C Balance and Growing Season GHG Emissions from Corn-Soybean Rotations on a Midwestern Mollisol (Gracenet Project)

Changes in management can reduce agriculture contributions to greenhouse gases (GHG). A field study was started in 2003 on a Typic Haplaquoll to determine the impact of tillage and N management on GHG emissions. There were four treatments for a corn-soybean rotation implemented: 1) chisel/disk tillage with N fertilizer applied as urea ammonium nitrate (UAN) before planting corn; 2) No-till with pre-plant UAN; 3) No-till with split application of UAN; and 4) No-till with winter rye as a cover crop with pre-plant UAN. There were four replicates, with corn and soybean phases planted each year. Starting in 2004, CO₂, CH₄ and N₂O emissions were measured throughout the growing season. CO₂ emissions in corn peaked in late June, and were significantly higher in the CD and cover crop system, while there was no difference between the no-till systems. For soybeans, CO₂ emissions peaked in late July, with the cover crop producing more than the other systems some years. Overall emissions during the growing seasons were higher in soybean than in corn, 20.7 and 14.4 Mg CO₂ ha^-1 yr^-1, respectively. The soil served as a sink for CH₄, with a mean of 11.6 kg CO₂ equivalents ha^-1 yr^-1 sequestered, with no difference between crops or within crops. N₂O emissions peaked in corn about a month after fertilizer application. There were no differences between corn and soybean, and the amounts emitted were trivial compared to the CO₂ emissions (a mean of 0.2 kg CO₂ equivalents emitted as N₂O). Overall mean global warning potential was higher for soybean than for corn, 5.1 and 2.4 Mg CO₂ equivalents emitted, respectively. C balance and fuel usage analysis will be presented. CO₂ emissions are reduced due to fewer tillage operations, so that implementing no-till can immediately reduce CO₂ emissions, despite lack of differences in the field.