High Atmospheric Nitrate Inputs and Nitrogen Turnover in Semi-arid Urban Catchments Krystin M. Riha, 1 Greg Michalski, 1,2 * Erika L. Gallo, 3,4 Kathleen A. Lohse, 4 Paul D. Brooks, 3 and Tom Meixner 3 1 Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, Indiana 47907, USA; 2 Department of Chemistry, Purdue University, 550 Stadium Mall Drive, West Lafayette, Indiana 47907, USA; 3 Depart- ment of Hydrology and Water Resources, University of Arizona, Tucson, Arizona 85719, USA; 4 Department of Biological Sciences, Idaho State University, Pocatello, Idaho 83209, USA ABSTRACT The influx of atmospheric nitrogen to soils and sur- faces in arid environments is of growing concern due to increased N emissions and N usage associated with urbanization. Atmospheric nitrogen inputs to the critical zone can occur as wet (rain or snow) or dry (dust or aerosols) deposition, and can lead to eutro- phication, soil acidification, and groundwater con- tamination through leaching of excess nitrate. The objective of this research was to use the d 15 N, d 18 O, and D 17 O values of atmospheric nitrate (NO 3 - ) (precipitation and aerosols) and NO 3 - in runoff to assess the importance of N deposition and turnover in semi-arid urban watersheds. Data show that the fractions of atmospheric NO 3 - exported from all the urban catchments, throughout the study period, were substantially higher than in nearly all other ecosystems studied with mean atmospheric contri- butions of 38% (min 0% and max 82%). These re- sults suggest that catchment and stream channel imperviousness enhance atmospheric NO 3 - export due to inefficient N cycling and retention. In contrast, catchment and stream channel perviousness allow for enhanced N processing and therefore reduced atmospheric NO 3 - export. Overall high fractions of atmospheric NO 3 - were primarily attributed to slow N turn over in arid/semi-arid ecosystems. A relatively high fraction of nitrification NO 3 - (30%) was found in runoff from a nearly completely impervious watershed (91%). This was attributed to nitrification of atmospheric NH 4 + in dry-deposited dust, suggest- ing that N nitrifiers have adapted to urban micro ni- ches. Gross nitrification rates based on NO 3 - D 17 O values ranged from a low 3.04 ± 2 kg NO 3 -N km -2 day -1 in highly impervious catchments to a high of 10.15 ± 1 kg NO 3 -N km -2 day -1 in the low density urban catchment. These low gross nitrification rates were attributed to low soil C:N ratios that control gross autotrophic nitrification by regulating gross NH 4 + production rates. Key words: urban ecosystems; N cycle; isotopes; gross nitrification; N deposition; runoff. INTRODUCTION Arid and semi-arid regions cover one-third of the Earth’s terrestrial surface and are experiencing disproportionate increases in population growth and land-use change (Ezcurra 2006). These Received 2 October 2013; accepted 2 May 2014 Electronic supplementary material: The online version of this article (doi:10.1007/s10021-014-9797-x) contains supplementary material, which is available to authorized users. Author contributions Riha: data collection, isotope analysis, data interpretation, lead author Michalski: isotope analysis, data interpreta- tion, lead author, advisor to Riha Lohse: field data collection, geochemical analysis, data interpretation, lead author Gallo: data collection, GIS analysis, data interpretation, co-author Brookes: field support, data col- lection, data interpretation, editor Meixner: data interpretation, meta data, modeling, editor. *Corresponding author; e-mail: gmichals@purdue.edu Ecosystems DOI: 10.1007/s10021-014-9797-x Ó 2014 Springer Science+Business Media New York