Potential Greenhouse Gas Emissions From Two Switchgrass Cultivars Under Different N Fertilization Rates.

Increased production and use of biofuels derived from perennial feedstocks such as switchgrass is expected to reduce the emission of fossil-based carbon dioxide (CO₂) into the atmosphere.  Management choices (e.g. cultivar, fertilization), however, could affect the emission of other important temperature-forcing greenhouse gases (GHGs) such as methane (CH₄) and nitrous oxide (N₂O).  Potential emissions of CO₂, CH₄, and N₂O and net N mineralization were measured from incubations of surface soils (0-5 cm, 5-10 cm) collected at the end of the growing season from a field experiment using two different cultivars of switchgrass (Cave-in-Rock, Trailblazer) under three N fertilization rates (0, 60, 120 kg N ha^-1).  Triplicate soil samples (50 g) were incubated under controlled laboratory conditions in the dark (25°C, 55% water-filled pore space) over a 28-day incubation period, and headspace gas samples were collected weekly for GHG analysis.  Potential N mineralization rates did not differ between switchgrass cultivars, but was significantly greater in soils fertilized at 120 kg N ha^-1 compared to lower N rates (P < 0.0001).  No treatment effects were significant for cumulative CH₄ production.  Cumulative CO₂ and N₂O production were greater in 0-5 cm soils compared to 5-10 cm (P < 0.0001).  Cumulative CO₂ production was highest in Cave-in-Rock soils fertilized at 120 kg N ha^-1 compared to the lower N rates (P < 0.01), but no N rate effect on C mineralization was apparent for Trailblazer soils.  Cumulative N₂O production, however, was highest in soils from both cultivars treated at the highest N rate.  Cumulative N₂O production in Trailblazer soils tended to be higher than Cave-in-Rock soils (P = 0.0910), but appeared to be related to greater residual soil N associated with decreased growth and lower overall biomass production by the Trailblazer cultivar.  Thus, optimizing N fertilization rate with the biomass production potential for individual switchgrass cultivars could minimize potential GHG emissions from these perennial biofuel feedstock systems.