Kinetics of arginine ammonification to estimate microbial activity of N mineralization in forest and cropland soils

Abstract The arginine mineralization assay has been used to estimate microbial potentials of soil ammonification, but the validity of saturating arginine dose must be assessed before applying the assay to forest soils as well as cropland soils. We studied the Michaelis–Menten kinetics of 14 C-radiolabeled arginine mineralization ( 14 CO 2 production) at wide range of arginine doses (20–420 mg N kg −1 soil). The affinity constants ( K M ) of concentration-dependent mineralization rates (16–89 mg N kg −1 soil) increased with increasing soil N concentration but were even lower than the standard addition (140 mg N kg −1 soil). Arginine was selectively mineralized within 24 h in cropland soils, while arginine ammonification was slower in the coniferous forest soils with high C to N ratios (C/N > 20). The wide variation (11–90%) in the mineralization of arginine-derived N in 24 h is caused by the differences in microbial activities of arginine degradation and NH 4 + release. This supports the validity of the arginine ammonification assay (140 mg N kg −1 soil addition and 24 h incubation) to obtain rough estimates of microbial potentials of ammonification in forest and cropland soils within the soil N concentrations studied ( −1 ). The ammonification assay showed the repressed activity of microbial N release in coniferous forest soils with high C/N ratios.