Convergence in Nitrogen Deposition and Cryptic Isotopic Variation across Urban and Agricultural Valleys in Northern Utah

The extent to which atmospheric nitrogen (N) deposition reflects land use differences and biogenic versus fossil fuel reactive N sources remains unclear yet represents a critical uncertainty in ecosystem N budgets. We compared N concentrations and isotopes in precipitation-event bulk (wet + dry) deposition across nearby valleys in northern Utah with contrasting land use (highly urban versus intensive agriculture/low-density urban). We predicted greater nitrate (NO3 ) versus ammonium (NH4 ) and higher δN of NO3 and NH4 + in urban valley sites. Contrary to expectations, annual N deposition (3.5–5.1 kgNha 1 yr ) and inorganic N concentrations were similar within and between valleys. Significant summertime decreases in δN of NO3 possibly reflected increasing biogenic emissions in the agricultural valley. Organic N was a relatively minor component of deposition (~13%). Nearby paired wildland sites had similar bulk deposition N concentrations as the urban and agricultural sites. Weighted bulk deposition δN was similar to natural ecosystems ( 0.6 ± 0.7‰). Fine atmospheric particulate matter (PM2.5) had consistently high values of bulk δN (15.6 ± 1.4‰), δN in NH4 + (22.5 ± 1.6‰), and NO3 (8.8 ± 0.7‰), consistent with equilibrium fractionation with gaseous species. The δN in bulk deposition NH4 + varied by more than 40‰, and spatial variation in δN within storms exceeded 10‰. Sporadically high values of δN were thus consistent with increased particulate N contributions as well as potential N source variation. Despite large differences in reactive N sources, urban and agricultural landscapes are not always strongly reflected in the composition and fluxes of local N deposition—an important consideration for regional-scale ecosystem models.