Dinitrogen Fixation Technologies for Alfalfa Improvement

Abstract

The energy crisis of the 1970s stimulated renewed research on the use of forage legumes as inexpensive sources of N in croping systems. New research programs were begun to improve the N assimilation of forage legumes, and to convert scientific knowledge about legume N2 fixation into technological choices that would support a profitable and sustainable agriculture. We assess the status of selected technologies that have resulted from this research. Four technologies, viz. alfalfa germplasm with enhanced activities of nodule enzymes, alfalfa germplasm unable to fix N2, alfalfa germplasm with improved N accumulation in herbage or roots, and improved rhizobial strains for inoculum have reached the developmental stage of field testing. Selection for activity of nodule enzymes resulted in experimental populations with enhanced or with reduced enzyme activity, but without differences in N2 fixation in the field. Several alfalfa germplasms incapable of N2 fixation have been released for use as non-N2 fixing controls in field experiments and in breeding programs. Nondormant alfalfa has been developed for enhanced N accumulation and storage, and one special purpose cultivar has been released. Molecular biology methods applied to inoculum improvement have yielded at least one rhizobial strain that enhanced yield in the greenhouse. These technologies can provide new cultivars for the forage legume seed industry, new controls for use in forage legume cultivar evaluation studies, and germplasm for use in testing models of soil nutrient transformations and soil management. These technologies will influence future crop management systems and soil conservation practices.