Impacts of N Fertilizer and Harvest Rate on Size and Stability of Switchgrass Soil and C and N Pools.

The environmental sustainability of bioenergy cropping systems depends upon multiple factors such as crop selection, optimization of agricultural practices, and resource conservation (e.g. management of carbon (C), nitrogen (N), and water). Perennial grasses, such as switchgrass (Panicum virgatum L.), show potential as a 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 soil organic C (SOC) and total N (TN) stocks in whole soil, roots, free-particulate, and mineral-associated soil organic matter (SOM) pools in a four year old switchgrass system fertilized at either 0 or 196 kg N ha^-1 and harvested either once or twice annually. Additionally, we use ¹³C Nuclear Magnetic Resonance (NMR) spectroscopy to study the bulk chemistry (carbohydrate, lignin, lipid, and protein) of the roots and free-particulate light fraction (LF) organic matter. NMR coupled with elemental (C and N) data allow us to estimate the relative decomposition and decomposability of the soil organic matter using various indices to infer the influence of crop management on the mechanisms of soil C storage and stabilization in switchgrass agriculture.  Switchgrass treatments showed SOC and TN accrual predominantly at depths greater than 15 cm and in the mineral-associated dense fraction (DF). Twice-annual harvesting caused a reduction of C and N accrual in the relatively labile roots and free-particulate organic matter pools. Nitrogen fertilizer significantly reduced the accrual of total SOC and TN as well as the stable, mineral-associated C and N pools.  Initial results found 0 kg N/ha fertilization rate and twice-annual harvests improved root and LF SOM stability.