The quantification and characterization of soil carbon pools in biofuel agriculture.
The shift toward renewable energy resources through agricultural biofuel production provides an opportunity to study and implement sustainable land management practices that improve soil health and a deeper understanding of mechanisms involved in terrestrial carbon cycling. Agricultural land management strategies such as fertilization, tillage, and crop selection affect soil organic carbon (SOC) storage through the quantity and quality of soil organic matter (SOM) fractions. A small perturbation in the SOC pool due to climate or land use change has major implications for food production, greenhouse gas emissions, and ecosystem health and stability. The goal of this research was to assess the impacts of fertilization and harvesting practices on whole soil C and N and develop approaches to quantifying soil health, using spectroscopic and thermal analyses of SOM fractions at a bioenergy field-scale plot at the Kellogg Biological Station in Hickory Corners, MI. This is one of the first comprehensive studies of twice-annual harvesting impacts on SOC and SOM fractions in agricultural bioenergy plots. Twice-annual harvesting resulted in smaller C stocks in root and low-density fraction of the soil organic matter (LF OM). The roots and LF OM are the primary substrate for organisms in the soil food web. Therefore, this dissertation also quantifies roots and LF OM in terms of Gibbs free energy and macronutrient inventories. The plots with the highest rates of nitrogen (N) fertilizer application (196kg N/ha) contained smaller SOC and TN stocks than the unfertilized control plots, particularly in the dense fraction of the soil (>1.8 g/cm3). Carbon and N in the dense fraction is associated with mineral particles and is more stable in soil than C and N of the LF OM. This fertilization rate increased N content in root and LF OM and reduced decomposability indices (lignin/N and C/N) and likely increased their susceptibility to decomposition. Our results indicate that fertilizer and harvesting practices affect SOC storage with a direct impact on short-term C cycling and measurable effects on the Gibbs free energy. The lessons learned in this research suggest that land management practices can be optimized for carbon storage, soil health, and sustainability outcomes.