Print PageWhile cities occupy a relatively small land area globally, atmospheric compounds generated from human-dominated ecosystems have significant impacts on protected lands worldwide. Atmospheric nitrogen (N) deposition alters ecological processes and properties, such as biogeochemical cycling, primary production, and community composition. Nitrogen is released into the atmosphere from agricultural and combustion activities as ammonia (NH3) and nitrogen oxides (NOx), and deposited to ecosystem surfaces as NOy (NOx, nitric acid (HNO3), nitrate (NO3-)) as well as ammonium (NH4+) and NH3. In arid urban ecosystems, such as Phoenix, Arizona USA, considerable uncertainty surrounds estimates of atmospheric N inputs due to difficulties in quantifying rates of dry deposition to natural ecosystems. We compared sampling methods for N deposition at locations within, upwind, and downwind of the Phoenix metropolitan area. Specifically, we compared deposition fluxes using the Community Multi-scale Air Quality (CMAQ) model (for 1996; Fenn et al. 2003), wet-dry buckets (years 2000 – 2005; Lohse et al 2008), ion-exchange resin (IER) collectors (bulk and throughfall, years 2006 – 2011), and inferential methods using passive samplers (atmospheric N concentrations x modeled deposition velocity; years 2010 - 2011). In addition, we measured soil inorganic N availability with IER soil probes co-located with IER deposition collectors. We expected soil N availability to increase with elevated atmospheric N inputs to the system.
Results/Conclusions
Rates of N deposition estimated with resin collectors, passive atmospheric samplers, and wet-dry buckets (4-6 kg N ha-1 y-1) are significantly lower than expected based on rates estimated by the CMAQ model (7-15 kg N ha-1 y-1). Furthermore, contrary to patterns in the CMAQ model, which predicted high rates of N deposition both within and downwind of Phoenix, resin and inferential methods showed that the elevated N deposition rate is restricted to the urban core. Passive sampler data suggest that higher rates of N deposition in the city are due to differences in atmospheric NOx concentration rather than HNO3 or NH3. While inorganic soil N availability is higher within Phoenix than outside of the city, available inorganic soil N is not as strongly correlated to N deposition fluxes as predicted. These results suggest the importance of further research to determine the fate of deposited N in dryland ecosystems. Overall, our findings highlight the need for mixed methods to quantify N deposition and its ecological consequences.
