Amidohydrolases in soils as affected by cropping systems

Abstract

Cropping systems influence the ecology of agricultural soils. The effects of crop rotation and N fertilization on the activities of four amidohydrolases (amidase, l-asparaginase, l-aspartase, and l-glutaminase) were studied in soils of two long-term cropping systems in Iowa: the Northeast Research Center (NERC) and the Clarion–Webster Research Center (CWRC) that were initiated in 1979 and 1954, respectively, and sampled in 1996 and 1997. Multicropping systems, especially meadow–oat-based systems, enhanced the activities of the amidohydrolases; monocropping systems, involving corn or soybean, reduced the activities of the four enzymes. Analysis of variance indicated that activities of the amidohydrolases were significantly affected by cropping systems at the NERC site in both years; but only amidase activity was significantly affected by N fertilization. With the exception of amidase, the activities of the other three enzymes were not affected by cropping systems and N fertilization in the soils at the CWRC site. The activities of the four amidohydrolases were significantly correlated with soil microbial carbon (C mic) ( r>0.447 and r>0.398, P<0.01), soil microbial nitrogen (N mic) ( r>0.445, P<0.01 and r>0.596, P<0.001), and the amount of N mineralized in 24 weeks of incubation at 30 °C ( r>0.427 and r>0.467, P<0.05) in soils of the CWRC and NERC sites, respectively. The specific activities of the four enzymes in soils under multicropping systems were greater than those under monocropping systems. The results suggest that the mineralization of organic N in the soil is mediated by microbial biomass through the action of these enzymes, and that the activities of the amidohydrolases can be used to predict N mineralization and monitor soil health and quality.