Decoupling of protein depolymerization and ammonification in nitrogen mineralization of acidic forest soils

Abstract Soil nitrogen (N) mineralization is generally limited by amino acid production by proteases (depolymerization). Protease activities can be stimulated under N limitation, but it cannot directly increase microbial N release (ammonification) in temperate forest soils. To analyze factors determining rate-limiting steps of soil N mineralization, we compared microbial potentials of depolymerization and ammonification by measuring casein degradation and arginine ammonification rates in casein- or arginine-amended and control soils, respectively. The microbial potentials of casein depolymerization were positively correlated with soil carbon (C)/N ratios, whereas arginine ammonification exhibited the opposite pattern. The soil N mineralization was limited by depolymerization in N-rich broad-leaved forest and cropland soils, while N mineralization could be limited by the low microbial potentials of ammonification for microbial N conservation in coniferous forest soils with C/N ratios > 20. Due to the increased depolymerization and microbial N conservation under N limitation, ammonification as well as depolymerization can be a rate-limiting step of N mineralization in fungi-dominated acidic forest soils. Contrasting responses of depolymerization and ammonification to N limitation and low pH can induce a shift in rate-limiting steps in soil N mineralization.