Title: Grazing Management Contributions to Net Global Warming Potential: A Long-Term Evaluation in the Northern Great Plains Authors
Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 16, 2009
Publication Date: May 3, 2010
Repository URL: http://hdl.handle.net/10113/42860
Citation: Liebig, M.A., Gross, J.R., Kronberg, S.L., Phillips, B.L., Hanson, J.D. 2010. Grazing Management Contributions to Net Global Warming Potential: A Long-Term Evaluation in the Northern Great Plains. J. Environ. Qual. 39(3):799-809. Interpretive Summary: Contributions of grassland ecosystems to net global warming potential (GWP) are largely unknown. Furthermore, no long-term evaluation of net GWP for grassland ecosystems in the northern Great Plains (NGP) of North America have been reported. Given this need, we sought to determine net GWP for three long-term grazing management systems (two native vegetation pastures and one seeded forage pasture) located in the NGP, a region possessing a semi-arid continental climate and abundant grassland resources. Five factors were assessed in each pasture for their contribution to net GWP: 1) CO2 emissions from N fertilizer production and application, 2) CH4 emissions from cattle, 3) change in soil carbon, 4) soil-atmosphere CH4 flux, and 5) soil-atmosphere N2O flux. Overall, the contribution of factors on net GWP decreased in relative impact in the order of, 1) change in soil carbon, 2) soil-atmosphere N2O flux, 3) CH4 emission from cattle, 4) CO2 emission associated with N fertilizer production, and 5) soil-atmosphere CH4 flux. Summing across factors, net GWP was negative for both native vegetation pastures, implying net CO2 uptake. This finding underscores the value of grazed, mixed-grass prairie as a viable agroecosystem to serve as a net CO2 sink in the NGP. Conversely, net GWP for the seeded forage was positive, implying net CO2 emission to the atmosphere. When GWP data were expressed per unit of animal production, the native vegetation pasture with a lower stocking rate was found to be most effective at achieving net reductions in greenhouse gas emissions among the three pastures.
Technical Abstract: 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 methane (CH4) and nitrous oxide (N2O). Furthermore, no long-term evaluation of net global warming potential (GWP) for grassland ecosystems in the northern Great Plains (NGP) 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 CH4 and N2O flux over three yr using static chamber methodology. Analysis of SOC indicated all pastures to be sinks for SOC, with sequestration rates ranging from 0.39 to 0.46 Mg C/ha/yr. All pastures were minor sinks for CH4 (<2.0 kg CH4-C/ha/yr). 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 CO2 sinks, while CWP was a net CO2 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.