Quantifying Carbon Exchange in Midwest U.S. Bioenergy Cropping Systems after Land-use Change: The Effects of Drought and Uncertainty in Model Predictions

When land-use was changed from natural ecosystems to tillage-based agroecosystems, terrestrial carbon storage was substantially reduced. To reverse this effect, agroecosystems today must use management practices that allocate significant amounts of carbon into the soil and protect that carbon from mechanisms that cause loss. Cellulosic bioenergy cropping systems are one type of land management that may contribute to carbon storage goals along with providing other ecosystem services. From 2010 to 2012, the net ecosystem carbon balance (NECB) of three potential Midwest cellulosic bioenergy cropping systems -- continuous maize (Zea maize L.), switchgrass (Panicum virgatum L.) and hybrid poplar (NM6, Populus nigra x P. maximowiczii) -- was quantified using a component-based modeling approach. Over the course of the experiment, the research site experienced a wide range of growing season drought conditions that illuminated significant differences between cropping systems. In addition, at the outset of the research project the hybrid poplar stands began showing signs of a defoliating fungus that eventually led to the removal of the hybrid poplar system from the final analysis because of incompatibilities with model structure. Final results showed reductions of ecosystem carbon in maize and maintenance of ecosystem carbon in switchgrass. Carbon removal during harvest was a significant contributor to the reduction of ecosystem carbon in maize and it was hypothesized that the legacy effects of past management and the high initial carbon content of the soil contributed to shifts in ecosystem carbon. A significant difference between the NECB of the two ecosystems was only found in the year without water stress, and the most severe drought conditions greatly increased the variability in NECB. The lack of differences in drought years were attributed to reduced maize harvest with increasing drought severity and increased model uncertainty during the most significant drought conditions. The fact that neither bioenergy cropping system increased carbon storage indicates these systems are not ideal for meeting goals to increase terrestrial carbon storage in Southern Wisconsin, assuming they are managed according to the methods described here.