Comparative Analysis of Soil Properties and Greenhouse Gas Flux Responses to Nitrogen Fertilization in Bioenergy Production Systems.

The US energy security agenda is promoting deployment of new bioenergy cropping systems on marginal lands, but the environmental impacts of this strategy are poorly understood. Our objective was to compare greenhouse gas emissions (GHG), and monitor changes in soil C and N pools of switchgrass (Panicum virgatum L.), Miscanthus x giganteus, and sorghum (Sorghum  bicolor L. Moench) grown for biomass to maize (Zea mays L.) and an unmanaged native prairie. Study site is located on low fertility soils at the Throckmorton Purdue Agricultural Center, and crop treatments were blocked into four field replicates with two rates of N fertilization. Treatments were fertilized according to best management practices with switchgrass and Miscanthus N rates of 0 and 100 kg ha^-1 N (Urea + Agrotain®), and sorghum and maize N rates of 0 and 150 kg ha^-1 (UAN). A weekly assessment of GHG in situ fluxes following the USDA GRACEnet methodology commenced in 2013 and 2014 prior to spring field activities through to fall harvest. Soil samples were collected prior to spring field activities and within a week following fertilizer application. Soils were tested for changes in total C, water-extractable C, total N, ammonium, and nitrate. Differences in N₂O emissions were identified between N rates in all systems. Differences between N rates dissipated by Week 8 and 10 following fertilizer application for annual and perennial systems, respectively. Compared to annual crops elevated CO₂ fluxes and water-extractable C were observed in the Miscanthus treatments. Soil nitrate values increased following N fertilization of Miscanthus and switchgrass, whereas soil ammonium concentrations were similar between N rates. In the annual crops both ammonium and nitrate soil values increased following N application. We found large differences in GHG emissions and soil C/N cycling among potential biomass feedstock production systems. This research informs our understanding of the environmental impacts of N fertilization on GHG emissions and belowground N and C transformations for these biomass production systems.