Pulses of Biogenic Nitrogen Cycling Lead to Atmospheric-Based Nutrient Spiraling in Southern California

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The cycling of nitrogen in arid lands, deserts, and areas with limited water and frequent high temperatures is challenging to study and does not fit within the current understanding of other areas such as forests or grasslands. In arid lands, nitrogen concentrations are generally low and many biological processes are limited by nitrogen availability during wet periods.  However, many arid land ecosystems also exhibit high rates of nitrogen loss more characteristic of areas with a nitrogen abundance.  During wet periods soils may emit high nitrogen concentrations. Because of these nitrogen pulses, arid lands may have an important role in atmospheric chemistry although the magnitudes or fate of emissions are not well known.  Return of the soil nitrogen pulses to the soil may be a previously unaccounted nutrient source contributing to downwind nitrogen deposition.  New research directed to characterizing arid land nitrogen dynamics across landscapes may resolve these apparent contradictions, contribute to more comprehensive understanding, and improve understanding of the broad-scale effects of nitrogen enrichment. The findings will be communicated to the federal and state agencies that are directly affected by atmospheric nitrogen including Joshua Tree National Park.  Training opportunities, targeted for underrepresented graduate and undergraduate students, include tools for trace gases, isotopes, genomes, and modeling.  The results will be communicated to the local community through UC Cooperative Extension and an annual public lecture series that targets K-12 educators throughout the region.  This project will develop and test a hypothesis of pulse-induced nutrient spiraling to explain the landscape distribution of nitrogen concentrations and sources in southern California.  The central hypothesis borrows from nutrient dynamics in stream systems where movement downstream leads to biogeochemical ?spirals?, as nutrients cycle from dissolved and biotic forms.  In arid lands, it is hypothesized that soil pulses of gaseous nitrogen emissions coupled with atmospheric transport and deposition forms such a spiral and propagates nitrogen enrichment across a landscape.   This hypothesis will be evaluated across a transect of decreasing nitrogen deposition extending from Riverside, CA to the eastern edge of Joshua Tree National Park where replicated experiments will be conducted that assess soil nitrogen pulses and identifies their sources through isotopic and genomic tools. These experiments will test predictions that pulses of soil nitrogen emissions have characteristic isotopic signatures and increasingly dominate soil nitrogen content downwind from urban sources.  The experimental results will be used to inform a regional atmospheric transport and chemistry model that will simulate nitrogen dynamics throughout southern California.  The model simulations will further test predictions that soil pulse emissions can influence atmospheric nitrogen chemistry and subsequent nitrogen deposition.