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The synchrony of cover crop decomposition, enzyme activity, and nitrogen availability in a corn agroecosystem in the Midwest United States

Cover cropping is commonly associated with increased soil microbial activity and soil health. Cover cropping in the Upper Mississippi River Basin has increased rapidly in the last decade due to the demonstrated ability that cover crops can increase soil health and decrease nitrate losses in tile-drained fields. One reason cover cropping is not adopted in the Midwest United States is the lack of knowledge on the synchrony of cover crop residue nitrogen (N) release and corn N demand. Therefore, the overarching goal of this research was to investigate soil enzyme activity and inorganic N availability during the decomposition period of common winter cover crops. Objectives were: i) to determine if soil enzyme activity (β-glucosidase and urease) is influenced by different cover crops (cereal rye (CR), hairy vetch (HV), and HV/CR mixture) and tillage systems (reduced tillage and no-tillage residue management) during the residue decomposition period, and ii) to determine the relationship between soil enzyme activity, inorganic N availability, and critical corn growth stages during cover crop decomposition. Cover crops in this study were selected due to contrast in residue carbon (C):N ratios, their commonality among regional farmers, and their differences in physiology. Spring tillage and no-tillage were investigated because of the differing levels of soil disturbance and residue incorporation. Enzyme activities were quantified at seven sampling dates following cover crop termination from April-October during the 2016 and 2017 corn growing seasons. Results revealed significant effects of sampling date (p < 0.001) and cover crop treatment (p < 0.05) on soil β-glucosidase activity and significant interaction effects between sampling date and cover crop treatment (p < 0.05) on potential soil urease activity. Tillage had a significant impact on soil β-glucosidase (p < 0.001) and urease (p < 0.05) activities in 2017, with higher activity observed in the no-tillage soil. These findings indicate that enzyme activity fluxes can be observed in agroecosystems one year after introducing conservation practices, such as cover crops and no-tillage. Greater than 50 % of C release from cover crop residue occurred from corn emergence to tasseling, and significantly greater β-glucosidase activity for CR-based treatments during corn peak N demand did not result in more soil inorganic N. This indicates a loss of plant-available N from the agroecosystem, which could be attributed to N immobilization by the soil microbial community. This study demonstrates a need to develop adaptive N fertilization management for corn that can overcome the potential impact of N immobilization during peak corn N demand following CR.

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