Soil C Dynamics for Bioenergy Agriculture Under Different Fertilization and Harvesting Treatments.

The benefits of growing switchgrass (Panicum virgatum, L.), a promising bioenergy crop, are  influenced by agricultural practices to improve aboveground yield and soil quality through soil organic matter accumulation. Therefore, we examined the belowground responses of two nitrogen fertilization rates (0 kg N/ha and 196 kg N/ha) and two harvest frequencies (annual and biannual) after three years of successive practices at the Kellogg Biological Station (KBS) in western Michigan as part of a Long Term Ecological Research (LTER) site.  We studied four soil depth intervals (0-5, 5-25, 15-30, and 30-60cm) in which we measured  soil and root carbon (C) and nitrogen (N) stocks, as well as the relative proportions of  C and N stored as free particulate organic matter (fPOM)versus mineral-associated matter. Soil C stock were 15% larger (P = 0.091) in the annually harvested, unfertilized treatment in comparison to the biannual, fertilized treatment. The total root C stock was greater  (P = 0.49) in the unfertilized plots, and the annual harvest also possessed around 40% more root C than the biannual harvest treatment.  The root and fPOM, examined using combustion elemental analysis and solid state ¹³C Nuclear Magnetic Resonance (NMR) spectroscopy, showed chemical properties consistent with greater organic matter stability (e.g. C/N and lignin/N) in the unfertilized treatments.   This study suggest that the excess use of N fertilizer and harvesting practices in  switchgrass bioenergy agriculture could have a potentially negative influence on belowground biomass,  soil C stocks, and organic matter stability.