Effects of Switchgrass Cultivars and Intraspecific Differences in Root Structure on Soil Carbon Inputs and Accumulation

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Switchgrass (Panicum virgatum L.), a cellulosic biofuel feedstock, may promote soil C accumulation compared to annual cropping systems by increasing the amount and retention of root-derived soil C inputs. However, these inputs and stabilization thereof may differ by cultivar, and it is uncertain which root traits favor soil C input and stabilization rates. The aim of this study was to assess how different switchgrass cultivars impact soil C inputs and retention, whether these impacts vary with depth, and whether specific root length (SRL) explains these impacts. We collected soil to a depth of 30 cm (10 cm increments) from six switchgrass cultivars with root systems ranging from high to low SRL. The cultivars (C4 species) were grown for 27 months on soils previously dominated by C3 plants, allowing us to quantify both total C and switchgrass-derived C accumulation in the bulk soil and in coarse particulate organic matter (CPOM), fine particulate organic matter (FPOM), silt-sized, and clay-sized fractions. The study led to two main results: (1) bulk soil C concentrations beneath switchgrass cultivars varied by 40% in the 0–10 cm soil depth and by 70% in the 10–20 cm soil depth, and cultivars with high bulk soil C concentrations tended to have relatively high C concentrations in the mineral soil fractions and relatively low C concentrations in the POM fractions; (2) there were significant differences in switchgrass-derived soil C between cultivars at the 0–10 cm depth, where soil C inputs ranged from 1.2 to 3.2 mg C g−1 dry soil. In addition, switchgrass-derived C was positively related to SRL when one outlier data point was removed. These results suggest that switchgrass cultivars differentially impact mechanisms contributing to soil C accumulation.