GRAIN LEGUME EFFECTS ON SOIL-NITROGEN, GRAIN-YIELD, AND NITROGEN NUTRITION OF WHEAT

Cereal crop yields frequently are greater when grown after soybean [Gl ycine max (L,) Merr.] than after continuous cereal cropping. Informati on on other grain legumes is limited in eastern areas of the northern Great Plains. The objectives of this field study were to determine (i) soil nitrate-N status in the spring following grain legumes, and (ii) grain legume effects on grain yield, grain yield components, and N nu trition of the subsequent hard red spring wheat (Triticum aestivum L. emend. Thell.) crop fertilized with 0, 75, and 150 kg N ha(-1). Two-ye ar crop sequences were evaluated on a Fargo silty clay (fine, montmori llonitic, frigid Vertic Haplaquoll) soil at Fargo, ND, and on a Perell a-Bearden silty clay loam (fine-silty, mixed, frigid Typic Haplaquoll, fine-silty, frigid Aeric Calciaquoll) soil at Prosper. Six grain legu me species were harvested for grain, and aboveground residues were rem oved in 1984 and were uniformly spread and incorporated into the soil in 1985. Spring soil nitrate-N level following legumes was 28% greater than that following N-fertilized wheat across three environments but 43% lower than that following fellow. Unfertilized wheat grain yields following grain legumes were equivalent to or greater than that follow ing a wheat crop fertilized with 75 kg N ha(-1) and similar to fallow at the same fertility level. Total N accumulation by wheat following g rain legumes was 9% greater than that following wheat but 13% lower th an that following fellow. Nitrogen-use efficiency for wheat following legumes, however, was up to 32% greater than that for wheat following fallow and up to 21% greater than that for continuous wheat. These stu dies indicate that grain legumes should be considered in cropping syst ems in higher moisture areas of the northern Great Plains to help main tain subsequent crop productivity.