Simulating Impacts Of Nitrogen Fertilization Using Daycent To Optimize Economic Returns And Environmental Services From Bioenergy Sorghum Production

Inappropriate nitrogen (N) fertilization rate could cause yield and economic losses and negative environmental impacts. This study was conducted to explore optimum N rate for a promising biofuel crop - bioenergy sorghum [Sorghum bicolor (L.) Moench]. The biogeochemical model, DAYCENT, was verified with an eight-year field trial and then used to simulate the long-term (35 years) effects of N fertilization on aboveground biomass carbon (C), soil organic C (SOC), carbon dioxide (CO2), and nitrous oxide (N2O) emissions. Associated with the simulated metrics, N use efficiency (NUE), net greenhouse gas (GHG) emissions, and net economic return to N (RTN) were calculated to determine the optimum N rate. The model was capable of reproducing the field measurements with r(2) of 0.57, 0.47, 0.55, and 0.34 for aboveground biomass C, SOC, CO2, and N2O, respectively. Projection with 0-350 kg N ha(-1) fertilization in increments of 70 kg N ha(-1) indicated positive responses of aboveground biomass C and SOC to increasing N but with little increase above 140 kg N ha(-1). Declining NUE and increasing net GHG emission at field scale were predicted as N rate increased. When considering GHG mitigation from fossil fuel replacement, net GHG emission decreased first and leveled off at a N rate of 70-140 kg N ha(-1) before increasing. Net economic RTN increased first and peaked when N rate was around 140 kg N ha(-1) before decreasing. Fertilization at 140 kg N ha(-1) was found to be optimal when using both GHG mitigation and economic criteria.