world of soil microbes has been revealed. It is not difficult at this point of our study to reach the conclusion that each of the individual habitats of soil microbes is nearly unique. Microsites are composed of a diversity of physical components assembled in varying degrees of structure ranging from an apparently structureless array of sand grains to the complex, ever‐changing combinations of mineral and organic matter termed soil aggregates. Furthermore, microbes encounter an infinite combination of soil organic compounds ranging from simple organic acids, amino acids, and monosacharides to the highly complex humic acids. This assemblage of mineral and physical components of the microbial world is further varied by the presence of a variety of inorganic substances, many of which provide the energetic basis for growth and development of a portion of the soil microbial community known as the chemolithotrophs. An additional layer of complexity of the soil system is provided by myriad potential members of the microbial community and the variations in their interactions. The complexity of the soil biological community is only partially revealed by this abbreviated illumination of the microbial world. Growth and development of individual microbial cells and the associated alteration of soil components through cellular energy metabolism and the by‐products of this activity appear to be nearly chaotic.
In contrast, microbial growth in test tubes is highly ordered and easily described by a variety of somewhat simple mathematical relationships. In soil, a highly productive cell may only be allowed one to a few cellular divisions before the stresses of space, nutrients, or biological interactions reduce the proliferation rate. In reality, it is difficult to envision a situation in soil where the maximal division rates commonly seen under the optimal conditions used in laboratory investigations could ever occur. Further, quiescent organisms may suddenly be spurred into a few rounds of seemingly unimpeded division by the influx of an energy source or temporary relief from growth‐inhibiting conditions, but this time of luxury can just as quickly be disrupted by exhaustion of the newly found growth sources or imposition of new impediments to proliferation.
The preceding chapters described the basic principles of the discipline of soil microbiology that provide some order for this highly disordered world in soil and for prediction of the behavior of the soil microbial community as the properties of their habitats are altered. With our continually growing appreciation of the complexity of the soil microbial community and the interactions between its members, it becomes readily apparent that the information revealed in these pages only serves as a guidepost along the path to a more complete understanding of the home of the soil microbes and the genetic potential contained therein. This pathway could be likened to a divided highway leading to the ultimate in understanding and wisdom in this discipline. The seemingly contradictory endeavors of delving into the minutiae of the system must be combined with just as diligent a drive to conceptualization of the role of the soil microbes in function of the total ecosystem. This latter system must extend far beyond the simple soil of a meadow or mineland reclamation site to the totality of terrestrial systems and perhaps beyond.
As our capacity to understand, and perhaps model, larger landscape units increases, our intellectual concept of an ecosystem has enlarged. Instead of being a limited area, reasonably homogeneous from the perspective of aboveground aspects, we are becoming more aware that a true ecosystem is a mixture of contiguous but clearly distinct systems (e.g. a swamp bordering a forest system) as well as a continuum of slowly changing systems. Nowhere is this concept of an ecosystem as a complex mixture of interacting system types more clearly demonstrated than in soil, where we find subsoil processes affecting surface soil systems that are further regulated by external inputs such as organic matter and moisture from the aboveground portions of the grander system. On a microscale, we can also develop a realistic conceptual scenario of complex interactions of a variety of microsites being summed to produce the product that we may describe as a particular soil system, such as a grassland.
With this more inclusive and less reductive view of ecosystem description comes an enhanced appreciation of the impact of soil microbiological processes on total terrestrial processes. Necessarily, the basic principles described in the preceding chapters were elucidated with examples of limited ecosystem types, sometimes simply the system that develops from a soil sample contained in a test tube. Thus, a somewhat disparate view of the impact of such processes as denitrification (and associated nitrous oxide production) on general terrestrial processes can be gained. It is easy conceptually to separate the internal portions of the soil aggregate wherein denitrification occurs from the external oxygenated regions of the same aggregate. More dramatically, conceptually, the denitrification occurring in a swamp, or even a constructed wetland, can readily be isolated from its impacts on the function of the surrounding systems, be they forest, agricultural lands, or even urban environments.
Taken in isolation, the concepts of soil microbiology could almost be considered to be evolving at a “snail's pace,” yet once we move our mental contemplation from the limited world of the specific microbe or process, a multitude of major challenges to the environmental microbiologist arises. Concerns such as predicting the impacts of global warming and the role of soil microbes as they interact with the organic resources on the product of greenhouse gases, maintenance of soil quality, not to mention the problems of reclaiming chemically polluted soils or soils damaged from mismanagement, demand an expansion of soil microbial concepts further than ever before. Solutions to these environmental problems and support for efforts such as global climate modeling can be derived to some degree from extrapolation of data and concepts derived from studies of more limited ecosystems. But the greater challenge to soil microbiologists resides in developing reliable models and data sets supportive of more general models of the total ecosystem (the entire terrestrial system), predictive of the impact of anthropogenic activities and of our reclamation efforts.
This treatise was introduced with the objective of presenting the basic principles of soil microbiology and illuminating them with examples from the primary literature accumulated over the short history of this discipline. Any soil microbiology textbook, no matter how complete, can only provide a snapshot of the current status of our studies. A viable scientific endeavor must be continually growing. Great gains in describing the soil environment have been made, but the activities of society have created environmental challenges that may baffle even the most learned soil microbiologist. The challenges of reclaiming severely damaged soil sites, the goals of properly managing our terrestrial home, and the necessity of manipulating the soil system to provide the needs of an ever‐growing, ever‐changing society demand refinement of our knowledge of the capacity of the soil microbes to adapt to their continually evolving physical home and our capability to manage this minute soil community to meet both their and our needs for survival.
Thus, those that have mastered the introduction to soil microbiology provided within these pages are now challenged to use the information as a form of intellectual energy to advance our knowledge base into the realms necessary to meet the demands of the twenty‐first century. As our society entered the more modern era, soil microbiology would have been viewed as a diversion into elucidation of the intriguing “wee beasties” of the soil upon which we walk. Soil microbiology must now play more of an integral role in the advancement and application of ecosystem‐sustaining stewardship practices.