INFLUENCE OF TILLAGE AND COVER CROP ON SOIL NITROUS OXIDE EMISSION IN CORN AND WINTER CEREAL RYE

Food production security and resiliency require combination of agricultural management practices that are environmentally friendly and economically viable. Cover crops and tillage are two typical management practices that influence corn (Zea mays L.) and soybean (Glycine max L.) production in Illinois and the Midwest, USA. Finding practices that could potentially reduce nitrous oxide (N2O) emissions and sequester carbon (C) in the soil can improve agricultural resiliency to climate change. Generally, shifting from reduced tillage (RT) to no-till (NT) improves soil structure and decreases C emissions or sequesters soil C but might increase N2O emissions. Including a legume cover crop such as hairy vetch (Vicia villosa L.) before corn is preferred to winter cereal cover crops (WCCCs) to avoid yield penalty in corn and ensure high grain production. Winter cereal cover crops such as winter cereal rye (Secale cereale) (WCR) could potentially decrease soil N2O emissions during fallow period by capturing residual N and reducing soil moisture. These conditions could change in soils with legacy tillage (RT vs. NT) effects due to changes in soil physical, chemical, and biological over time. We utilized a medium-term (six-year-old) trial to test several hypotheses. We hypothesized that RT increases the soil temperature, accelerates soil organic matter mineralization, and especially in combination with hairy vetch could increase soil N in the soil leading to increased corn grain yield and N2O emission (Chapter 1). We also hypothesized that WCR takes up residual N after harvesting corn, decrease soil N, use soil moisture, and therefore, could decrease soil N2O emission (Chapter 2). For study 1 (Chapter 1), our objective was to evaluate the influence of cover crop (hairy vetch) vs. a no CC control and tillage systems (RT vs. NT) on (i) corn yield, N uptake, removal, and N balance; (ii) N2O emissions during corn season; (iii) yield scaled N2O emissions on a long-term (eight years) tillage × cover cropping system during the corn growing season in 2019 and 2021. We also analyzed factors that influence N2O emissions via principal component analysis in corn season. In corn growing seasons, we found that corn grain yield was higher in RT than NT reflecting on more N in the soil in RT than NT. Hairy vetch increased corn grain yield, soil N, and N2O-N indicating increased corn grain yield by hairy vetch N contribution let to higher N loss. Yield-scaled N2O-N emissions in NT-2019 (3696.4 g N2O-N Mg-1) were twofold higher than RT-2019 (1872.7 g N2O-N Mg-1) and almost fourfold higher than NT-2021 and RT-2021 indicating in a wet year like 2019, yield-scaled N2O-N emissions were higher in NT than RT. Principal component analysis indicated N2O-N fluxes were less driven by soil N and more by environmental conditions and N balances reflecting on N application at planting in this trial. . The objectives for chapter 2 were to evaluate the legacy effect of tillage (RT vs. NT) and cover crops (WCR vs. a no cover crop control) on soil nitrate-N (NO3-N), volumetric water content (VWC), temperature, and N2O emission trends during a fallow period after corn in a six-yr trial. In spring 2020 we also estimated WCR biomass and N uptake as affected by tillage practices and compared WCR biomass to weeds in the no cover crop treatment. In rye growing season, winter cereal rye biomass was 55% higher than weeds in the fallow treatment. A linear positive relation between WCR biomass and N uptake (R2= 0.93) and C accumulation (R2 = 0.99) indicates WCR captures more N and adds more C inputs than weeds. Winter cereal rye biomass was also higher in RT than NT reflecting on higher soil temperature and N availability in RT than NT. Soil VWC was lower in WCR plots and there was a negative linear relation between days of the year (DOY) and VWC (R2 = 0.6). Despite all these differences, soil N2O-N values were mainly less than 5 g N2O-N ha-1d-1 in all sampling dates regardless of tillage or cover crop treatment. We conclude that in poorly drained Alfisols with claypan and fragipans, NT is not an effective strategy to decrease N2O-N fluxes. Hairy vetch benefits corn grain yield and supplement N but that increases N loss through N2O-N emissions. We concluded that we should focus on decreasing N2O emissions early in corn season since majority of N is lost during that time sometimes 300 times higher than those reported during the WCR phase. Some changes in management practices that could reduce N2O losses are shifting from upfront N application to sidedress N management, terminating hairy vetch at or even after corn planting, and combine these efforts with enhanced efficiency fertilizers that control nitrification and denitrification