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

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 (C ₄ species) were grown for 27 months on soils previously dominated by C ₃ 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.