Root biomass and soil carbon response to growing perennial grasses for bioenergy

Background: Dedicated bioenergy crops such as switchgrass (Panicum virgatum L.), miscanthus [Miscanthus x giganteus (Mxg)], indiangrass [Sorghastrum nutans (L.) Nash], and big bluestem (Andropogon gerardii Vitman) can provide cellulosic feedstock for biofuel production while maintaining or improving soil and environmental quality. To better understand bioenergy crop effects on soils, we studied changes in soil properties of a Tomek silt loam under inorganic fertilization of switchgrass after 4 years and warm-season grass monocultures and mixtures after 6 years in eastern Nebraska. Methods: The first experiment had two study factors: two switchgrass harvest dates (August and November) and nitrogen (N), phosphorus (P), and potassium (K) fertilization rates. Nitrogen fertilizer levels (0, 60, and 120 kg N ha(-1)) were the main plots, while P levels (0, 22, and 44 kg P ha(-1)) were the split plots and K levels (0, 11, and 22 kg K ha(-1)) were the split-split plots. The second experiment included six bioenergy feedstocks comprised of four monocultures [switchgrass (cv. Shawnee and an experimental strain tracked as Kanlow N1), indiangrass (Chief), and miscanthus (Mxg)] and two mixtures [big bluestem (Goldmine) + indiangrass (Warrior) + switchgrass (Shawnee) and big bluestem (Bonanza) + indiangrass (Scout) + switchgrass (Shawnee)]. Soil samples were analyzed for root biomass, soil organic C (SOC), total N, bulk density, aggregate stability, and pH. Results: In the first experiment, inorganic fertilization and harvest dates had no effect on switchgrass root biomass, SOC pools, soil aggregate stability, and other properties. In the second experiment, cumulative root biomass under Chief indiangrass monoculture was lower than that under other grass monocultures and mixtures except miscanthus. These results suggest that inorganic fertilization and harvest dates do not affect soil properties in the short term, but Chief indiangrass monoculture may have lower root biomass than other grasses. Conclusions: Overall, fertilization management did not induce changes in root biomass and soil properties, but Chief indiangrass monoculture had lower cumulative root biomass compared with mixtures and switchgrass monocultures, suggesting that cultivar selection will affect root biomass accumulation. Further monitoring is needed to determine long-term changes in root biomass and soil properties under these bioenergy crop systems.