Integrating Crimson Clover (Trifolium incarnatum L.) and Cowpea (Vigna unguiculata L. [Walp]) Into Grain Sorghum (Sorghum bicolor L. [Moench]) Cropping Systems to Improve Crop Yield, Soil Nitrogen, and Soil Carbon.

Legumes fix N and can reduce inorganic N fertilizer inputs for primary crops and also contribute C to soil organic matter, which benefits soil productivity and sustainability. At the Texas AgriLife Research Center in Overton, TX (32^o 17’ N; 94^o 58’ W; Elev: 147 m), ‘Dixie’ crimson clover (Trifolium incarnatum L.) (CLO) and ‘Iron-and-Clay’ cowpea (Vigna unguiculata L. [Walp]) (COW) were cover-cropped and inter-cropped, respectively, with ‘Pioneer 84G62’ grain sorghum (Sorghum bicolor L.) (SOR). Four N fertilizer rates of 0, 45, 90, and 135 kg ha^-1 randomly assigned within four replications on a Lilbert loamy fine sand (Loamy, siliceous, semiactive, thermic Arenic Plinthic Paleudult). This study intends to identify a cropping system that increases soil organic C, decreases inorganic N fertilizer, and maintains SOR grain yields. In 2010, CLO did not affect SOR grain yield (923 kg ha^-1) or total biomass (8.7 Mt DM ha^-1); whereas intercropped COW decreased SOR grain yield 76% and decreased total biomass 17%. An extreme drought in 2011 reduced SOR grain (98%) and total biomass (63%) production, but maintained similar trends as 2010. The CLO and COW contributed 108 and 100 kg N ha^-1, respectively, in 2010 and 57 and 35 kg N ha^-1 in 2011. There were no treatments impacting soil N (1.1 g kg^-1) or C (13.2 g kg^-1) in 2010, but both CLO and COW (p<0.05) increased soil N and C in the top 15 cm of soil by 14% (CLO) and 13% (COW) after two years. Despite substantial N contributions from CLO and COW and increased soil N and C, low rainfall in both years created moisture limitations and increased competition between legumes and SOR. The COW exhibited rapid, vining growth and better drought tolerance than SOR and may be better suited as a rotational crop instead of an intercrop under dryland conditions.