McSwiney, Claire (Michigan State University, W.K. Kellogg Biological Station, 3700 East Gull Lane, Hickory Corners, MI, 49060; Phone: 269- 671-2212; Email: cmcswiney@kbs.msu.edu)

Global Warming Impact of Irrigated Continuous Corn Fertilized at Different N Rates

C.P. McSwiney *, G.P. Robertson

In past studies at the W.K. Kellogg Biological Station in southwest Michigan we determined that N₂O fluxes measured across a high-resolution N gradient were moderately low (less than 50 g N₂O-N ha^-1 day^-1) up to 101 kg N ha^-1 additions (grain yield maximum), after which fluxes increased sharply. In 2003, N, as granular urea, was applied at nine levels from 0-292 kg N ha^-1 yr^-1 to 8 replicate fields in continuous corn and then incorporated. Four replicates were irrigated to alleviate water stress in the corn crop. From these results we calculated the N rate at which we gained the greatest mitigation potential for the global warming impact (GWI) of these cropping systems by including contributions from all sources of radiative forcing in these systems – fertilizer, fuel, lime, pesticides, soil C change, and trace gas fluxes. We found that by fertilizing at the N level required to maintain maximal yields, we gained a 34% reduction in GWI compared to the N level that is considered best management practice. When crops were grown in a corn-soybean-wheat rotation, there was an additional 66% reduction in GWI. The GWI increases precipitously at N rates that are higher than required to maintain yields, primarily driven by N₂O fluxes from the farm field and CO₂ produced during N fertilizer manufacture. By using less N fertilizer and maintaining fields in rotation, growers can significantly reduce the GWI of their farming operations.