Convergence in nitrogen deposition and cryptic isotopic variation across urban and agricultural valleys in northern Utah

Abstract

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 δ15N of NO3− and NH4+ in urban valley sites. Contrary to expectations, annual N deposition (3.5–5.1 kg N ha−1 yr−1) and inorganic N concentrations were similar within and between valleys. Significant summertime decreases in δ15N 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 δ15N was similar to natural ecosystems (−0.6 ± 0.7‰). Fine atmospheric particulate matter (PM2.5) had consistently high values of bulk δ15N (15.6 ± 1.4‰), δ15N in NH4+ (22.5 ± 1.6‰), and NO3− (8.8 ± 0.7‰), consistent with equilibrium fractionation with gaseous species. The δ15N in bulk deposition NH4+ varied by more than 40‰, and spatial variation in δ15N within storms exceeded 10‰. Sporadically high values of δ15N 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.

Incorporating <sup>15</sup>N into the outputs of SMOKE version 4.6 as the emission input dataset for CMAQ version 5.2.1 for assessing the role emission sources plays in controlling the isotopic composition of NO<sub>x</sub>, NO<sub>y</sub>, and atmospheric nitrate

“…Comparing with the SMOKE simulation, the measurements of δ 15 N values of NO3in the United States from previous researches (Elliott et al, 2009;Garten, 1992;Hall et al, 2016;Occhipinti, 2008;Russell et al, 1998) shows the similar monthly variations and seasonal trend (Fig. 12, bottom).…”

“…Finally, we compared the emission model's predicted NOx δ 15 N values at 82 NADP sites in the Midwest (Fig. S8) with measurements of NO3d 15 N values (Elliott et al, 2009;Garten, 1992;Hall et al, 2016;Occhipinti, 2008;Russell et al, 1998). The δ 15 N values of NOx simulated by SMOKE at these sites show large monthly variations and a seasonal trend (Fig.…”

Atmospheric Inputs of Nitrogen, Carbon, and Phosphorus across an Urban Area: Unaccounted Fluxes and Canopy Influences

“…A similar proportion of organic N in total deposition has also been found in urban areas in studies around the world (Araujo et al, ; Chen et al, ; de Souza et al, ; González‐Benitez et al, ; He et al, ; Izquieta‐Rojano et al, ; Li et al, ; Matsumoto et al, ). There have been a handful of studies in the U.S. measuring organic N deposition in small towns in Connecticut and North Carolina, in winter precipitation in Salt Lake City, Utah and in Miami, Florida; four of these studies either do not report organic N deposition rates or do not report organic rates in the urban sites individually, and the others report the percentage of organic N in total N as below 20% (Brezonik et al, ; Hall et al, ; Keene et al, ; Luo et al, ; Nadim et al, ; Zamora et al, ). The rate of atmospheric deposition of organic N is thus currently unknown in many large urban areas throughout the U.S, and as the majority of urban studies only consider atmospheric inputs of inorganic N, estimates of total N inputs into these ecosystems are likely to be underestimated.…”

Using Traffic Density and Foliar Chemistry Variables to Understand Interactions Between Air Pollution and Household Income

“…Winter snow is the dominant form of precipitation, and there is some rain in the spring and fall. The SLV is bounded by the Great Salt Lake to the north, the Wasatch mountains to the east, Oquirrh mountains to the west and Transverse Mountains to the south (Alder et al., 1998; Bush et al., 2008; Hall et al., 2016). Elevation on the valley floor ranges between 1,285 and 2,304 m above sea level (Cobley et al., 2018).…”