Quantifying Biochemical and Energetic Reservoirs in Soil Organic Matter Under Fertilization and Harvest Treatments.

The environmental sustainability of agricultural systems depend upon multiple factors such as crop selection, optimization of agricultural practices, and resource conservation (e.g. management of carbon (C), nitrogen (N), and water). We quantified biochemical (lipids (g), carbohydrates (g), etc.) and energetic (Gibb’s Free Energy (joules)) inventories of roots and free-particulate light fraction organic matter (LF OM) in a four year old switchgrass system fertilized at either 0 or 196 kg N ha^-1 and harvested either once or twice-annually. These values were calculated from ¹³C Nuclear Magnetic Resonance (NMR) spectroscopy and a Molecular Mixing Model (MMM) alongside decomposition state variables (lignin:N, Alkyl C:O-Alkyl C) to quantify the influence of management practices on nutritional and energetic inputs into the soil system and assess the impacts on soil health. We found annually harvest plots contained larger energetic and biochemical stocks, distributed deeper within the profile, and less altered by decomposition than twice-annual harvested plots. The fertilization treatment increased N content and digestibility indices in the labile root and LF OM. Future research aims to quantify root and LF OM energetic yields using thermal combustion analysis to support our biochemical analysis as a tool for evaluating agricultural management impacts on the energetic yields of SOM pools for implications in nutrient cycling, GHG mitigation, and soil health assessments.