Modeling microbial exchanges between forms of soil nitrogen in contrasting ecosystems

Although nitrogen (N) is often linked to carbon (C) in organic molecules, C is carried from the atmosphere to the soil through plants while N is carried from the soil to plants by microbial transformations. Many schemes have been proposed to describe the microbial conversion between organic and inorganic forms of N but current models do not fully represent the microbial control over these conversions. This study followed the transfer of 15N between plant materials, microorganisms, humified compartments and inorganic forms in 6 very different ecosystems along an altitudinal transect. The microbial conversion of the 15N forms appeared to be strongly linked to that found previously for 14C forms since the parameters and relationships defined for C were appropriate for modeling the N cycle. The only difference was in the flows between microbial and inorganic forms. The CO2-C loss was modeled using the equation for microbial respiration. Inorganic N appears also closely associated with microorganisms, which, depending on their C : N ratio and those of the available substrates, regulate the N mineralization and immobilization processes. Applications at earth scale can use the approximation that the microbial C : N ratio does not vary with time, but for this study, microorganisms cannot be treated always as homeostatic as their C : N ratio can decrease during incubation and increase with altitude when C storage increases. The MOMOS model has been validated for the C cycle, and it also appears to be valid for microbial conversion of N forms. It uses a relatively small number of well-defined, climate-dependent parameters, and it should fill a gap in the range of current models based on a direct microbial control for describing C and N flows in ecosystems.