Cover crops (CC) are planted in between cash crops to improve soil quality and to supply nitrogen (N) to cash crops through biological N fixation or soil N scavenging. Most producers use single species CC, in part because potential benefits of using mixtures of three or more CC species are poorly understood. A three-year study was initiated at Painter, Virginia to observe effects of CC mixtures on a no-till (NT) corn (Zea mays), wheat (Triticum aestivum L.), and soybean (Glycine max) rotation to measure CC performance, N cycling, cash crop yield, and soil quality in a sandy, low organic matter soil. Twelve treatments were created with conventional tillage (CT), NT, no CC control, and monoculture or CC mixtures of 3 to 9 species. Corn was grown in year 3 in all 12 treatments and four N rates were applied (0, 56, 112 and 156 kg N ha-1). Cover crop biomass, N accumulation, CC C:N ratio, and corn and soybean yield were measured annually. Soil bulk density, compaction, infiltration rate, pH, electrical conductivity, soil respiration, earthworm counts, soil microbial respiration, and soil microbial biomass carbon (C) after three years of CC. Cover crop biomass production varied significantly each year (5633 kg ha-1 in year 1, 755 kg ha-1 in year 2, 5370 kg ha-1 in year 3) due to climate and agronomic parameters, but a CC mixture always produced the highest biomass at termination. Nitrogen accumulation was strongly correlated with biomass production (R2= 0.94) and followed the same trend due to all CC having C:N < 30:1. Corn and soybean yields in years 1 and 2 were not significantly different, but corn yield was significantly affected by treatment and N fertilizer rate in year 3. After 3 years, soil respiration, earthworm populations and soil microbial biomass C increased in CC compared to CT without CC. However, infiltration rate, bulk density, microbial respiration, pH did not improve or declined compared to CT. In conclusion, adding CC mixtures to crop rotations shows promise for producing high CC biomass, accumulating N, and increasing crop yields, while improving some soil quality parameters on sandy low organic matter soils. / Doctor of Philosophy / Cover crop (CC) are planted in between cash crops to protect the soil from erosion, improve soil quality, and supply N to next cash crop through biological N fixation or soil N scavenging. Traditionally, CC were single species, but new CC methodologies utilize mixtures of three or more species planted together to protect soils as well as produce high biomass to suppress weeds, conserve soil moisture, and improve soil quality. A long-term study was initiated in fall 2014 in Painter, VA to observe CC mixture effects on no-till (NT) corn (Zea mays), wheat (Triticum aestivum L.), and soybean (Glycine max) rotations on CC performance, N cycling, cash crop yield, and soil quality of a sandy, low organic matter soil. Twelve treatments were created that compared NT rotations with CC monocultures, CC mixtures of 3-9 species, and without CC. In the third year corn was grown in all 12 rotations and four N rates were applied (0, 56, 112 and 156 kg N ha-1). To evaluate CC mixture performance in rotations, CC biomass, CC N accumulation and corn and soybean yield was measured over three years. To evaluate changes in soil quality, nine soil physical, chemical and biological soil properties were measured after three years of NT and CC. Biomass production varied significantly each experimental year (5633 kg ha-1 in year 1, 755 kg ha-1 in year 2, 5370 kg ha-1 in year 3) due to climate and agronomic differences, but CC mixtures were the highest biomass producing CC each spring and accumulated the highest amount of N. Cover crop mixtures had equal corn and soybean yield as CC monocultures. In year 3 corn yield and was greater in treatments with CC than in treatments without CC and was greater in legume dominated monocultures and mixtures than majority grass CC mixtures and monocultures. After 3 years of CC and NT, some soil quality parameters improved. Indicators of soil biology (soil respiration, earthworm populations, and soil microbial biomass C) increased in CC treatments. However, some soil physical and chemical properties (infiltration rate, bulk density, pH and EC) did not improve. In conclusion, adding CC mixtures to crop rotations shows promise for producing high CC biomass, accumulating N, and increasing crop yields, while also improving some soil quality parameters that are important for agricultural systems.

Master of Science / Department of Agronomy / P. V. Vara Prasad, / Kraig L. Roozeboom / Replacing fallow with cover crops can provide many benefits, including improved soil quality and reduced nitrogen fertilizer requirements. The addition of cover crops into no-till systems has become popular in recent years as a means of increasing cropping system intensity and diversity. A primary concern of producers in the Great Plains is the possibility that cover crops may reduce the amount of soil water stored in the profile for the next grain crop, potentially reducing yields. Multi-species cover crop mixtures that enhance the ecological stability and resilience of cover crop communities may produce greater and more consistent biomass than single species. Field experiments were established in 2013 and 2014 near Belleville and Manhattan, KS following winter wheat (Triticum aestivum L.) harvest to evaluate the effect of cover crop species and species complexity on changes in soil profile water content and water use efficiency. Along with a chemical fallow control, ten cover crop treatments were tested: six single species, two-three component mixes, a mix of six species, and a mix of nine species. Volumetric water content was measured using a neutron probe and a Field Scout TDR 300. Similar data were collected in 2014 from an experiment established in 2007 comparing fallow, double-cropped soybean, and four cover crop types (summer and winter legumes and non-legumes) in a no-till winter wheat-grain sorghum-soybean cropping system near Manhattan, KS. Results from both studies showed that grasses produced the most dry matter with the highest water use efficiency (up to 618 kg cm-1). Fallow lost up to 7.9 cm less water than all cover crop treatments throughout cover crop growth and in the fall, but captured up to 3.4 cm less moisture in the spring than the cover crops that added residue to the soil surface. Brassica species extracted water from deeper in the soil profile than the other cover crop species. Species complexity affected water use only relative to the proportions and productivity of their individual components, with no advantage in water use efficiency for the more complex mixtures.