The nitrogen cycle in the Broadbalk wheat experiment : a model for the turnover of nitrogen through the soil microbial biomass

A simple four-compartment model is presented for the turnover of N through the soil microbial biomass, based on measurements made in the Broadbalk Continuous Wheat Experiment at Rothamsted. The model comprises an input compartment (containing the N in roots and other plant debris entering the soil plus the inorganic N immobilized during the decomposition of this debris), feeding two organic compartments, the microbial biomass compartment and the humus compartment, which in turn feed the soil inorganic N compartment. The model was fitted to data from the plot receiving 192 kg N ha−1 yr−1 by iterative adjustment of the five independent parameters it contains. The value thus modelled for the turnover time of N in the soil microbial biomass is 1.52yr, the corresponding annual flux through the biomass being 125 kg N ha−1 yr−1. Of this flux, 59 kg comes from the input compartment and 66 kg from the humus compartment.