The Role of Symbiotic Nitrogen Fixation on Nitrogen Availability, Competition, and Plant Invasion into the Sagebrush Steppe
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In the semi-arid sagebrush steppe of the Northeastern Sierra Nevada, resources are both spatially and temporally variable, arguably making resource availability a primary factor determining invasion success. Nitrogen fixing plant species, primarily native legumes, are often relatively abundant in sagebrush steppe and can contribute to ecosystem nitrogen budgets. <italic>Lupinus argenteus</italic> (Pursh), a native legume abundant in high elevation areas of western North America, is one of the most common native legumes in sagebrush steppe. <italic>L. argenteus</italic>responds positively to disturbance and prior studies indicate that it can increase the availability of soil nitrogen. Thus contribution of nitrogen by <italic>L. argenteus</italic> can potentially have a large effect on maintaining native species diversity and productivity of sagebrush ecosystems. However, if a non-native seed source is present, increased nitrogen associated with <italic>L. argenteus</italic> can create conditions favorable for invasion by non-native species. This study examined the role of <italic>L. argenteus</italic> on resource availability in the sagebrush steppe and the implications for invasion with four interrelated studies. Results indicate that <italic>L. argenteus</italic> can modify available soil nitrogen and increase productivity in sagebrush ecosystems both through rhizodeposition and litter decomposition. Further, modification of the local resource pool by <italic>L. argenteus</italic> can alter competitive outcomes among native and non-native species and can increase plant establishment and growth of both native and non-native species. However, higher establishment and growth rates give the non-native a greater advantage. The ability of <italic>L. argenteus</italic> to increase nitrogen availability can serve to promote resilience of native ecosystems, but also may create an avenue for invasion.