Differences in Active Fraction Soil Carbon Under Annual and Perennial Biofuel Crops.

Perennial cellulosic cropping systems have been promoted as sustainable alternatives to grain-based biofuel crops due to their ability to produce large amounts of biomass with reduced inputs. A potential benefit of such crops is carbon sequestration. We investigated the active C fraction in eight different crops on two contrasting soil orders using two different methods: long-term laboratory incubations and particulate organic matter evaluated through physical size fractionation. We also examined perennial fine root productivity determined through in-growth cores. All samples were collected in the fall of 2013 from the Great Lakes Bioenergy Research Center (GLBRC) Biofuel Cropping System Experiment in Arlington WI (Mollisol soils) and Kellogg Biological Station MI (Alfisol soils), established in 2009. The experiment is a randomized complete block design with ten biofuel cropping systems (consisting of annual row crops, monoculture perennials, and perennial mixed grass systems) and has five replications. Overall, C mineralization rates were significantly different across cropping systems at both sites. At Arlington, short-term carbon mineralization rates were significantly greater (P< 0.0001) in all perennial systems compared to the annual row-crop systems, with the exception of poplars. Differences in short-term C mineralization rates between annual and perennial systems at KBS were not as apparent, with only the old field and native prairie systems having significantly greater rates (P< 0.05). Soil order had no overall effect on short-term C mineralization rates (P=0.13), but mineralization differed significantly by soil order in the switchgrass and prairie systems (P<0.0001). Perennial fine root biomass differed across soil order (P<0.03), with greater biomass found in Alfisols compared to Mollisols. Differences in root biomass were evident across cropping systems at both sites (P<0.02), with diverse perennials generally having greater root biomass compared to monoculture perennials. Perennial fine root biomass trends did not follow the same patterns as C mineralization rates.