Environmentally Sustainable Bioenergy Biomass Production: Interactive Effects of Growth Stimulating Hormones and Nitrogen Fertilization.

Cost of commercial fertilizers (particularly nitrogen) accounts for a substantial portion of the total production cost of cellulosic biomass, and can be a major obstacle to biofuel production. In a greenhouse study, we evaluated the feasibility of co-applying Gibberellins (GA) and reduced nitrogen (N) rates to produce a bioenergy crop less expensively. In a preliminary study, we determined the minimum combined application rates of GA and N required for efficient biomass (Sweet sorghum, Sorghum bicolor) production. Co-application of 75 kg ha^-1 (one-half of the recommended N rate for sorghum) and a modest GA rate of 3 g ha^-1 optimized dry matter yield (DMY), and N and P uptake efficiencies, resulting in a reduction of N and P leaching. Organic nutrient sources such as manures and biosolids can be substituted for commercial N fertilizers (and incidentally supply P) to further reduce the cost of nutrient supply for biomass production. Based on the results of the preliminary study, we conducted a second greenhouse study using Sweet sorghum as a test bioenergy crop. We co-applied organic sources of N (manure and biosolids) at 75 and 150 kg N ha^-1 (representing 50 and 100% N rate respectively) with 3 g GA ha^-1. The crop was grown for 8 wk on Immokalee fine sand with minimal native fertility. After harvest, sufficient water was applied to soil in each pot to yield ~1.5 L (~0.75 pore volume) of leachate, and analyzed for total N and soluble reactive P. Reduced (50%) N application rate together with GA optimized biomass production. Application of GA at 3 g ha^-1, and the organic sources of N at 50% of the recommended N rate, decreased nutrient cost of producing the bioenergy biomass by ~$375 ha^-1 (>90% of total nutrient cost), and could reduce offsite N and P losses from vulnerable soils.