Article pubs.acs.org/est
Nitrogen Deposition in and near an Urban Ecosystem Neil D. Bettez* and Peter M. Groffman Cary Institute of Ecosystem Studies, P.O. Box AB, Millbrook, New York 12545, United States ABSTRACT: Excess nitrogen (N) is a serious water-quality problem in most of the estuaries in the United States, especially those downstream of developed coastal basins. Understanding sources of N is a key first step in managing and mitigating N pollution. While the major sources of this N, atmospheric deposition, wastewater, fertilizer, and other agricultural sources are well-known, their relative importance as N sources to particular estuaries is not. Much of this uncertainty is due to difficulties associated with estimating the amount of atmospheric N deposition. Here, we show that deposition is 47% higher in urban and 22% higher in suburban areas compared to nonurban areas and that this deposition is primarily due to dry deposition. Moreover, this deposition is not being measured by the current deposition monitoring networks that were designed to measure broader regional patterns causing an underestimation of N inputs in urban areas.
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INTRODUCTION In the U.S., N deposition estimates are made using data collected by the National Atmospheric Deposition Program (NADP), which consists of ∼250 sites that measure wet deposition, and by the Clean Air Status and Trends Network (CASTNET), which consists of ∼90 sites that measure dry deposition and are usually colocated with a subset of NADP sites. These networks were set up to monitor temporal longterm regional trends in precipitation chemistry dry deposition. Therefore, sampling sites were specifically located in areas uninfluenced by local pollution sources and thus pointedly avoided urban areas (e.g., “sites should be located >10 km away from suburban/urban areas with a population of 10 000”).1 As a result, the current monitoring networks are missing a key component of the total deposition, which is emitted and deposited in close proximity to urban areas. Indeed, undercounting of N input from deposition has been documented in several continental scale input/output budgets (e.g., refs 2 and 3) in which only ∼40% of the emissions are accounted for in deposition measurements. At a regional scale, Elliot et al. (2007) used 15N to track the contribution of different NOx sources to deposition collected at NADP sites, and found that despite mobile sources (mostly vehicle emissions) being the dominant NOx source, the δ15N values of the wet nitrate deposition were instead more strongly correlated with the surrounding stationary source NOx emissions.4 N deposition sources are currently underestimated not only because of where monitoring stations are located but also because of what the stations measure. Although nearly all forms of NHx deposition (NH3 gas and particulate NH4+) are due to NH3 emissions, the species (gaseous or particulate) collected is related to the proximity of the collectors to the emission source. The NADP measures wet NO3− and NH4+, and CASTNet measures nitric acid vapor (HNO3), and particulate NO3− and NH4+. Although the NADP has begun to set up an ammoniamonitoring network (AMoN), which currently has 66 sites, it is © XXXX American Chemical Society
still in the early stages of development, therefore, NH3 is not currently being monitored. Because they do not measure NH3, the current monitoring networks are likely to miss an increasing percentage of the deposition. Ammonia, which has a high deposition velocity, is usually deposited as dry deposition near the source, while NH4+ and particulates are deposited further away.5 Although NH3 is the major form of air pollution from agricultural sources, it is also produced by motor vehicles. Unlike NOx (NO and NO2), which is primarily the result of combustion, lightning and biogenic emissions from soils are responsible for ∼4% of emissions;6 NH3 is a secondary pollutant in motor vehicle emissions that results from the over reduction of NO by the catalytic converter. As a result of stricter emission standards for NO, the over reduction to NH3 is increasingly common and NH3’s percentage of overall vehicle nitrogen emissions has been increasing.7 As NOx emissions are reduced further, NH3 levels, and therefore the amount of nearsource deposition, are predicted to increase as a result of the role NOx emissions play in the chemical transformation of NH3 to NH4+ and particulate NH4+.8 NH3 is not regulated or widely monitored. The EPA uses an estimate of ∼70 mg NH3−N km−1 per vehicle for their emission models,9 but others have found higher levels (∼118 mg N-NH3 km−1) under urban driving conditions, resulting in vehicles, at least in urban areas, being an important source of NH3.7 The great challenge with assessing and managing atmospheric sources of N is that there is a disconnection between production and deposition. This challenge is especially critical in areas where there are regulatory mandates to reduce N delivery to receiving waters (e.g., total maximum daily loads).10 Recent studies have documented that a substantial portion of Received: February 11, 2013 Revised: April 30, 2013 Accepted: April 30, 2013
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Figure 1. Map of Gwynn Falls watershed.
the importance of any underestimation can be evaluated in terms of overall sources and delivery to receiving waters. In this study, our goals were to investigate how N deposition is influenced by urbanization. To accomplish this, we measured atmospheric N inputs to watersheds with different land uses (forested, suburban, urban, and agricultural) near Baltimore, MD, using ion-exchange resins to measure bulk and throughfall deposition for one year from June 2010−June 2011. We then compared these results to the nearest monitoring sites used to estimate regional wet and dry deposition and calculated how much unaccounted for N is being deposited in the City of Baltimore.
mobile source emissions (especially NH3 which has a high deposition velocity) are being deposited near where they are produced and thus are not being measured by the NADP/ CASTNet networks. Modeling work by Dennis et al. (2006) using the Community Multiscale Air Quality (CMAQ) model found that local deposition of total ammonia can be significant and that 15−30% of emissions are deposited locally.11 Local deposition gradients occur at multiple scales. Lovett et al. (2000) found deposition to be twice as high near New York City than in areas 150 km away from this urban center,12 while others working adjacent to individual roads have found gradients that are only a few hundred meters in length of 15N in leaves13 and tree rings.14 In a study along a transect away from a major highway in Germany, Kirchner et al. (2005) found up to 3× higher NH3 and NO2 concentrations and 2× higher N deposition for sites near the highway than at sites 500 m away.15 Similarly, Bettez et al., in a study that focused on quantifying the effects of near-source deposition from mobile source emissions along a moderately sized road on Cape Cod, found that sites 10 m away from roadways had 1.5−2× more deposition compared to sites in the forest interior away from roads. This near-source deposition of mobile source emissions can result in significantly increased deposition near roads and can cause large underestimates in the amount of N deposition to the watershed by up to 25%. It is therefore critical that road deposition studies be carried out in a watershed context so that
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METHODS AND MATERIALS Site Description. We conducted this research in the Gwynns Falls (76°30′, 39°15′) and Pond Branch watersheds, which are used as study and reference watersheds by the Baltimore Ecosystem Study, a component of the U.S. National Science Foundation Long-Term Ecological Research (LTER) network (Figure 1). Gwynns Falls is a 17 150 ha watershed containing 16 subwatersheds, which span an urban−rural gradient from downtown Baltimore to the rural suburban fringe of Baltimore. Pond Branch is a 40 ha forested watershed ∼10 km to the east of the upper Gwynns Falls. In this study, we measured deposition in Pond Branch, and several subwatersheds in the Gwynns Falls: Glyndon, which is a 94 ha primarily suburban watershed, McDonough, which is a 6.1 ha watershed B
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Figure 2. Significant differences in bulk and throughfall.
dominated by row crop agriculture, and Rognel Heights, which is a 1.8 ha sewershed (i.e., an area defined by a storm sewer drain network). Nitrogen Deposition. We estimated total deposition (wet and dry) using throughfall, the precipitation that passes through the tree canopy, which captures wet as well as any dry deposition collected on the foliage. This method overcomes many of the difficulties associated with measuring dry deposition and has been used extensively by researchers as an approximate estimate of total deposition.16,17 We corrected the values upward by 16% (based on the Integrated Forest Study regression of net canopy uptake and throughfall total N to account for canopy uptake).16 We measured bulk and throughfall deposition using a mixed bed (1:1 cation/anion) ion-exchange resin sampler similar to those used by Fenn et al., (2004).18 Deposition samplers consisted of a polyethylene funnel (20 cm diameter with polyFil Nu- foam to keep out debris) attached (using hot glue) to the female slip end of a 1.27 cm PVC (slip × threaded) fitting. A length of threaded PVC pipe (30.5 cm long × 1.27 cm diameter) filled with ion-exchange resin (US Filter NR-6, which is equivalent to MR-3 (Dowex)19 and Amberlite IRN 15020). The pipe was loosely plugged at both ends with Poly-Fil NuFoam to retain the resin and attached to a Micro Spray Adapter (1.27 cm female pipe thread × 0.79 cm female pipe thread) on the other. We shielded the resin tubes from the sun and adjusted the height of samplers to ∼1 m by resting the funnels in a 3.17 cm diameter × 75 cm long piece of PVC pipe that was attached to a stake driven into the ground. We measured bulk deposition, which is a mixture of mostly wet and some dry deposition, by placing ten samplers in open fields at three sites: (1) Beltsville NAPD/CASTNet site (39° 1′42.24″N, 76°49′1.51″W); (2) University of Maryland Baltimore County (UMBC) (39°15′15.64″N, 76°42′8.56″W); (3) The McDonough School (39°23′46.70″N, 76°46′17.09″), which is adjacent to agricultural watershed and the official BES LTER meteorological station. We measured throughfall by
placing twenty-five samplers under the forest canopy at four sites: Pond Branch (forested), Glyndon (suburban), McDonogh (agricultural), and Rognel Heights (urban). Samplers were deployed from June 1, 2010−June 1, 2011 for periods of 3−5 months, after which we brought them back to the lab, eluted the resins with three 200 -ml 2 M KCL rinses, and analyzed for NO3− (method #10-107-04-1-B) and NH4+ (method #12-107-06-1-B) using a Lachat QuikChem FIA analyzer in the Rachel L. Carson Analytical Facility at the Cary Institute of Ecosystem studies. Statistical Analysis. The statistical program JMP 9.0.2 was used for all data analysis.21 We used a one-way analysis of variance (ANOVA) followed by a Tukey HSD post hoc analysis to test for differences (p < 0.05) in N deposition among sites.
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RESULTS Our estimates of bulk deposition ranged from 6.3 (±.21) to 7.0 (±.23) kg N ha−1 yr−1 and were very similar to the 2011 NADP/CASTNet site of 7.1 kg N ha−1 yr−1 for wet and dry deposition.22 Our estimates of total (wet and dry) deposition based on canopy corrected estimates of throughfall were 9.1, 11.1, 13.3, and 13.6 kg N ha−1 yr−1 for the forested, suburban, urban, and agricultural sites, respectively. Among all our sites, we measured significant differences (F(6, 114) = 18.76, p < 0.0001) in throughfall but not bulk deposition (Figure 2). Although throughfall deposition at the forested site was not significantly different from the bulk deposition estimates at Beltsville, UMBC, or McDonough or from the throughfall estimates at the suburban site there was significantly less deposition at the forested site than there was at the urban and agricultural sites. Relative to the NAPD/CASTNet site estimates of wet and dry deposition, our estimates of wet and dry deposition were 57% higher (4.0 kg N ha−1 yr−1) at the suburban site and 88% higher (6.2 kg N ha−1 yr−1) at the urban site. Compared to our estimates of deposition at the forested site, deposition was 22% higher (2 kg N ha−1 yr−1) at the suburban site and 47% higher (4.2 kg N ha−1yr−1) at the urban C
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there is a need for a higher resolution of monitoring sites for ammonia, which has a higher deposition velocity and is therefore more likely to deposited locally and not measured by monitoring sites located away from urban areas. These increased inputs are likely to have numerous direct and indirect impacts on carbon and nitrogen cycling in urban areas. Higher N inputs will result in higher outputs, particularly in urban watersheds, which retain a lower percentage of the N inputs compared to forested watersheds.31 This has implications for coastal eutrophication, especially in the Chesapeake Bay region, which is highly urbanized. While increased N inputs from local deposition have been shown to have an impact on watershed nitrogen cycle processes in a suburban/rural watershed on Cape Cod,29 these impacts are likely to be greater in more urban watersheds, such as Baltimore, given the magnitude of other anthropogenic N inputs such as lawn fertilizer (i.e., ∼ 96 kg N ha−1 yr−1 31), and pet waste (i.e., ∼1.4 kg N household−1 yr−1 from ref 32. Indirectly, increased N inputs from deposition are likely to impact other biogeochemical processes ranging from uptake of CO2 by trees33 to fluxes of the greenhouse gases nitrous oxide34 and methane uptake by lawns and forests.35 Currently, the majority of Americans live within 100 km of the coast,36 and many of the largest cities in the U.S are situated on estuaries. Urbanization is increasing nationally and globally and its impacts, especially in coastal areas, are significant. Many of the current models and monitoring programs do not work well in urban areas because they fail to take into account localized human influences. This research addresses one example of this; the impact of mobile sources, primarily highway vehicles, on local deposition and highlights how urbanization affects N cycling. Our results suggest that efforts to assess and control N delivery to coastal areas are likely to fall short of proposed goals unless the effects of urbanization on deposition are taken into consideration.
site. If we scale the urban deposition estimates to the entire city of Baltimore (238.5 km2), these results indicate that we are underestimating N inputs due to deposition by 101 168 kg yr−1 compared to what they would be if we used the deposition estimate from the forested site and 148 847 kg yr−1 if we use the deposition estimate from the Beltsville NADP/CASTnet site.
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DISCUSSION In this study, we find that deposition is related to land use. Similar to other studies, we measured higher deposition at the agricultural site compared to deposition at the nonagricultural sites. This increased deposition was likely due to the volatilization and redeposition of N applied as fertilizer and or manure 23 as well as deposition associated with emissions from nearby roads (see below). We also measured higher deposition at the urban site than the forested site. These results confirm that there is important heterogeneity in deposition in areas of complex land use that are not accounted for in existing national monitoring networks. Our results also suggest that increased deposition associated with urbanization is due to increased amounts of dry deposition, which we estimated as the difference between throughfall and bulk deposition. The source of the dry deposition is likely near-source deposition of nitrogen (NO, NO2, NH3) emissions associated with urbanization such as stationary sources (e.g., power plants, incinerators) and mobile sources (e.g., highway vehicles and off highway vehicles such as construction equipment, planes, boats, and trains). There were several roads near the suburban site with moderate to high traffic volumes in 2010: MD 128, which is ∼0.5 km to the north had an annual average daily traffic (AADT) of 16 442 vehicles day−1, US 795, which is ∼1.2 km to the west had an AADT of 60 210 vehicles day−1, MD 30, which is ∼0.5 km to the west had an AADT of 8,931 vehicles day−1. At the urban site, these sources are even more numerous. On an areal basis, the emissions from all of these sources can be significant. According to the latest (2008) National Emissions Inventory,24 total emissions in the City of Baltimore were 271 kg N ha−1 yr−1. Although there are several large stationary sources such as electrical generating facilities and solid waste incinerators in downtown Baltimore (5−6 km from the sampling site) these account for less than 20% of the all the N emissions for the city. The majority of the emissions in the City of Baltimore are due to mobile sources, which are responsible for 73% of all the nitrogen emissions. Indeed, there are multiple highways passing near the urban site. In 2010, US 695, which is ∼4.5 km to the southwest had an AADT of 201 490 vehicles day−1, US 95, which is ∼4.5 km to the southeast had an AADT of 182 473 vehicles day−1, US 83, which is ∼5.4 km to the east had an AADT of 119 562 vehicles day−1, and MD RT 40, which is ∼1 km to the south had an AADT of 53 241 vehicles day−1.25 It is likely that these highways are influencing the urban N budget in our study. Numerous studies (including at least one in Baltimore) have used tunnels to show that motor vehicles are significant sources of N emissions,26−28 while others have used transects away from roadways to show that N deposition is elevated near roads.29,30,15 Here, we evaluate the importance of this deposition at the whole-city scale. We suggest that the impact of atmospheric deposition and the role of urbanization on local N budgets be taken into account when designing monitoring networks and constructing management plans to reduce coastal N loading. In particular,
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was funded by National Science Foundation: NSFEAR Award No. #0847838 to N.D.B and NSF-DEB Award No. #1027188 to P.M.G. Special thanks to Dan Dillon from the Cary Institute, who provided valuable assistance and advice in the field; Dan Jones from UMBC for assistance in the field; Lisa Martel and Kate Shepard from the Cary Institute who processed and analyzed samples in the laboratory; and Milada Vomela from the Rachel Carson Analytical Facility at the Cary Institute who analyzed nutrient samples.
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