Seasonal Nitrous Oxide and Methane Fluxes from Grain- and Forage-Based Production Systems in Wisconsin, USA

Agriculture in the midwestern United States is a major anthropogenic source of nitrous oxide (N2O) and is both a source and sink for methane (CH4), but the degree to which cropping systems differ in emissions of these gases is not well understood. Our objectives were to determine if fluxes of N2O and CH4 varied among cropping systems and among crop phases within a cropping system. We compare N2O and CH4 fluxes over the 2010 and 2011 growing seasons from the six cropping systems at the Wisconsin Integrated Cropping Systems Trial (WICST), a 20-yr-old cropping systems experiment. The study is composed of three grain and three forage cropping systems spanning a spectrum of crop diversity and perenniality that model a wide range of realistic cropping systems that differ in management, crop rotation, and fertilizer regimes. Among the grain systems, cumulative growing season N2O emissions were greater for continuous corn (Zea mays L.) (3.7 kg N2O-N ha(-1)) than corn-soybean [Glycine max (L.) Merr.] (2.0 kg N2O-N ha(-1)) or organic corn-soybean-wheat (Triticum aestivum L.) (1.7 kg N2O-N ha(-1)). Among the forage systems, cumulative growing-season N2O emissions were greater for organic corn-alfalfa (Medicago sativa L.)-alfalfa (2.9 kg N2O-N ha(-1)) and conventional corn-alfalfa-alfalfa-alfalfa (2.5 kg N2O-N ha(-1)), and lower for rotational pasture (1.9 kg N2O-N ha(-1)). Application of mineral or organic N fertilizer was associated with elevated N2O emissions. Yield-scaled emissions (kg N2O-N Mg-1) did not differ by cropping system. Methane fluxes were highly variable and no effect of cropping system was observed. These results suggest that extended and diversified cropping systems could reduce area-scaled N2O emissions from agriculture, but none of the systems studied significantly reduced yield-scaled N2O emissions.