Nitrogen (N) fertilizer inputs for intensive corn-based cropping systems can increase nitrate (NO3-–N) concentrations in groundwater. Nitrogen transport in surface water to the Gulf of Mexico is also an on-going issue for the upper Mississippi river basin, especially areas with large corn and soybean acreage. Education and policy efforts have focused on improvement in N application rate, timing, management, and crop N use efficiency. However, additional means to reduce N loss are needed. Recent efforts have refocused on practices to keep soluble NO3-–N and soil-bound N in fields. One practice receiving attention and agency cost share is cover crops.

Corn N fertilization results in residual NO3-–N in the soil profile after crop maturity. The amount is dependent on factors such as N application rate, rainfall, soil texture, and crop yield. Residual NO3-–N is subject to off-season loss (fall to early spring) as there is no active crop (in a corn- soybean row crop system) to assimilate N. Cover crops have been shown to help retain NO3-–N and protect the surface soil from erosion. In some situations, cover crops have increased yield of the row crop. Many cover crops can be used, but due to seed availability, cost, and winter hardiness, winter rye (Secale cereal L.) has been a common choice.

Many questions arise as producers consider implementing a cover crop system, including the potential need to adjust corn N fertilization rate. Results of prior research with cereal cover crops has been inconsistent in regard to N supply and effect on corn fertilization rate requirement, with differences related to soil properties such as texture and organic matter (Kessavalou and Walters, 1997; Vyn et al., 2000; Andraski and Bundy, 2005). Of particular interest is what happens to the N taken up by the cover crop. It is known that temperature and precipitation affect the decomposition of the cover crop biomass (Ruffo and Bollero, 2003), but more research is needed to understand N cycling to the soil as cover crop biomass degrades. Is it immobilized by microbial processing of the cover crop biomass due to high carbon content, or does it add to plant-available N during corn N uptake? This is not as important of an issue with cereal cover crops preceding soybean, but is for corn. The objectives of this research were to determine long- term corn response to applied N, corn N fertilization requirement, nutrient cycling, and crop productivity in a corn-soybean rotation when grown in sequence with a winter rye cover crop. This report only covers the initial and partial second year of the cover crop system, with the expected to continue for several years. expected to continue for several years.