Negative Effect of Grazing on Activities of Nitrogen-Transforming Enzymes in Soils.

Our previous study suggested that grazing might have negatively impacted carbon-transforming enzyme activities in soils. Would a similar trend be found on nitrogen-transforming enzyme? In this study, the effect of long-term (>30 years) cropping systems on activities of amidohydrolases were evaluated; and the relationship between enzyme activities and selected soil properties were assessed. Five evaluated cropping systems were monocrop soybean (MS), double-crop wheat (for grain)-full season soybean (DC), double crop wheat (for grazing)-early season soybean (GS), and modified double-crop, a two-year rotation including MD1 (early season soybean-winter wheat-full season soybean and MD2 (full season soybean- early season soybean- winter wheat). Based on evaluation of 60 soil samples and up to 30 cm depth, cropping systems significantly affected the activities of amidase, urease, L-asparaginase and L-glutaminase in the surface 0-10 cm soils. These enzyme activities were significantly correlated with organic carbon (r=0.51*** ̶ 0.80***) and total nitrogen (r=0.40** ̶ 0.85***), but not soil pH. Enzyme activities involved in both carbon and nitrogen cycling were inter-correlated, with r values ranging from 0.31** to 0.80***. Amidase activity was the highest of the enzymes tested, followed by L-glutaminase, urease, and then L-asparaginase. Activities of the tested enzymes were consistently lower in GS than other cropping systems, which supported previous observation that grazing might have negatively impacted soil’s capacity to cycle nutrients. Of the cropping systems tested, the highest enzyme activities were found in MD2, signifying the importance of crop residue input in preserving functional capacity of a soil. The obtained results exemplified the need to implement sustainable soil management practices that preserve soil health while maintain productivity.