Minimum available N requirement for microbial biomass P formation in a regosol

A soil incubation experiment was conducted to determine the effects of N application on microbial biomass C and P and to estimate the minimum requirement of available N for microbial biomass P formation. A granitic regosol soil was amended with N (as (NH4)2SO4) at rates of 0, 200, 400 or 800 mg N kg−1, C (as rice straw) at 2100 mg C kg−1 and P (as KH2PO4) at 500 mg P kg−1 soil. With increasing N application up to 200 mg N kg−1 soil, microbial biomass C significantly increased and remained constant or slightly decreased at higher N rates, while microbial biomass P increased up to 400 mg N kg−1 soil and remained constant or slightly increased at the highest N contents. The concentration of P in microbial biomass (assuming that dry biomass contained 50% C) increased with increasing N rate and ranged from 32 to 76 mg g−1. Among the P fractions in soil, microbial biomass P and inorganic P (available P) fractions increased with increasing N rates, whilst the Ca–P fraction decreased. The critical P concentration in microbial biomass (defined as that required to achieve 80% of the maximum synthesis of microbial biomass C) was estimated to be 60±4.1 mg P g−1 biomass. The corresponding minimum amount of available N in the soil required to increase the biomass was estimated as 425±12 mg N kg−1 soil. The specific respiration of the microbial biomass was little affected by the N concentration and was very high even above an N concentration considered to be the optimum for microbial biomass C and P but also microbial activity.