Microbial growth in soil and nitrogen turnover: a theoretical model considering the activity state of microorganisms

A new model describing microbial growth in soil and considering carbon and nitrogen turnover is proposed. The approach we present modifies the classical equations for microbial growth by the introduction of a new state variable (r), which determines the microbial activity. The activity factor in turn controls microbial growth and death rates as well as the decomposition rate of insoluble organic matter and the immobilization rate of mineral N. The N-to-C ratios of microbial biomass are flexible dependent on the availability of N. The efficiency of microbial biosynthesis and maintenance respiration are controlled by the N-to-C ratio and activity state of the biomass. The behaviour of the model is investigated for conditions of batch and continuous input of substrate at different amounts of microbial activity and C and N in the soil. The proposed model structure is capable of reproducing such features of microbial turnover in soil as a transition of microbial population from an active to a dormant state in the case of C or N limitation, priming action on the decomposition of insoluble soil organic matter, and the reduced efficiency of microbial biosynthesis in the case of a N deficit.