The effect of water and nutrient availability on nitrogen contribution by the native sagebrush legume Lupinus argenteus

Nitrogen fixing species can contribute to nitrogen budgets of ecosystems, but background resource levels influence nodulation, fixation, and plant growth. Understanding how nitrogen fixing species respond to varying levels of resources can provide insights into their functional role under different site conditions and following disturbance. We conducted a greenhouse experiment to examine the separate and interacting effects of water and nitrogen on the growth, nodule activity, and nitrogen contribution of Lupinus argenteus, a legume native to sagebrush steppe. Plants were grown under a completely randomized block design with three levels of water (200ml every 3, 6, or 12d) and four levels of nitrogen (0, 5mM, 20mM, 100mM) availability. Biomass was significantly affected by both water and nitrogen and was greatest under high water and intermediate nitrogen levels. All plants were nodulated, but plants grown without nitrogen produced the largest and most active nodules. Organic nitrogen was present in the rhizosphere of all plants, regardless of resource condition. Thus, nodule activity of L. argenteus is reduced but not inhibited under elevated nitrogen, even with low availability of water. Also, L. argenteus is able to influence nitrogen availability through rhizodeposition of organic nitrogen. These results combined with potential overwinter decomposition of low C:N tissue, indicate that L. argenteus can provide substantial inputs of nitrogen to this system. The ability of L. argenteus to affect nitrogen availability and cycling indicates that it has the potential to influence community composition and plant invasion processes within the sagebrush steppe.