Switchgrass and Maize Contributions to Deep Soil C After 9 Years: Soil Fractions to Plant Biomarkers.

Switchgrass (Panicum virgatum L.) is a perennial, cellulosic biofuel feedstock capable of growing under a wide variety of climatic conditions on land marginally suited to cultivated crops.  Switchgrass root production is greater than maize (grown using a tillage production system) and contributes to greater SOC sequestration rates under switchgrass. However, researchers observed similar soil C sequestration rates after 9 years in a study evaluating N fertilizer and harvest management treatments on non-irrigated switchgrass (harvested in Aug. and Oct.) and no-till maize (with and without stover removal) (Follett et al. 2012). To assess the form and stability of SOC under switchgrass (Cave-in-Rock) and maize we sampled these plots to 150 cm in 2007 and fractioned the soils into labile (POM) or stable (silt+clay) soil fractions, and assessed ^δ13C abundance. We then assessed plant biomarkers through the profile using cupric oxide extraction.  Through the entire 0-150 cm profile, switchgrass had greater root biomass and a greater root C:N ratio than maize.  Both the POM and silt+clay fractions in the 0-30 cm depth incorporated the higher switchgrass C:N ratio, suggesting that root-derived contribution to SOM under switchgrass may derive directly from lignin rather than root exudates. Nitrogen and harvest treatments altered the ^δ13C of the POM and silt+clay fractions surprisingly deep in the soil profile (to 120 cm). Lignin biomarkers under maize were more decomposed than under switchgrass, reflecting greater root turnover. These results illustrate complex, plant-specific interactions of nitrogen fertilization and harvest timing on root biomass and soil fraction C.