Integrating Legumes as Cover Crops and Intercrops into Grain Sorghum Production Systems

Legumes fix N, reduce fertilizer inputs for primary crops, and contribute C to soil organic matter (SOM), which benefits soil productivity and sustainability. Objectives of this study were to (i) maintain grain sorghum [Sorghum bicolor (L.) Moench] yields while decreasing N fertilizer requirements and build SOM using a temperate legume cover crop and tropical legume intercrop; (ii) quantify soil water usage between legume treatments and N fertilizer rates; and (iii) evaluate treatment effects on residue decomposition of each crop. In Overton, TX, crimson clover (Trifolium incarnatum L.) and cowpea [Vigna unguiculata L. (Walp)] were used as green manure and an intercrop, respectively, with grain sorghum. Four N fertilizer rates of 0, 45, 90, and 135 kg ha(-1) were randomly assigned within a four replicate split-split plot design on a Lilbert loamy fine sand. From 2010 through 2012, intercropped cowpea decreased sorghum grain yield between 77 and 93% compared to monocropped sorghum. Clover green manure increased sorghum grain yield 21% over winter fallow in 2012, but not significantly (P = 0.22). After 3 yr, soil organic C (12.4 g kg(-1)) and soil N (1.1 g kg(-1)) were 18% (P = 0.01) and 21% (P = 0.01) higher, respectively, for clover green manure compared to fallow at 0 to 15-cm depth. Crop residue decomposition was relatively stable across years and only intercropping showed small increases in decomposition of grain sorghum residue. Overall, clover green manure proved to be a more sustainable choice than intercropping cowpea from both a yield and soil perspective.