Forage Legume Effects on Soil Nitrogen and Grain Yield, and Nitrogen Nutrition of Wheat

Information on the effect of hayed legumes on soil fertility and subsequent crop productivity is needed to evaluate the use of legumes in place of summerfallow in the northern Great Plains. The objectives of these studies were to determine (i) the seeding-year hay yield, and root and crown N content of five forage legumes, (ii) the spring soil NO⁻₃-N status following the forage legumes, and (iii) the forage legume effects on grain yield, grain yield components, and N nutrition of the subsequent hard red spring wheat (Triticum aestivum L.) crop at 0, 75, or 150 kg of fertilizer N ha⁻¹. Field experiments were conducted on a Fargo silty clay (fine, montmorillonitic, frigid Vertic Haplaquolls) soil at Fargo, ND and on a Perella-Bearden silty clay loam (fine-silty, mixed, frigid Typic Haplaquolls, fine-silty, frigid Aerie Calciaquolls) soil at Prosper, ND from 1984 to 1986. The legume species were not significantly different in hay yields, and root and crown N content across environments, although alfalfa (Medicago sativa L.) and sweetclover [Melilotus officinalis (L.) Lam.] had 35 to 83% greater N contents than other species. Fall regrowth, crowns, and roots were incorporated into the soil in late fall. Soil NO⁻₃-N in the spring following legumes was greater than following fertilized wheat, but less than following fallow across environments. Grain yield and N uptake of wheat following forage legumes generally were equal to those following fallow, but greater than those following wheat. Nitrogen use efficiency of unfertilized wheat following forage legumes was greater than following fallow or wheat in 1986, but equal in 1985. Grain yield to total N uptake ratio and N partitioning to grain following legumes were greater than following fallow or wheat in two of four environments. These results suggest that including 1-yr forage legumes in crop sequence would be a better option than fallow in the higher moisture areas of the northern Great Plains. Research was supported in part by the North Dakota Agric. Exp. Stn.