Biomass has low energy density relative to fossil fuel; consequently, transitioning to bioenergy will incur large changes in land use. Agricultural practices have the potential cause extensive changes in the net exchange of greenhouse gases between the land surface and the atmosphere, and to further alter climate-regulating services by changing albedo and latent heat flux. We have employed three approaches to address how land use change associated with expanding production of bioenergy crops affects of the climate regulating services of ecosystems: 1) experimental manipulation of agro-ecosystems; 2) process-based modeling of land use change; 3) theoretical considerations of the climate regulating value of ecosystems.
Results/Conclusions
Comparative measurements revealed that replacing annual row crops with perennial bioenergy feedstocks in the Midwest US increases cumulative soil respiration, but this increase is derived primarily for autotrophic respiration associated with the extensive root systems of perennial grasses. Corresponding increases in belowground carbon allocation by perennial grasses, suggest that bioenergy crops will have a climate benefit beyond the displacement of fossil fuels. The application of a process-based model indicates that replacing corn acres for ethanol production with high-yielding perennial grasses will transition the rain fed Midwest from a source to a sink for greenhouse gases. This transition is not limited to replacing annual with perennial crops, as an analysis in the southeastern US indicates that displacing pasture with energy cane has similar greenhouse gas benefits. To determine global consequences of land use change caused by the expansion of bioenergy crops, a metric that quantifies the climate regulating value (CRV) of ecosystems was developed. CRV is strongly related to total carbon stocks, therefore displacing high carbon systems with low carbon systems typically degrades ecosystem CRV. Judicious use of bioenergy crops can potentially increase the CRV of land surfaces, but raises the specter of competition with food supplies. Resolving the appropriate allocation of land to food/fiber versus fuel remains a critical concern.