Nodulation and Acetylene Reduction by Certain Rangeland Legume Species under Field Conditions

Abstract

Acetylene reduction rates, N2[C2H2], were obtained to estimate nitrogen fixation by several introduced and native range legume species. The N2[C2H2] fixation rates of excised root segments with attached nodules were measured in the field for legumes from two mountain grassland sites, one native sagebrush-dominated site, and three cultivated former big sagebrush study sites. Sampling was conducted in the driest part of the growing season. Even during this high stress period, nodules were present and active in some legume species. Medicago sativa plants were particularly notable because of their capability of being nodulated and ability to reduce acetylene in dry soils when other legumes were not active. In addition, under the most favorable environmental conditions in this study, nodules from M. sativa reduced acetylene most actively at a rate of 26.2 micromoles ethylene/h/g nodule fresh weight. Although nodulation was generally less successful in New World than Old World lupine species, Lupinus mutabilis was capable of reducing acetylene at a rate of 4.97 micromoles ethylene/h/g nodule fresh weight, even in a severely water stressed environment. These results suggested that some legume species may be capable of fixing significant amounts of nitrogen on semiarid range sites.