A comparison of cellulosic fuel yields and separated soil-surface CO2 fluxes in maize and prairie biofuel cropping systems

We investigated potential use of prairie as a Midwestern cellulosic feedstock by conducting a 9 ha side-by-side comparison of maizeand prairie-based systems on fertile soils in Boone County Iowa, USA. Investigated systems were: maize-soybean rotation grown for grain only, continuous maize grown for grain and stover both with and without a winter rye cover crop, and a 31-species reconstructed prairie grown with and without spring nitrogen fertilization for fall-harvested biomass. We compared amounts of cellulosic biomass produced and harvested, ethanol conversion ratios of biomass as measured by both dietary and detergent methodology, as well as each method’s estimated ethanol yields per unit land area. From 2009-2013, the highest producing system was N-fertilized prairie, averaging 10.4 Mg ha yr above-ground biomass with average harvest removals of 7.8 Mg ha yr. The unfertilized prairie produced 7.4 Mg ha yr, with average harvests of 5.3 Mg ha yr. Lowest cellulosic biomass harvests were realized from continuous maize systems, averaging 3.5 Mg ha yr when grown with, and 3.7 Mg ha yr when grown without a winter rye cover crop, respectively. Un-fertilized prairie biomass and maize stover had equivalent dietary conversion ratios at 330 g ethanol kg dry biomass, but N-fertilized prairie was lower at 315. Detergent method over-prediction depended upon both growing environment and biomass source. Harvested biomass accounted for >90% of ethanol yield variation. Over four years prairie systems averaged 1287 L cellulosic ethanol ha yr more than maize systems, with fertilization increasing prairie ethanol production by 865 L ha yr . On both a biomass-per-unit-land area and ethanol-per-unit biomass basis, the reconstructed prairies met or surpassed cellulosic standards set by maize cellulosic systems, indicating prairies could function as an industrial cellulosic feedstock in the Midwest.