A Masterful Underview

Authors


Biological Diversity and Function in Soils . Bardgett, R. D., M. B.Usher, and D. W.Hopkins , editors . Cambridge University Press , New York , NY . 425 pp . ( 411 + xiv ). $65.00 (paperback) . ISBN 0-521-60987-9 .

Bardgett et al. present an impressive array of 20 chapters, authored by more than 40 researchers in the field of soil ecology, including topics of biological diversity and function in soils. The book takes a vigorous whole-systems approach, noting the many interlinkages between above- and belowground processes. The volume is the product of a symposium of the British Ecological Society held in the spring of 2003.

Biological Diversity and Function in Soils is the logical successor to a symposium held 19 years earlier on the topic of ecological interactions in soil, also sponsored by the British Ecological Society. That volume was almost entirely process oriented, mentioning aspects of biological diversity only briefly. So much of the biota was undescribed then (the situation is somewhat improved for some of the mesobiota and macrobiota), but even now the work of describing bacteria and fungi is in its infancy. Molecular tools provide insights into the wide array of microbes (over 25 phyla in the Eubacteria and the number is increasing) and fauna, many of which remain undescribed in temperate and tropical locales.

One generation ago, soil ecology was considered rather esoteric. It would have been difficult then to imagine a feature issue of Science (11 June 2004) with the theme “Soils, the Final Frontier.” The articles in Science emphasized the emerging concept of soils as central organizing centers for terrestrial ecosystems. There have been additional issues of Science featuring the Mars explorers and their findings. Unfortunately, we know only a bit more about soils on planet Earth.

The book is arranged in six parts, beginning with an insightful introduction by Wall, Fitter, and Paul (Part I), who provide an extensive overview of the field of soil biodiversity. After presenting a figure showing the major influences on research in soil biodiversity, the authors condense their message into 10 “take-home messages” as follows:

  • 1The terrestrial world is brown and black, not green, which emphasizes that soils contain twice the total carbon of vegetation and more ecology occurs below than above ground.
  • 2The world seems primarily microscopic. This is too mild; the world is primarily microbial dominated.
  • 3We do not know their names or what they do. Even now, no soils are known in which all species are described.
  • 4Food webs do not follow traditional rules. Many organisms are generalists and omnivory seems to be the rule. System behavioral traits follow from these facts.
  • 5Indirect effects can dominate, and are hard to quantify. The high species richness in soils results in multiple species interactions that are mostly indirect. Are species essentially interchangeable as a consequence?
  • 6Scale is a dividing issue between above- and belowground ecology. Spatial and temporal scales of above- and belowground organisms are markedly different. Many soil phenomena have been misinterpreted by applying aboveground macroscale viewpoints to the array of biota involved. Allowing that the biota involved span many scales in space and time and viewing them as “integrators” of phenomena is only gradually becoming appreciated.
  • 7Soil legac(ies) imprint soil biodiversity (and can override plant effects). This is a big factor that is little known or appreciated yet. Thus, soil organic matter (SOM) at 50 cm depth in the field can be up to 1400 years older than the SOM at the surface but can decompose at similar rates to surface SOM when incubated in the laboratory.
  • 8Soils and their biota are not isolated in terrestrial ecosystems: they have multiple landscape connections. Humans and academic researchers tend to view soils and biota separately, but they do so at the peril of missing the big picture. Soils and sediments have many interconnections yet are often considered separately, unfortunately.
  • 9Small creatures have biogeography too. Latitudinal and landscape patterns of soil biodiversity are largely unknown, but information is starting to accumulate.
  • 10Decomposition is one of the two major life-generating processes. Decomposition is paired with photosynthesis as one of the two key ecosystem processes. The diversity of organisms involved in decomposition dwarfs that of photosynthetic organisms, and much of the remainder of the book hangs on that.

Part II, “The Soil Environment,” contains chapters on “The Habitat of Soil Microbes,” with innovative three-dimensional analyses of soil aggregates; “Twenty Years of Molecular Analysis of Bacterial Communities in Soil” (O'Donnell et al.), noting the exponential increase in information that is lagged yet by an equivalent amount understanding of all that the array of microbes do; and “Carbon as a Substrate for Soil Organisms” (Hopkins & Gregorich). This latter chapter takes a “soil metabolomic” approach to complex and composite substrates, noting the many benefits of stable-isotope probing to link organisms with their substrates.

Part III, “Patterns and Drivers of Soil Biodiversity,” gets to the core of the subject. With an impressive array of synthesis papers, virtually any one would merit a full page of commentary in a review. I cover just a few high points. Bardgett, Yeates, and Anderson, in “Patterns and Determinants of Soil Biological Diversity,” note the paucity of data, with little support for the notion that soil biodiversity at local scales conforms to either productivity-diversity or disturbance-diversity relationships. They note that belowground communities could differ from their aboveground counterparts in that soil biodiversity is not so strongly regulated by competition and competitive exclusion does not occur with increased resource availability in soil. This finds further support in a later article by Setälä et al., who discuss trophic structure and functional redundancy in soil systems. Bardgett et al. make the trenchant observation that although the results of numerous studies show that disturbances resulting from agricultural intensification can result in reduced soil biodiversity, they do not lend support for the notion that biological diversity in soil is optimized at intermediate levels of disturbance.

David Wardle, in “How Plant Communities Influence Decomposer Communities,” presents two examples of aboveground, human-induced changes affecting the composition of the soil food web across several trophic levels, including key ecosystem functions carried out by the soil biota. These studies led Wardle to suggest that reductions in plant diversity do not cause predictable changes in drivers of soil biodiversity, such as microhabitat diversity or favorability of environmental conditions (soil fertility). De Ruiter, Neutel, and Moore in “The Balance between Productivity and Food Web Structure in Soil Ecosystems,” follow up on earlier leads by noting several system-level properties of soil food webs. They note the key components in food web structure, namely the lengths and weights of trophic interaction loops (a pathway of interactions [not feeding rates] from a species through the web back to the same species without visiting the species more than once; hence a closed chain of trophic links). This synthetic construct enables ecologists to compare and contrast productivity, energy flow, and interaction strengths of microbes, microbivores, omnivores, and predators.

The role of redundancy in many of these groups is the subject of later chapters in the volume. Standing et al. consider “Rhizosphere Carbon Flow: a Driver of Soil Microbial Diversity?” and note the powerful tools of nucleic-acid-stable-isotope probing (SIP) with 13C incorporated into microbial biomass. They comment that the techniques for linking microbial diversity to function can be extended further by application of SIP to mRNA gene probing. This approach will allow the activity associated with specific genes to be quantified.

Part IV, “Consequences of Soil Biodiversity,” contains six chapters with a wealth of new information in impressive syntheses. Schimel, Bennett, and Fierer cover “Microbial Community Composition and Soil Nitrogen Cycling: Is there Really a Connection?” These authors note that some processes are physiologically “narrow” (e.g., fixation and denitrification) and should be sensitive to microbial community composition. In contrast, internal turnover processes, such as mineralization and immobilization, involve “aggregate” processes that have classically been considered insensitive to community composition. However, the latter can be broken into individual components that may be sensitive to microbial community composition, considering exoenzyme and microsite phenomena. The kinetics of exoenzymes may regulate microbial carbon and nitrogen limitation and hence community composition. Microsite phenomena in turn appear to regulate system-level nitrogen cycling in nitrogen-poor soils, with the effects scaling nonlinearly to the whole system. Thus, different organisms will live and function in different types of microsites. Taking this more microsite-oriented approach will enable research linking microbial populations and the nitrogen cycling processes they carry out.

Robinson, Miller, and Deacon, in “Biodiversity of Saprotrophic Fungi in Relation to Their Function: Do Fungi Obey the Rules?” consider the conundrum of apparent fungal redundancy. They note that decomposition rate depends more on fungal species composition and its functional repertoire and less on species richness alone. Leake et al., in “Is Diversity of Mycorrhizal Fungi Important for Ecosystem Functioning?” tackle the problem of redundancy in mycorrhiza by noting that mycorrhizal associations are multifunctional, exhibiting complementarity. Because of the high specificity and dependency in many mycorrhizal associations, especially ones involving mycoheterotrophic plants, they suggest that the extent of functional “redundancy” is low. Setälä, Berg, and Jones, in “Trophic Structure and Functional Redundancy in Soil Communities,” offer several examples of the extent of generalism, omnivory, and highly heterogeneous nature of soil organisms, all playing major roles in explaining the high degree of functional complementarity in decomposer communities.

Van der Putten, in “Plant–Soil Feedback and Soil Biodiversity Affect the Composition of Plant Communities,” shows how different components of the soil subsystem, including plant pathogens, can have variable and even opposite effects on plant community composition depending on the productivity level considered. Spatial and temporal scales are ever important, and he demonstrates how central they are to soil ecological studies. McCarthy et al., in “Response of the Soil Bacterial Community to Perturbation,” tackle the problem of myriads of bacterial species apparently coexisting by focusing on two measurable parameters: the total number of bacterial cells and the abundance of the most abundant species in a given microhabitat.

Part V, “Applications of Soil Biodiversity,” addresses key problems of interest to land managers in tropical environments (Giller et al.). Also included are “Restoration Ecology and the Role of Soil Biodiversity” (Harris, Grogan, & Hobbs), “Soil Biodiversity: Stress and Change in Grasslands under Restoration Succession” (Brussaard et al.), and “Soil Biodiversity, Nature Conservation and Sustainability” (Usher). The main take-home lesson for the reader is that there is a wide range of useful techniques for land managers that can be employed now. Usher points out, rather poignantly, that there is no “charismatic fauna or microflora” with which the general public can resonate; hence, there is little pressure for conservation of soil organisms. That situation should be changed but will require further education of public and land managers alike.

Part VI, “Conclusion,” is a masterful synthesis by Karl Ritz entitled “Underview: Origins and Consequences of Below-Ground Diversity.” His main points are that we must be both wide ranging and far seeing in our dealings with soil diversity in terms of soil structure and the consequent community structure that is governed by many factors. He notes that soil biodiversity must be viewed in relation to the functional repertoire of the biota, potential and realized interactions between components, and functional redundancy. Biodiversity per se in most soils seems to be of little functional significance. The functional repertoire of the soil biota is much more pertinent. Although knowledge is growing apace, a unifying framework for soil biodiversity is not yet within reach, but he suggests that soil architecture, because it varies over space and time, may be the key to providing such a framework.

In its totality, this volume with its wide range of topics, is very thought provoking and a must-purchase reference for all terrestrial ecologists. It should be at the top of the list for all graduate students coming through ecology and natural resource advanced degree programs.

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