Using Legumes to Reduce Nitrogen Fertilization and Enhance Soil Nitrogen and Carbon for Grain Sorghum Cropping Systems.

The addition of legumes to a grain sorghum (Sorghum bicolor L.) cropping system has the potential to reduce chemical nitrogen (N) fertilizer rates through two mechanisms—N fixation and biomass incorporation. Legume biomass incorporation also has the potential to increase soil organic carbon (C). The objectives of this field experiment were to compare impacts of both a cool- and warm-season legume on grain sorghum yield, soil water use by the crops, and soil organic C and N. The experiment was conducted on a Lilbert loamy fine sand (2-5% slope) at the Texas AgriLife Research Center in Overton. A split- split plot design planted 'Dixie' crimson clover (Trifolium incarnatum L.) to half of the plots as a winter cover-green manure crop and included 4 replications of N rates (0, 45, 90, and 135 kg ha^-1) applied to grain sorghum. ‘Pioneer 84G62’ grain sorghum was drilled in 0.76 m rows alone or in combination with 'Iron-and-Clay' cowpeas (Vigna unguiculata L. [Walp]). Combustion analysis and a capacitance probe were used to evaluate C, N, and moisture in the soil. Preliminary results showed no impact of the crimson clover cover crop on soil C, soil N, or grain yield after the first year. The cowpea did not reduce soil moisture or total biomass. Interseeded 'Iron and Clay' cowpea was a highly competitive cultivar that significantly reduced grain sorghum yields of seed (745.8 kg ha^-1, p < 0.0001) and biomass (9.57 Mt ha^-1, p = 0.0002) by 568.8 kg ha^-1 and 6.19 Mt ha^-1, respectively, compared to sole cropped sorghum. N rates had no significant impact on grain or biomass yield of either crop. The aggressive nature of cowpea and its competitive effects on grain sorghum negate any of its benefits for sorghum production and suggest these two crops be planted separately in cropping rotation.