DROUGHT EFFECTS ON NITROGEN FIXATION RATES IN RANGELANDS OF THE SOUTHWESTERN UNITED STATES

The southwestern US is an N fixation hotspot in which woody N fixers represent a greater percentage of woody plant basal area than most other areas of the US. Many areas across Texas and the US southwest have experienced periods of major drought conditions in the past several decades; the frequency and severity of droughts such as these are predicted to increase in the future. Understanding the role that drought plays in controlling rates of N fixation is critical to understanding inputs of N into these ecosystems in the future, and to parameterizing the role of N fixation in maintaining critical ecosystem services in the rangelands of this region.The ecophysiology of N fixers strongly suggests that drought, and water availability in general, plays an important role in determining rates of N fixation, and thus rates of N input into ecosystems. Other work has investigated the interactions between N availability and light availability on N fixation rates, demonstrating the need to investigate variation in multiple environmental parameters concurrently. The next step in understanding how drought may affect N fixation rates is to examine how WUE efficiency in N fixers changes with changing water availability in conjunction with variability in other environmental parameters, specifically light and N availability.Objective 1: Quantify the relative effects of drought versus light limitation and excess soil N availability on N fixation, WUE, and drought tolerance across several N-fixing tree species.Objective 2: Quantify how woody N fixers affect ecosystem services (e.g., forage quality, plant biodiversity, and soil C) in rangelands across a precipitation gradient, and how these effects correlate with N-fixer WUE and proxies of N fixation.Objective 3: Quantify how the effects drought on N-fixation rates in Prosopis glandulosa are driven by local adaptation versus acclimation by growing seeds collected from across the rainfall gradient crossed with Rhizobia collected from across the rainfall gradient.