Converting conventional agriculture to poplar bioenergy crops: soil greenhouse gas flux

Conversion of agricultural fields to bioenergy crops can affect greenhouse gases (GHG) such as carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). Soil GHG emissions were measured seasonally in poplar bioenergy and agricultural fields at three Northwestern US locations. A forest stand was also used at one location for comparison. A portable gas analyzer was used to measure CO₂ efflux and CH₄ and N₂O fluxes were first measured with chambers and later with gradients. Agricultural soil had 17% larger CO₂ efflux rates than poplar soil. Chamber fluxes showed no differences in CH₄ uptake but did show higher N₂O fluxes in poplar than agricultural soil. Gradient CH₄ uptake rates were highest in agricultural soil in the summer but showed no N₂O flux differences. Forest soils had smaller quarterly CO₂ efflux rates than agricultural soils and greater CH₄ uptake rates than poplar soils. The largest GHG contributor to soil GHG flux was CO₂, with those being ∼1000 times larger than CH₄ flux rates and ∼500 times larger than N₂O flux rates based on CO₂ equivalences. Converting conventional agricultural cropland to poplar bioenergy production does not have adverse effects on soil greenhouse gas flux and these results could be useful for modeling or life cycle analysis of land use conversion.