Date of Award
Master of Science
Plant and Soil Science
Growing winter cereal cover crops (WCCCs) has been identified as an effective in-field practice to reduce nitrate-nitrogen (N) and total phosphorus (P) losses to Upper Mississippi River Basin, USA. In this region, however, growers are reluctant to plant WCCCs prior to corn (Zea mays L.) due to soil N immobilization and corn establishment issues. Two strategies to minimize these issues are (i) incorporating legumes and brassicas into WCCCs as mixtures and (ii) precision planting of cover crops. The objective of chapter 1 was to (i) evaluate the effect of cover crop mixtures vs a no-cover crop control on soil health indicators and (ii) assess the impact of precision planting of cover crops on soil nutrient availability, soil nutrient stratification, soil permanganate oxidizable carbon (POXC) and soil organic carbon (SOC) stocks “on” and “off” the corn row over three depths (0-5, 5-20, and 20-90 cm). Treatments were (i) a no-cover crop control (NCC); (ii) no cover on corn row, hairy vetch (V) on middle row, and winter cereal rye (WCR) on the outside row of corn (NOVR); and (iii) oats (Avena sativa) and radishes (Raphanus sativus) on the corn row, V on the middle row, and WCR on the outside row (ORVR). Our results indicated NCC had lower SOC stocks than the NOVR and ORVR only at 0-5 cm depth. Soil POXC was more sensitive to cover crop management than SOC, and POXC concentrations were higher in ORVR at 5-20 cm than the NCC control. At 0-5 cm depth, cover cropping increased Bray-1 soil test P (STP). Soil test P declined over depth reflecting its immobility in the soil. Mehlich-3 soil test K (STK) was higher in cover crop treatments than the no-cover crop control at 0-5 cm depth. Soil test K was higher on corn row indicating that the oats and radish mix and corn residue decomposition releases K detectable in soil as Mehlich-3 K. Soil test sulfur was similar among treatments but higher at 20-90 cm depth reflecting S leaching and/or potential anion exchange capacity at depth that can lead to subsoil sulfate-S accumulation. These results indicate cover cropping in the fragipan belt / Alfisols of the Upper Mississippi River Basin can benefit soil after six years, but soil C benefits are limited to surface soil depths.In Chapter 2 the objectives were to (i) evaluate the biomass, nutrient concentration, and uptake of precision planted cover crop mixtures; (ii) assess whether precision planted cover crops influence corn stand density, grain yield, yield components, and nutrient balances; identify the best economically viable precision planted mixture prior to corn. Treatments were (i) a no-cover crop control (NCC); (ii) no cover on corn row, hairy vetch (V) on middle row, and winter cereal rye (WCR) + annual rye (AR) on the outside row of corn (RVSKIP); and (iii) no cover on corn row, clover (C) on the middle row, and WCR + AR on the outside row (RCSKIP). Results indicated that RVSKIP was always high yielding, with high N uptake, and low C:N ratio (25) suggesting it could release N throughout the corn growing season without immobilizing N. Cover crops influenced corn population only in one site-yr but that did not result in lower corn grain yield reflecting corn potential for filling the plant gap by creating larger ears with heavier grain (TKW). Similar corn grain in all cover crop treatments was mainly due to adding optimum N as fertilizer. We concluded that overall, cover cropping could benefit soil over a six-year period but to optimize their benefit to corn, adjustments to N should be made. Therefore, future research should focus on revisiting corn N requirement especially in cover crop mixtures with high percentage (>50%) of legumes in the mixture to determine the fertilizer value of the cover crops.
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