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(1) Organism trophic structure and interactions and soil nutrient levels were assessed for a semi-arid grassland steppe monthly between April and October 1982. The observed trophic interactions with resultant changes in soil nitrogen were interpreted on the basis of simple predator-prey microcosm studies and models. (2) Seasonal responses in trophic interactions and N mineralization-immobilization processes were: (i) in the spring, predator groups (protozoa and nematodes) increased as their food sources (bacteria and fungi, mainly) increased, subsequently reducing microbial biomass; (ii) reduced decomposer and increased grazer numbers occurred concomitant with increases in soil inorganic N; (iii) increased decomposer numbers (fourfold increase in bacterial numbers in September, for example) corresponded to decreased levels of soil inorganic N. (3) The complex, highly interconnected, and multi-linked …