US energy policy mandates the production of 36 billion gallons of ethanol annually by the year 2022—a policy that will have significant impacts on regional biogeochemistry and greenhouse gas exchange. Perennial grasses hold promise as second-generation biofuel crops that could provide ligno-cellulosic material for ethanol while restoring ecosystem services in intensively managed agro-ecosystems. Here, we address the question of whether perennial grass biofuel crops could help to mitigate climate change through carbon sequestration at an ecosystem level.
At the ‘Energy Farm’ in Urbana, Illinois, we quantified numerous components of the carbon cycle in three potential perennial grass bioenergy crops—switchgrass, miscanthus, and a native prairie mix—and a row crop control (corn-corn-soy rotation). Specifically, over the first three years of crop establishment, we measured above- and below-ground production, soil respiration (partitioned into bulk soil and root-associated respiration), and net ecosystem exchange of carbon (using the eddy-covariance technique).
Results/Conclusions
Belowground carbon cycling was significantly enhanced in perennial grass bioenergy crops relative to the row crop control. The perennial grasses allocated more carbon belowground, resulting in higher root biomass and root-associated respiration. CO2 efflux through soil respiration was also greater in the perennial grass agro-ecosystems; three years of continuous soil respiration measurements reveal that the perennial grass crops had approximately 40% higher soil respiration than the row-crop control. Partitioning this into bulk-soil and root-associated respiration reveals that the difference is driven primarily by the higher root-associated respiration of the perennial grasses. On balance, the increase in belowground carbon inputs exceeds the increase in carbon loss such that the perennial grass crops are acting as carbon sinks. This implies that replacing corn with perennial grasses as an ethanol source would have climate benefits through belowground carbon sequestration.