Ecosystem response to the urban atmosphere in the Sonoran Desert.
Hall, Sharon *,1, Grimm, Nancy1, Kaye, Jason2, 3, Allen, Jonathan3, 1 Arizona State University, Tempe, AZ, USA2 The Pennsylvania State University, University Park, PA, USA3 Arizona State University, Tempe, AZ, USA
ABSTRACT- Cities occupy just 2% of terrestrial land cover, but 50% of the worlds population currently lives urban areas. Ecosystems downwind of cities are exposed to numerous compounds in the urban atmosphere that can alter ecological properties and processes, including species composition, plant productivity, and biogeochemical cycling. We explored the effects of the urban atmosphere on ecosystem processes in native desert ecosystems within and surrounding Phoenix, AZ, the sixth largest and one of the fastest growing cities in the United States. Preliminary results show that organic matter, soil carbon, and soil carbon/nitrogen (C/N) ratios increased in plant interspaces across the gradient (from upwind to core to downwind), supporting the idea that urban deposition decreases the spatial heterogeneity of soil nutrients that characterizes desert ecosystems. Further, ratios of nitrogen/phosphorus (N/P) in foliar tissue of Larrea tridentata, the dominant perennial shrub, were higher in urban core and downwind sites compared to upwind sites, suggesting that the urban atmosphere may alter the importance of N vs. P as the element most limiting to primary production. However, we were surprised to find that other common indices of soil microbial activity (respiration, net N mineralization, net nitrification) were not significantly different across the gradient within interspaces or beneath shrubs, even during the monsoon season when soils were moist and plants were active. Pulse rainfall events after long dry periods may quickly mobilize accumulated surface inorganic N pools, thus reducing their availability to heterotrophic and autotrophic microorganisms in these desert soils.
Key words: Sonoran Desert, nutrient cycling, atmospheric deposition
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