Effect of Cover Crops and Nitrogen Application Timing on Nutrient Loading through Subsurface Drainage.

It has been estimated that nitrate (NO₃-N) leaching from artificially drained agricultural fields in the Upper Mississippi River Basin accounts for approximately 65% of nitrogen (N) delivered annually to the Gulf of Mexico.  Therefore, the objectives of this study were to investigate the impact of N fertilizer application timing and cover crops (CC) adoption on the load and concentration of NO₃-N in subsurface drainage leachate.  This experiment was conducted at the Illinois State University Nitrogen Management Field Station, in Lexington, IL.  Treatments include a zero control (no N fertilizer and no cover crop), fall dominated N application (70% fall, 30% spring) with and without CC, and a spring dominated N application (20% fall, 80% spring) with and without CC.  A total rate of 225 kg N ha^-1 was applied to all treatments, except the zero control.  Automated water samplers (ISCO 6712) were programmed to collect samples that were analyzed for nitrate, ammonium, and dissolved reactive phosphorus concentrations.  Spring cover crop sampling revealed an average biomass of 1165 kg ha^-1 and an average N uptake of 42.5 kg N ha^-1.  Over the course of the study, CC integration resulted in a 40-44% reduction in tile NO₃-N load despite N management system.  Cumulative tile leachate for NO₃-N (kg ha^-1) for the first year revealed the following treatment trends: fall (54.0) > spring (44.4) > fall N + CC (39.2) > spring N + CC (38.5).  The average flow-weighted NO₃-N (mg/L) concentrations concluded that fall (9.19) > fall CC (8.95) > spring (8.80) > spring CC (8.10).  The Illinois Nutrient Reduction strategy presents a target of reducing NO₃-N losses by 15% by 2025.  This data demonstrates that CC have the potential to significantly reduce NO₃-N losses to similar levels in both spring and fall dominated N management systems.