Harvest strategy and N fertilizer effects on bioenergy sorghum production

Bioenergy sorghum (Sorghum bicolor (L.) Moench) has the potential to be a very important cellulosic feedstock if it can be produced without degrading soil quality. Two important factors for achieving that goal are N management and the amount of residue (i.e. carbon) returned to the soil. This study evaluated two N rates (0 or 280 kg ha(-1) yr(-1)) and three levels of residue return (0, 25%, or 50%) on Weswood silty clay loam near College Station, TX USA. Biomass sorghum was grown continuously from 2009 through 2014. Maximum dry biomass yield (23 Mg ha(-1)) was produced with added N and 25% residue return in a year with above average precipitation. Overall, N fertilization increased biomass yield by 43 to 104%, while residue return enhanced yield from < 1 to 23% during the six-year study. Averaged for the six years, biomass production for the 0, 25%, and 50% residue return treatments was 16, 20, and 18 Mg ha(-1), respectively. Returning 25% of the crop residue significantly increased K uptake in both the 1st and 6th years. Sorghum fertilizer N uptake efficiency (FNUE) with residue return by 2014 was significantly increased compared to 2009 values. Non-limiting N fertilization and 25% residue return significantly increased NO3-N, P, K, and soil organic C (SOC) concentrations in surface (0 to 5 cm) samples and soil total N (TN) and K concentrations within the 60 to 90 cm layer. This study confirms that N fertilization will be required to achieve high biomass sorghum yield and suggests that developing a harvest strategy to return 25% of the crop residue will be sufficient to maintain soil quality.