Soil Carbon and Nitrogen Responses to Nitrogen Fertilizer and Harvesting Rates in Switchgrass Cropping Systems

The environmental sustainability of bioenergy cropping systems depends upon multiple factors such as crop selection, agricultural practices, and the management of carbon (C), nitrogen (N), and water resources. Perennial grasses, such as switchgrass (Panicum virgatum L.), show potential as a sustainable bioenergy source due to high yields on marginal lands with low fertilizer inputs and an extensive root system that may increase sequestration of C and N in subsurface soil horizons. We quantified the C and N stocks in roots, free particulate, and mineral-associated soil organic matter pools in a 4-year-old switchgrass system following conversion from row crop agriculture at the W.K. Kellogg Biological Station in southwest Michigan. Crops were fertilized with nitrogen at either 0, 84, or 196 kg N ha(-1) and harvested either once or twice annually. Twice-annual harvesting caused a reduction of C and N stocks in the relatively labile roots and free-particulate organic matter pools. Nitrogen fertilizer significantly reduced total soil organic C and N stocks, particularly in the stable, mineral-associated C and N pools at depths greater than 15 cm. The largest total belowground C stocks in biomass and soil occurred in unfertilized plots with annual harvesting. These findings suggest that fertilization in switchgrass agriculture moderates the sequestration potential of the soil C pool.