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Understanding biotic. influences on soil processes is a major research frontier made challenging by organismal diversity, variation in distribution, and variety of interactions. Nevertheless, two fundamental influences can be recognized: assimilation/ dissimilation (uptake, metabolism, wastes, death) and physical ecosystem engineering (non-assimilatory/dissimilatory, organismally-induced, structurally-mediated changes in energy and materials in the abiotic environment). Because many organisms can engineer soils, predicting their effects is particularly challenging. Here we use Hans Jenny's State Factor Equation as a flexible, integrative tool for understanding these effects. We distinguish organismal influences via engineering from those of assimilation/dissimilation, explicitly placing engineers into the equation as independent state factors. We then ask: What abiotic state factors does an engineer affect? What relationships among state factors does it change? How does this affect soil processes? Using examples from our work, we illustrate use of this conceptual framework for a physical process-soil erosion; a chemical process-desalination; and a biogeochemical process-denitrification. We show that the framework can be used to: Identify conditions for small or large engineering effects on soil processes; assess engineer impacts on multiple soil processes; compare effects of different engineers on a given soil process; and integrate effects of multiple engineers on a single soil process. (c) 2006 Elsevier Masson SAS. All rights reserved.