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Nitrogen-fixing species contribute to ecosystem nitrogen budgets, but background resource levels influence nodulation, fixation, and plant growth. We conducted a greenhouse experiment to examine the separate and interacting effects of water and N availability on biomass production, tissue N concentration, nodulation, nodule activity, and rhizodeposition of Lupinus argenteus (Pursh), a legume native to sagebrush steppe. Plants were grown in a replicated, randomized design with three levels of water and four levels of N. Additional water and N increased biomass except at the highest N level. All plants formed nodules regardless of treatment, but plants grown without N had the largest, most active nodules. Organic N was deposited into the rhizosphere of all plants, regardless of treatment, indicating that Lupinus can influence N availability while actively growing. even under water stress. High tissue N concentrations and low C:N ratios indicate that Lupinus also can provide substantial amounts of N through litter decomposition. The ability of Lupinus to affect N availability and cycling indicates that it has the potential to significantly influence N budgets and community composition within the sagebrush steppe. (C) 2009 Elsevier B.V. All rights reserved.