Land Use Effects on Net Greenhouse Gas Fluxes in the US Great Plains: Historical Trends and Model Projections

We present maps showing regional patterns of land use change and soil C levels in the US Great Plains during the 20^th century and time series of net greenhouse gas fluxes associated with different land uses. Net greenhouse gas fluxes were calculated by accounting for soil CO₂ fluxes, the CO₂ equivalents of N₂O emissions and CH₄ uptake, and the CO₂ costs of N fertilizer production. Both historical and modern agriculture in this region have been net sources of greenhouse gases. The primary reason for this, prior to 1950, is that agriculture mined soil C and resulted in net CO₂ emissions. When chemical N fertilizer became widely used in the 1950's agricultural soils began to sequester CO₂-C but these soils were still net greenhouse gas sources if the effects of increased N₂O emissions and decreased CH₄ uptake are included. The sensitivity of net greenhouse gas fluxes to conventional and alternative land uses was explored using the DAYCENT ecosystem model. Model projections suggest that conversion to no-till, reduction of the fallow period, and use of nitrification inhibitors can significantly decrease net greenhouse gas emissions in dryland and irrigated systems, while maintaining or increasing crop yields.