Seasonal changes of soil carbon fractions and enzyme activities in response to winter cover crops under long‐term rotation and tillage systems

Quantifying changes in soil biochemical properties is important in understanding the response to soil management practices. Our objectives were to evaluate the single and combined effects of three different crop rotations (maize ( Zea mays )‐soybean ( Glycine max ) (2 years), maize‐soybean‐oat ( Avena sativa ) (3 years), maize‐soybean‐oat‐winter wheat ( Triticum aestivum ) (4 years)), and two winter cover‐type (cover crop and no cover crop) managements under long‐term conventional‐till (CT) and no‐till (NT) systems on water‐extractable carbon and nitrogen fractions, microbial biomass carbon (MBC) and enzymatic activities. The experimental site under silty clay loam in southeastern South Dakota was sampled before planting in early spring, after planting in early summer, and during the maize grain‐filling stage in early autumn of 2017, from the surface 0–7.5‐cm depth in 2017. In general, the cover crops had 9, 17 and 19% higher geometric mean of enzyme activities than the no‐cover‐crop plots at pre‐planting, after planting and the grain‐filling stage of maize, respectively. Although there were not many differences between “NT vs. CT” and “2‐year vs. 3‐year vs. 4‐year rotation” treatments, additive effects between “tillage and cover crops” and “rotation and cover crops” were observed. The MBC and β‐glucosidase activity were 31 and 54% higher, respectively, with cover crop vs. no cover crop under 4‐year rotation after planting of maize. Similarly, significant interactions between “cover crop and tillage” for hot water‐extractable nitrogen and urease activity at pre‐planting, and β‐D‐glucosidase after planting of maize, were observed. At the grain‐filling stage, the hot water‐extractable contents were significantly greater under cover crop as compared to the no‐cover‐crop plots. Furthermore, this study also concluded that seasonal fluctuations are important for understanding the management impacts on soil carbon fractions and biochemical properties. Highlights Soil microbial properties responded differently to tillage, rotation and cover crops during the maize growing season. Residues produced by cover crops during winter fallow impacted soil functioning and interacted with other management factors (rotation and tillage system). Rotation impacted the carbon availability after planting, whereas tillage affected the enzyme activities. Winter cover crops influenced labile soil organic matter (SOM) and enzymatic properties after 4 years.