Date of Award
8-1-2024
Degree Name
Master of Science
Department
Forestry
First Advisor
Williard, Karl
Abstract
The Midwestern United States is a nationally and globally important producer of agricultural products and uses intensive practices to achieve high grain yields. However, intensive agriculture is a major contributor of nitrogen and phosphorus export to the Mississippi River and the hypoxic zone in the Gulf of Mexico. Cover cropping is a recommended conservation practice for providing soil cover throughout the winter and taking up nutrients that may otherwise be lost in bare fallow systems, but the associated costs limit widespread adoption of this practice. Double cropping, which involves growing two crops in one year, is functionally similar to cover cropping and can be harvested for an additional income, but the water quality impacts of applying fertilizer to maximize yields and the systemic impacts of intensification with another crop on corn-soybean rotations are not well understood. This two-year, plot scale study in Carbondale, Illinois was designed to assess nutrient leaching, referring to nitrate-N, ammonium-N, and dissolved reactive phosphorus (DRP), nutrient availability, and crop yields when using bare fallow, cereal rye (Secale cereale) cover crops, or winter wheat (Triticum aestivum) double crops with varying nitrogen fertilizer rates and timings in the winter seasons of corn-soybean rotations. Four blocks with randomly assigned treatments comprised of two treatment factors were used. These treatment factors included rotations with either bare fallow or cover crops in alternate winters and winter wheat fertilizer management intensity with a high fertilizer treatment level, grower recommended rates applied at planting, tillering, and jointing, a medium fertilizer treatment level, grower recommended rates applied at tillering and jointing, a low fertilizer treatment level, with reduced nitrogen rates applied at tillering and jointing, and a no fertilizer treatment level, which was used as either corn-soybean or corn-cover crop-soybean-cover crop control. Additional nutrient inputs from fertilizers in the winter wheat seasons did not significantly increase nitrate-N, ammonium-N, or DRP leaching in the 2021-2022 winter wheat sampling season and nitrate-N and ammonium-N leaching was significantly less in some or all the winter wheat plots compared to the control plots. Winter wheat yields and nitrogen uptake in 2022 were significantly greater in medium fertilizer plots while yield-based nitrogen leaching and partial nitrogen balances were significantly greater in high fertilizer treatments, indicating that delayed fertilization in winter wheat can improve nitrogen use efficiency and yields. Soybean yields were significantly greater in plots without winter wheat due to a longer growing season, but plant available ammonium-N concentrations, which were greater in winter wheat plots, also had a significant negative relationship with soybean yields, indicating that this may have impeded biological nitrogen fixation. Using cover crops in alternate winters reduced nitrate-N leaching by 106% and plant available nitrate-N concentrations by 107% in the season as well as the subsequent corn season by 66% and 90%, respectively, compared to the bare fallow plots, and the decreased plant available nitrate-N concentrations in cover crop plots caused a 6% yield penalty in the corn harvest. Despite yield penalties to cash crops from winter crops, the use of double crops was the only factor that significantly impacted total crop yields. The use of cover crops in alternate winters was the most significant factor in nutrient leaching, demonstrating that these practices can be used to increase total crop yields without contributing significantly to nutrient export. For farmers concerned with the costs of cover cropping, double cropping is a practice that can provide some of the same ecosystem services while also providing an additional financial incentive.
Access
This thesis is Open Access and may be downloaded by anyone.