Legume Contributions to Soil and Yield of High-Biomass Sorghum Cropping Systems.

Conversion of cellulosic biomass feedstocks to ethanol offers a sustainable alternative to petroleum based fuels. By adding legumes to biomass cropping systems we hope to reduce costly N fertilizer inputs, increase soil C, and reduce environmental concerns with leaching and runoff. This split-split plot study used high biomass sorghum (Sorghum bicolor L.) (SOR) ‘ES 5200’ with ‘Dixie’ crimson clover (Trifolium incarnatum L.) (CLO) cover crop, ‘Iron and Clay’ cowpea (Vigna unguiculata L. [Walp]) (COP) intercrop/rotational crop, and three N fertilizer rates (0, 45, 90 kg ha^-1). Research was conducted at the Texas AgriLife Research Center in Overton, TX on a Lilbert loamy fine sand (Loamy, siliceous, semiactive, thermic Arenic Plinthic Paleudult). During the first year, CLO contributed 108 kg ha^-1 of N as a green manure crop, but did not affect SOR biomass or soil C and N levels. The addition of 45 kg N ha^-1 increased (p=0.04) SOR biomass by 18% over 0 kg N ha^-1, however 90 kg N ha^-1 (8.7 Mt ha^-1) did not further increase yield. SOR only plots (12.1 Mt ha^-1) produced higher (p<0.001) total biomass than the SOR/COP intercrop (8.2 Mt ha^-1). Sole-cropped COP plots yielded 6.0 Mt ha^-1 of dry matter and contributed 133 kg ha^-1 N to the soil. Dry conditions in 2010 led to drought stress of SOR and moisture competition with the intercropped COP and SOR, and contributed to the decrease in total yield and SOR height by 62% and 18%, respectively. COP proved to be well-adapted to the dry conditions, but too competitive as an N source for intercropping with high biomass SOR.