In the Midwestern Corn Belt, bioenergy crop production systems are anticipated to play a significant role in reducing U.S. net carbon emissions. Bioenergy systems not only have the potential to reduce net carbon emissions by providing a non-fossil renewable fuel source, but also through removal of atmospheric CO2 via enhanced carbon sequestration in soils. First generation biofuels such as corn grain ethanol may only offer marginal net carbon reductions over conventional fuels, although improved management practices such conservation tillage and cover cropping often improve carbon storage potential. Second generation cellulosic biofuel feedstocks such as perennial grasses are expected to offer greater net carbon reductions due to reduced external inputs and improved soil carbon sequestration. As such, the 2007 U.S. Energy Independence and Security Act created the Renewable Fuel Standard which mandates the production of 36 billion gallons of biofuel by 2022, of which 16 billion gallons must be derived from cellulosic feedstocks.

Although future increases in biofuel production in the U.S. are written in law, many of the ecological ramifications of such a large shift in land use remain unknown. Fortunately, in addition to creating the Renewable Fuel Standard, the 2007 U.S. Energy Independence and Security Act created funding for biofuel research and development in the form of three bioenergy research centers. While the majority of the centers’ research focuses on technical problems related to plant feedstock deconstruction and conversion to liquid fuel, the Great Lakes Bioenergy Energy Research Center (GLBRC) is unique in that it also focuses on the sustainability of bioenergy systems. Life-cycle analyses are a central component of bioenergy sustainability research, as they allow potential bioenergy systems to be assessed on a level playing field in terms of their comprehensive environmental and economic impacts.

Our research aims to improve our understanding of carbon cycling within model bioenergy cropping systems by providing direct estimates of annual carbon balances, elucidating the mechanism of SOC accrual and stability, investigating plant and management-induced microclimate feedbacks, and determining the drivers of belowground plant metabolic carbon loss.

We are using two methods to evaluate the annual net ecosystem carbon balance of continuous corn and switchgrass cropping systems, which are two likely candidates for first and second generation biofuels, respectively. The resulting data can be used directly to improve life-cycle analyses, corroborate other carbon balance estimates, and inform bioenergy economics and decision making.

We will determine changes in multiple SOC pools following five years of biofuel crop establishment at two contrasting sites in order to determine where and why SOC is being accumulated or lost at the pedon scale. This information will be valuable for improving the mechanistic functioning of ecosystem SOC models, and the results will also be pertinent to land managers who are interested in attaining short-term SOC accrual.

We investigate a potentially important cropping system feedback whereby differences in plant phenology, structure, and management between corn and switchgrass systems may induce differential soil microclimates, thereby altering SOC decomposition via heterotrophic respiration. Such soil microclimate effects could provide partial explanation of anticipated differences in net ecosystem carbon balance and SOC accrual, and thus soil microclimate effects may be an important consideration when modeling ecosystem plant and soil dynamics.

We explore how the temporal variability of plant dynamics and environmental conditions affect patterns of belowground autotrophic respiration on seasonal and diurnal timescales. An integrated understanding of the controls of in situ autotrophic respiration will improve our understanding of plant carbon partitioning between biomass production and metabolic respiration, will allow for validation of autotrophic respiration in ecosystem models, and can be used to indirectly model heterotrophic respiration fluxes. Altogether, this research will provide applied and basic knowledge that can be directly used to inform ecosystem models, land management practices, economic assessments, and policy decisions related to carbon balance and storage in model bioenergy cropping systems.

Lead scientists: Adam von Haden, Chris Kucharik