Grazing Management Contributions to Net Global Warming Potential: A Long-Term Evaluation in the Northern Great Plains.

The role of grassland ecosystems as net sinks or sources of greenhouse gases (GHG) is limited by a paucity of information regarding management impacts on the flux of nitrous oxide (N₂O) and methane (CH₄).  Furthermore, no long-term evaluation of net global warming potential (GWP) for grassland ecosystems in the northern Great Plains (NGP) of North America has been reported.  Given this need, we sought to determine net GWP for three grazing management systems located within the NGP.  Grazing management systems included two native vegetation pastures [moderately grazed pasture (MGP), heavily grazed pasture (HGP)] and a heavily grazed crested wheatgrass [Agropyron desertorum (Fisch. ex. Link) Schult.] pasture (CWP) near Mandan, ND.  Factors evaluated for their contribution to GWP included, 1) CO₂ emissions associated with N fertilizer production and application, 2) literature-derived estimates of CH₄ production for enteric fermentation, 3) change in soil organic carbon (SOC) over 44 yr using archived soil samples, and 4) soil-atmosphere N₂O and CH₄ fluxes over three yr using static chamber methodology.  Analysis of SOC indicated all pastures to be significant sinks for SOC, with sequestration rates ranging from 0.39 to 0.46 Mg C ha^-1 yr^-1.  All pastures were minor sinks for CH₄ (<2.0 kg CH₄-C ha^-1 yr^-1).  Greater N inputs within CWP contributed to annual N₂O emission nearly three-fold greater than HGP and MGP.  Due to differences in stocking rate, CH₄ production from enteric fermentation was nearly three-fold less in MGP than CWP and HGP.  When factors contributing to net GWP were summed, HGP and MGP were found to serve as net CO_2equiv. sinks, while CWP was a net CO_2equiv. source.  Values for GWP and greenhouse gas intensity (GHGI), however, indicated net reductions in GHG emissions can be most effectively achieved through moderate stocking rates on native vegetation in the NGP.