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 las (NH4)(2)SO4) at rates of 0,200, 400 or 800 mg N kg(-1), C las rice straw) at 2100 mg C kg(-1) and P las 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. (C) 1999 Elsevier Science Ltd. All rights reserved.