Effects of cropping systems and microbial biomass on arylamidase activity in soils

The activity of arylamidase (EC 3.4.11.2) was recently detected in soils. This enzyme catalyzes the hydrolysis of an N-terminal amino acid from peptides, amides, or arylamides. Recent work suggests that this enzyme is involved in N mineralization in soils. The impacts of crop rotations and N fertilization on the activity of arylamidase were investigated in soils of two long-term cropping systems at the Northeast Research Center (NERC) in Nashua and the Clarion-Webster Research Center (CWRC) in Kanawha, Iowa, to assess the effect of the microbial biomass C (C_mic) and N (N_mic) contents on the activity of this enzyme. Surface soils (0-15 cm) were taken in 1996 and 1997 from four replicate field plots in corn, soybean, oats, or meadow (alfalfa) that received 0 or 180 kg N ha^-1 before corn. Results showed that arylamidase activity was significantly affected by crop rotations in both years at the NERC site, whereas it was affected by N fertilization only in 1997. Because of the high degree of spatial variability and sampling time, no such relationship was found for the samples from the CWRC site. The greatest activity values were obtained in multi-cropping systems in meadow or oats, and the lowest values in continuous corn or soybean systems. The enzyme activity was significantly correlated with Cmic (r>0.40, P<0.01) and N_mic (r>0.56, P<0.001) in both years at the two sites. The amounts of N mineralized during 24 weeks of incubation at 30°C in soils of the plots receiving 0 or 180 kg N ha^-1 were significantly correlated with arylamidase activity at the NERC site(r=0.88, P<0.001 and r=0.55, P<0.01, respectively), but not in soils from the CWRC site.