Soil Organisms as Engineers: Microsite Modulation of Macroscale Processes

Summary

Soil invertebrates and microorganisms are proximate factors regulating carbon and nutrient mineralization. It has proved intractable, however, to relate the activities of species in the community to rates of biogeochemical fluxes at the ecosystem level. A major reason for this is that the metabolic contributions of individual species affect soil processes orders of magnitude below the scales at which the ecosystem is defined and process measurements are made in the field. In addition, however, soil organisms also have indirect effects of longer duration and extent where they act as modulators of carbon, nutrient, or water fluxes. By changing the physical controls over biogeochemical processes, invertebrates and microorganisms can regulate much larger fluxes than their direct effects on soil processes. The mechanisms often involve the formation of durable artifacts, such as stabilized aggregates, burrows, or buried organic matter, which continue to function in the absence of the organisms creating them. The effects of these artifacts, most notably those created by earthworms and termites, are cumulative and can therefore be manifested in macroscale measurements. It is shown, however, that the net effects of these activities may not be detectable on larger areas, or over longer time periods, because sink/source processes operate within patches at the scale, or domain, at which the species populations function. Perturbations to the system can also synchronize these activities in space or time so that invertebrate activities become apparent at higher levels of organization. It is concluded that the role of invertebrates should be more explicitly considered in ecosystem studies and that this is facilitated by a “top-down” approach to defining the scales at which their regulatory effects on biogeochemical processes are emergent.