Changes in Soil Organic Carbon, Aggregate Distribution and Microbial Community Structure in Bioenergy Cropping Systems.

The 2007 Energy Independence Security Act (EISA) mandates the production of 16 billion gallons per year of cellulosic biofuel by 2022. These biofuels will be required to have life cycle assessment (LCA) greenhouse gas (GHG) emissions at least 60% below those of gasoline/diesel. Careful management and selection of bioenergy crops could reduce the GHG emissions from ethanol production by encouraging carbon (C) sequestration in soils. However, removal of crop residue from agricultural soils could result in decreases in soil organic carbon (SOC) and soil quality. The objectives of this study were to evaluate the impact of potential perennial and annual cellulosic ethanol crops on soil physical and biological properties and to measure changes in SOC in these crops over time. Plots containing switchgrass, miscanthus, big bluestem, corn, photoperiod sensitive sorghum, sweet sorghum and grain sorghum were established in Manhattan, KS in 2007. Root stocks and water-stable aggregates (WSA) were measured after the 2013 growing season. Phospholipid and neutral lipid fatty acids (PLFA and NLFA) were also measured to assess microbial community structure between crops. SOC was measured to a 120 cm depth in 2009 and 2014 in order to assess changes in SOC between crops over time. Perennial grasses had significantly higher total root stocks than the annual crops. Differences between perennial and annual root stocks were greatest at 5-60 cm. No significant differences were found in total PLFA between crops, but total PLFA decreased significantly with depth.  The concentration of PLFA and NLFA biomarkers for arbuscular mycorrhiza and fungi in perennial crops were significantly higher than those in annual crops. The fungi:bacteria ratio was significantly higher in perennial crops. SOC and WSA results are pending.