Field Emissions of Greenhouse Gases From Contrasting Biofuel Feedstock Production Systems Under Different N Fertilization Rates.

Management choices (crop type, fertilization rate) could affect agricultural soil emissions of important temperature-forcing greenhouse gases (GHGs) such as carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O).  Soil GHG emissions were measured in situ over the 2010 growing season at a biofuel feedstock study established in 1998 in eastern Nebraska (Mead, NE).  Three replicates of three no-tillage cropping systems (continuous corn, soybeans, switchgrass var. Cave-in-Rock) by three N rates (0, 60, 120 kg N ha^-1 y^-1) were randomized across the study site. Soybeans were a transitional crop during switchgrass cultivar replacement and did not receive N treatments in 2010.  Weekly soil GHG fluxes were measured from April to September 2010 with vented, non-steady-state, closed chambers used by the USDA-ARS’s GRACEnet (Greenhouse gas Reduction through Agricultural Carbon Enhancement network).  Soil gas fluxes were highly variable across the site, and both daily and cumulative GHG rates varied in response to precipitation.  All soils tended to consume CH₄ over the growing season.  Cumulative CO₂ production did not differ between N rates in any cropping system.  Cumulative N₂O production tended to increase with fertilizer rate, with significant increases at 120 kg N ha^-1 y^-1 in the switchgrass and soybean systems.  The absence of a fertilizer effect under continuous corn indicated that plant N uptake at all fertilizer levels left little residual soil N for gaseous N₂O losses.  Before the onset of significant mid-season rainfall, soybeans successfully utilized residual N remaining in the 60 kg N ha^-1 y^-1 treatment plots fertilized during the previous year, but biomass production at that time was not sufficient to immobilize residual soil N from the high N treatment.  Thus, the timing of rainfall, stage of crop growth, and N rate interacted to produce significantly higher N₂O losses at the high N rate in both soybean and switchgrass systems.