Conceptual Organization of Soil Organisms

Soil ecologists rely upon several classification schemes to conceptually link the diversity of soil organisms to ecological functions and processes. One of the more useful classifications is based upon size and uses body diameter to distinguish soil biota into microflora (<2 mm), microfauna (2-100 mm), mesofauna (0.1-2 mm), macrofauna (>2 mm), and megafauna (>20 mm). Organisms in different size classes have different spatial effects in soils and interact with different soil processes. Individually, microflora impact relatively small spatial scales and are important in the breakdown of organic residues and the production of exudates that form soil aggregates; microfauna are important in regulating these processes. Meso- and macrofauna operate at larger spatial scales, and physically breakdown plant residues, create pore structure, and mix and redistribute mineral and organic soil components (Figure 3).

Much success is also found from viewing soil organisms from a food web or trophic perspective. Problems associated with in situ identification of microbes and the degree of unnamed species of soil fauna makes the use of broader taxonomic groups necessary, including functional groupings of organisms. Soil organisms are commonly grouped based upon feeding habits, including microbe feeders (microbi-vores), those that feed on litter and organic matter (detritivores), those that feed off or parasitize plant roots (herbivores), or those that feed on other soil animals (carnivores). Soil food web dynamics are elucidated through a mixture of lab, field, and modeling exercises. The use of radioactive and stable (both pulses additions of labels and natural abundances) isotopic tracers and statistical techniques (such as interaction strength and path analysis) have been important in determining the spatial and temporal dynamics of soil food webs. Generally, it is thought that the interaction of bottom-up (productivity based) and top-down (predatory) controls lend to the stability of soil food webs. Soil food web analyses have been important in demonstrating the roles of microbe feeders (e.g., nematodes and protozoa) on regulating both microbial populations and decomposition and nutrient dynamics in soil ecosystems. Experimental research suggests that there are two distinct pathways in the soil food web: (1) a 'fast' bacterial pathway, and (2) a 'slow' fungal pathway. A third pathway via the activity ofroot grazers (e.g., plant parasitic nematodes) is also possible, but is theoretically less developed. Observations in experiments and field studies of nematode-trapping fungi and ectomycorrhizal fungi that prey upon collembolans complicate the trophic structure traditionally conceptualized in soil food webs, and while they have not yet been integrated into soil food web models, they point to the degree of trophic linkages possible in soils (Figure 4).

A third approach integrates spatial scale and trophic perspectives, linking specific spatial scales to soil processes. Ecosystem engineers, such as earthworms and termites, can indirectly influence the cycling of nutrients through direct impacts on soil structure. A second group is considered the litter transformers (micro- and macro-arthropods) and fragments or comminutes litter into smaller pieces, thereby increasing the surface area available to microbial decomposition. Members of a third group are part of a 'micro-food web', and include microbes and microfaunal predators (primarily nema-todes and protozoa). Each level in this conceptual scheme influences ecosystem properties through actions at different size, space, and timescales.

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