Cereal rye cover crop inputs disproportionately stimulate soil carbon‐ and phosphorus‐ but not nitrogen‐hydrolytic enzyme activities in long‐term maize–soybean rotations

Abstract

Long‐term crop rotations alter residue inputs and nutrient balances, yet how these translate to soil enzyme activities mediating carbon (C), nitrogen (N), and phosphorus (P) cycling remains unresolved. We assayed soil enzyme activities under grain‐ and silage‐maize ( Zea mays L.) with and without a cereal rye ( Secale cereale L.) winter cover crop in a 21‐year maize–soybean ( Glycine max L.) rotation field experiment in Iowa. Under the silage rotation, cover cropping increased C‐ and P‐hydrolytic activities: β‐glucosidase (+61%), cellobiohydrolase (+140%), phosphodiesterase (+82%), and phosphomonoesterase (+40%), which persisted after normalization to soil organic C, indicating a nonlinear, substrate‐induced response to residue inputs. In contrast, most N‐hydrolytic enzyme activities (protease, glycine‐, alanine‐, and methionine‐aminopeptidases) were unaffected by crop rotation or cover cropping, with only increased leucine aminopeptidases activity (+25%) under silage relative to grain maize, despite estimated negative N balances. These findings suggest C and P cycling mediated by enzyme activities are coupled through residue inputs, whereas N cycling is partially decoupled. More broadly, our results indicate that soil enzyme activities may reflect biogeochemical equilibrium states rather than simple indicators of nutrient limitation in intensively managed Mollisols of the US Midwest.