| Increasing use of inorganic nitrogen fertilizer in intensive cropping systems has in tremendously increased yields. Simultaneously, the appearance of nitrate in ground water as a result of agricultural practices is of major public concern. Deeply rooted legumes, like alfalfa (Medicago sativa L.), may provide an opportunity to remediate contaminated sites by removing leached nitrate from the subsoil. Alfalfa acquires N through two major pathways: absorption of inorganic N from the soil solution and symbiotic N{dollar}sb2{dollar} fixation from the atmosphere in association with Rhizobium meliloti. The objectives of this thesis research were to evaluate subsoil NO{dollar}sb3{dollar}-N removal by several alfalfa germplasms and to investigate underlying physiological aspects involved in the regulation of nitrogenase activity as affected by NO{dollar}sb3{dollar} exposure.; I developed a system that allows NO{dollar}sb3{dollar} application in the field to the lower portion of the root zone, independent of the upper portion, and assessed NO{dollar}sb3{dollar} uptake and N{dollar}sb2{dollar} fixation of alfalfa using {dollar}sp{lcub}15{rcub}{dollar}N tracer techniques. Ineffective Agate, an ineffectively nodulated, non-N{dollar}sb2{dollar}-fixing, alfalfa germplasm, removed in its herbage up to 100% more NO{dollar}sb3{dollar} from the subsoil than a number of N{dollar}sb2{dollar}-fixing alfalfa germplasms differing in root system architecture and leaf morphology.; In a vertical split root system I characterized the effect of localized exposure of alfalfa root systems to NO{dollar}sb3{dollar} on symbiotic N{dollar}sb2{dollar} fixation and NO{dollar}sb3{dollar}-N uptake. My results showed that: (1) nodule mass was dependent on the amount of NO{dollar}sb3{dollar}-N taken up and the concentration of NO{dollar}sb3{dollar} within the nodulated root zone; and (2) specific nitrogenase activity was not affected by concentration of NO{dollar}sb3{dollar} surrounding the nodules, but only by the amount of NO{dollar}sb3{dollar}-N taken up.; In another experiment, I decreased symbiotic efficiency of the alfalfa/Rhizobium meliloti symbiosis by reducing partial pressures of N{dollar}sb2{dollar} surrounding the nodulated root systems. These results showed that the rate of decline in nitrogenase activity caused by NO{dollar}sb3{dollar} was independent of the amount of N{dollar}sb2{dollar} fixed.; These experiments demonstrated the benefit of alfalfa, and especially ineffective alfalfa, for bioremediation of NO{dollar}sb3{dollar}-contaminated sites and extended our knowledge about underlying physiological aspects involved in the regulation of nitrogenase activity as affected by NO{dollar}sb3{dollar} exposure. |
