Cover Crop, Compost, and Rotation Effects on Net Mineralization in Rotated and Continuous Corn Systems.

Cover crops and compost can provide supplemental nitrogen (N) to row crops; however, N credit values for these organic sources are difficult to predict annually. Our objective was to investigate the amount and timing of N release from cover crops and/or compost in a corn-soybean-wheat rotation vs. a continuous corn system during 2006 and 2007 using a long-term experiment initiated in 1993 at the W.K. Kellogg Biological Station, Hickory Corners, MI.  Three cultural management systems were used: conventional (historically received synthetic N fertilizer); and two systems that received composted dairy manure at a rate of 4 Mg/ha/yr (one of these two systems did not receive compost during this evaluation to investigate recent vs. long-term additions). For cover crops, red clover was frost-seeded into wheat and cereal rye was planted after harvest in continuous corn. We used above-ground plant N accumulation of unfertilized corn as a bioassay for net soil N mineralization to estimate N credits of these organic sources. To address the timing of N release, we measured corn biomass at V6 and leaf chlorophyll at R1, R3, and R5. The growing seasons were distinctly different as precipitation was adequate throughout 2006, while an extreme drought occurred during vegetative growth in 2007. In 2006, yield of unfertilized corn was significantly increased following red clover and/or compost, however, the effect of crop rotation was minor. In 2007, there was not a consistent yield increase for corn following red clover; however, there was a significant yield response to compost as well as crop rotation. Interestingly, unfertilized corn following red clover accumulated approximately 40 to 45 kg of N per ha both years. Results indicate that N release and subsequent plant uptake from organic sources occurred during grain fill in 2007, too late to fully benefit crop yield.