Clover and Cowpea Impacts On Biomass Yield, SOC, and Soil Total N in a High-Biomass Sorghum Production System.

Before the advent of cheap, synthetic fertilizers, legumes were commonly used as green manure crops for their ability to fix N. As fertilizer prices increase, legumes are becoming more economically viable and can enhance cropping system sustainability. In this study, crimson clover and cowpea were integrated into a high-biomass sorghum production system to evaluate impacts on N content, C content, and yield of sorghum and their impacts on total soil N and soil organic carbon (SOC). Cool-season crop treatments included crimson clover green manure (CLGM) and winter fallow (FALL) followed by three warm-season crop rotations (cowpea green manure: CPGM; cowpea-sorghum intercrop: CPSR; sorghum monocrop: SORG) and three N fertilizer treatments (0, 45, 90 kg N ha^-1). The CLGM increased sorghum biomass yield (16.5 Mt DM ha^-1) 28% in 2012, but had no effect in the first two years. Sorghum yield was reduced between 39% and 62% with CPSR compared to SORG, whereas CPGM increased monocropped sorghum yield 56% and 18% each year following the incorporation of the cowpea green manure. Nitrogen fertilizer had no effect on sorghum biomass yield. Soil N and SOC were largely unaffected by treatments, but showed a significant drop in the first year, likely due to tillage. Decrease in soil N over time indicated mineralization of organic N and may explain why no N fertilizer response was observed in sorghum biomass yield. Cowpea showed potential as a green manure crop, but proved to be too competitive for successful intercropping in high-biomass sorghum production systems.