Linking food webs to ecosystem processes: Piecewise linear models of soil microcosms

A piecewise linear approach to link decomposer biomass and soil nitrogen dynamics is proposed. Dynamics of a detritus–fungus–nematode food chain were studied in a microcosm experiment and modelled to evaluate the role of fungal-feeding nematodes in N mineralization. The nematode dynamics switched from exponential growth to a resource-limited phase. At the same time the amount of ammonium nitrogen in soil started to increase, following an initial immobilization phase. Both nematode and ammonium dynamics could be described by two-phase linear equations. Two nitrogen models were compared. A model in which the nematode accounted for all mineralization could be fitted to the ammonium data, but failed to predict nematode dynamics. In contrast, a model with no mineralization by the nematode explained both nematode and ammonium dynamics. Thus, the results support a view that despite their relatively high N output fungal-feeding nematodes are not quantitatively important in N mineralization. The biomass dynamics of pine seedlings and associated mycorrhizal fungi were modelled in a second experiment. A nutrient-limited model with constant plant growth rate described the plant–mycorrhiza system adequately. Together these models provide a theoretical basis for analyzing experimental model systems (microcosms).