Corn (Zea mays L.) and soybean (Glycine max L.) production in the Midwest US can result in significant nutrient leaching to groundwater and surface waters, which contributes to eutrophication and hypoxia in the Gulf of Mexico. A promising strategy to control nutrient leaching and sediment runoff loss during winter fallow period is the use of cover crops (CCs). In southern Illinois, CCs are not widely adopted by farmers due to economic constraints and the lack of scientific data that supports benefits of incorporating CCs into the corn-soybean rotation. Therefore, this doctoral dissertation addresses the critical question of the feasibility of the use of CCs in southern Illinois and is divided into three overarching research studies with different objectives divided into six research chapters. Research study 1 was a field experiment conducted from 2013 to 2017 to examine the effect of CCs (CC vs noCC) under two tillage systems [(no-tillage (NT) and conventional tillage (CT)] on aboveground plant attributes [dry matter yield, C:N ratio and nitrogen uptake (N uptake)], crop yields, available soil N content and N leaching in the vadose zone. The experimental layout was a randomized design with three rotations including corn-noCC-soybean-noCC [CncSnc], corn-cereal rye (Secale cereale L.) –soybean-hairy vetch (Vicia villosa R.) [CcrShv], and corn-cereal rye-soybean-oats+radish (Avena sativa L. + Raphanus sativus L.) [CcrSor] and two tillage systems. Soil samples collected after corn or soybean harvest and CC termination were analyzed for standard soil fertility parameters. Pan lysimeters installed below the ‘A’ horizon with depth varying from 22 to 30 cm were used for measuring soil solution nutrient concentration on weekly or biweekly basis depending on the precipitation. In NT system, the corn yield was 14% greater with CcrShv compared to CncSnc, whereas no significant difference existed in corn yield due to CC treatments within CT. Both CC treatments under NT reduced soybean yield by 24 to 27% compared to noCC. The rotations CcrShv and CcrSor with hairy vetch and oats+radish as preceding CCs resulted in 89% (37.73 vs 19.96 kg ha-1) and 68% (33.46 vs 19.96 kg ha-1) more nitrate-N (NO3-N) leaching than the CncSnc during cash crop season 2015. During the CC season in spring 2016, cereal rye CC in CcrShv and CcrSor reduced the NO3-N leaching by 84% (0.68 kg ha-1) and 78% (0.63 kg ha-1) compared to the CncSnc, respectively, under the CT system. Overall, our results indicated that the CT system had greater N leaching losses compared to NT system due to higher N availability in the tilled soil profile. The goal of the second research study was to understand the mechanisms of N cycling by CCs. Therefore, we applied 15N labeled urea fertilizer (9.2% atom) to corn that followed hairy vetch and noCC in May 2017 to evaluate the contribution of fertilizer and soil organic matter to N leaching and quantify the 15N content of surface runoff after storm events. During the 2017 corn season, repeated soil samples were collected and analyzed for 15N fertilizer recovery in soil at three depths. 15N recovery was higher in the corn that had hairy vetch as the preceding CC than the corn that had noCC by 13.13 and 3.68 kg ha-1 on soil sampling events of 7 and 21 days after planting of corn, respectively, at the depth 15-30 cm. Overall, the cumulative loss of 15NO3-N during corn season 2017 was <2% of the applied fertilizer. The contribution of NO3-N from soil organic matter to leaching was 61% higher for the corn rotation with hairy vetch CC compared to corn rotation with noCC (1.12 vs 0.69 kg ha-1). Research study 3 evaluated the effects of CCs (cereal rye and hairy vetch) and topography (shoulder, backslope, footslope) on corn-soybean production, soil and water quality, nutrient and sediment export in agricultural headwater streams in a paired-watershed experimental design. The crop rotation followed in the CC-watershed was corn-cereal rye-soybean-hairy vetch whereas the rotation in the noCC watershed was corn-noCC-soybean-noCC. Use of hairy vetch CC increased N uptake at shoulder, backslope and footslope positions by 110.90, 85.02, and 44.89 kg ha-1, respectively, when compared to noCC treatment. The corn yield at the shoulder position was increased by 69% in CC-watershed compared to noCC watershed in 2017 likely by providing large N additions following decomposition. Cereal rye increased soybean yield by 17% and 8% at the shoulder and backslope position, respectively in 2016. Additionally, the effects of topography and CCs on soil N levels and N leaching in the watersheds were also evaluated. The NO3-N concentrations measured using suction cup lysimeters in CC-watershed were reduced by 2.54 mg L-1 (67%) when compared to noCC watershed. During the hairy vetch CC season, the reduction in NO3-N concentrations in soil solution was only seen at the footslope position. The excessive N at footslope positions may have been immobilized or denitrified due to soil waterlogging from higher water availability at the footslope. Forty-two and 18 storm events were collected during a 4-yr calibration period and CC-treatment period, respectively. Predictive regression equations developed from the calibration period were used for calculating TSS, NO3-N, NH4-N, and DRP losses of surface runoff for the CC-treatment watershed. The CCs reduced TSS and discharge by 33% and 34%, respectively in the CC-watershed during the treatment period. However, EMCs for NO3-N, NH4-N, and DRP did not decrease. Overall, CCs are a recommended conservation practice for farmers who want to enhance the long term profitability of their production systems, while building soil health and protecting downstream water quality. The CCs have the potential to reduce nutrient leaching, peak/total discharge, improve soil quality and crop yields. However, reduction in leaching will depend on the type of CC (legume vs non-legume crops) used in rotation, the time of termination of CCs, cover crop establishment and number of years under CCs. Synchronizing N availability from CCs to N uptake by cash crop is important for reducing nitrate leaching and increasing crop yields.

The diverse cropping system of eastern Virginia's coastal plain offers limited opportunity to establish winter annual cover crops (WCC) for nitrogen (N) scavenging. The winter fallow niche after double-crop or full-season soybean (Glycine max L. Merr.) encompasses the majority of acres left fallow. Our objective was to evaluate interseeded WCC N scavenging performance following soybean and N supplying capacity to subsequent corn (Zea mays L.). Field studies were conducted at four different locations in each of the two study years. The experimental design was split plot with cereal rye, hairy vetch, and RV mix WCC as main plots and ten fertilizer nitrogen (FN) rates in a factorial arrangement (0 and 45 kg FN ha-1 as starter; and 0, 45, 90, 135, and 180 kg FN ha-1 at sidedress) to corn as subplots. The highest N uptake for cereal rye at winter dormancy was 18 kg N ha-1, but the average was 6-7 kg N ha-1. At WCC termination average N uptake for cereal rye was 35 and 40 kg N ha-1 in 2013 and 2014, respectively. Average biomass dry matter (DM) at WCC termination for cereal rye, cereal rye + hairy vetch mix (RV mix), and hairy vetch was 2356, 2000, and 1864 kg ha-1 in 2013; and 2055, 2701, and 692 kg ha-1 in 2014, respectively. Average cereal rye N uptake was 35 kg N ha-1 in 2013 and 40 kg N ha-1 in 2014. Significant differences for residual soil nitrogen were most apparent for soil nitrate (NO3-N) at lower depths (15-30 and 30-60 cm) during WCC termination and in the upper 0-15 cm during corn growth stage (GS) V4 of both years. Corn grain yield plateau following hairy vetch WCC was 0.7 and 0.6 Mg ha-1 higher than when following cereal rye WCC at zero and 45 kg ha-1 starter FN, respectively. Average agronomic optimum FN rates (AONR) were 26 and 9 kg ha-1 lower following hairy vetch than cereal rye WCC at zero and 45 kg ha-1 starter FN, respectively. Estimated hairy vetch FN reductions by FN replacement and AONR difference methods were 48 and 18 kg FN ha-1 in plots receiving zero starter FN; and 58 and -43 kg FN ha-1 in plots receiving 45 kg ha-1 starter FN. In-season soil N tests did not offer adequate information in order to predict sidedress FN reductions. These findings suggest that cereal rye and RV mix have the potential to scavenge and conserve residual soil N and hairy vetch is more than capable to supply PAN to subsequent corn when interseeded into soybean. / Master of Science

Current interest in cover cropping is focused on enhancing ecosystem services beyond soil conservation. Cover crop (CC) species function uniquely in their effects on ecosystem services when grown in monoculture or mixtures. This research integrated field experiments and a literature synthesis to evaluate the role of cover crops in improving nitrogen (N) management and simultaneously providing multiple ecosystem services. Legume CC fertilized with poultry litter (PL) could replace 101 to 117 kg N ha-1 of fertilizer in corn (Zea mays L.) production. Rye (Secale cereale L.) CC fertilized with PL had a negligible effect on corn production. Biculture fertilizer equivalence ranged between -12 to +75 kg N ha-1. Fertilizer equivalence of legume-containing treatments increased across time. Without CC, fall-applied PL failed to supply N to corn. Ecosystem services of CC and PL illustrate complex species functions. Bicultures produced more total biomass than monocultures in year 1 but less than rye in year 2. Bicultures were as effective in suppressing weeds as rye, produced corn yield similar to legume, and by the second year had similar amounts of available soil N as the legume. Poultry litter effects and interspecific effects cover crop species biomass differed. Rye yield increased, while legume yield decreased slightly in biculture. Poultry litter increased legume N content and a decrease in legume C:N, while rye N content and C:N were unaffected. The synthesis corroborates that mixed and biculture cover crops yield more than the individual component species. Overyielding was transgressive in 60% of cases studied. Mixture effects varied by species: rye and brassica yield increased, while legume decreased in mixtures. The effect of mixed CC on crop yields varied by crop species and management practices, though generally crops increased 8 to 18% overall. This work can be applied to the design of complex CC and PL systems that optimize individual species functions to enhance ecosystem services. / Ph. D.