Carbon storage and nitrous oxide emissions of cropping systems in eastern Washington: A simulation study

Conservation tillage is an agricultural strategy to mitigate atmospheric greenhouse gas (GHG) emissions. In eastern Washington, we evaluated the long-term effects of conventional tillage (CT), reduced tillage (RT) and no-tillage (NT) on soil organic carbon (SOC) storage and nitrous oxide (N2O) emissions at three dryland and one irrigated location using the cropping systems simulation model CropSyst. Conversion of CT to NT produced the largest relative increase in SOC storage (ΔSOC, average yearly change relative to CT) in the top 30 cm (11.8 in) of soil where ΔSOC ranged from 0.29 to 0.53 Mg CO2e ha−1 y−1 (CO2e is carbon dioxide [CO2] equivalent of SOC; 0.13 to 0.24 tn CO2e ac−1 yr−1). The ΔSOC were less with lower annual precipitation, greater fallow frequency, and when changing from CT to RT. Overall, ΔSOC decreased from the first to the third decade after conversion from CT to NT or RT. Simulations of ΔSOC for the conversion of CT to NT based on a 0 to 15 cm (0 to 5.9 in) soil depth were greater than the ΔSOC based on a 0 to 30 cm depth, primarily due to differences among tillage regimes in the depth-distribution of carbon (C) inputs and the resultant SOC distribution with depth. Soil erosion rates under CT in the study region are high, posing deleterious effects on soil quality, productivity, and aquatic systems. However, an analysis that includes deposition, burial, and sedimentation on terrestrial and aquatic systems of eroded SOC indicates that the substantial erosion reduction obtained with RT and NT may result only in minor additional SOC oxidation as compared to CT. Simulated N2O emissions, expressed as CO2 equivalent, were not very different under CT, RT, and NT. However, N2O emissions were sufficiently high to offset gains in SOC from the conversion of CT to RT or NT. Thus, reducing tillage intensity can result in net C storage, but mitigation of GHG is limited unless it is coupled with nitrogen (N) fertilizer management to also reduce N2O emission.

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