Greenhouse gas contributions and mitigation potential of agricultural practices in northwestern USA and western Canada

Concern over human impact on the global environment has generated increased interest in quantifying agricultural contributions to greenhouse gas fluxes. As part of a research effort called GRACEnet (Greenhouse Gas Reduction through Agricultural Carbon Enhancement Network), this paper summarizes available information concerning management effects on soil organic carbon (SOC) and carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4) fluxes in cropland and rangeland in northwestern USA and western Canada, a region characterized by its inherently productive soils and highly variable climate. Continuous cropping under no-tillage in the region increased SOC by 0.27 ± 0.19 Mg C ha−1 yr−1, which is similar to the Intergovernmental Panel on Climate Change (IPCC) estimate for net annual change in C stocks from improved cropland management. Soil organic C sequestration potential for rangelands was highly variable due to the diversity of plant communities, soils, and landscapes, underscoring the need for additional long-term C cycling research on rangeland. Despite high variability, grazing increased SOC by 0.16 ± 0.12 Mg C ha−1 yr−1 and converting cropland or reclaimed mineland to grass increased SOC by 0.94 ± 0.86 Mg C ha−1 yr−1. Although there was generally poor geographical coverage throughout the region with respect to estimates of N2O and CH4 flux, emission of N2O was greatest in irrigated cropland, followed by non-irrigated cropland, and rangeland. Rangeland and non-irrigated cropland appeared to be a sink for atmospheric CH4, but the size of this sink was difficult to determine given the few studies conducted. Researchers in the region are challenged to fill the large voids of knowledge regarding CO2, N2O, and CH4 flux from cropland and rangeland in the northwestern USA and western Canada, as well as integrate such data to determine the net effect of agricultural management on radiative forcing of the atmosphere.