Evaluating the source of streamwater nitrate using δ<sup>15</sup>N and δ<sup>18</sup>O in nitrate in two watersheds in New Hampshire, USA

Pardo, Linda H.; Kendall, Carol; Pett‐Ridge, Jennifer; Chang, Cecily C. Y.

doi:10.1002/hyp.5576

Cited by 129 publications

(125 citation statements)

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“…Rainfall NO 3 − was highly enriched in 18 O and slightly depleted in 15 N (Table 1 and Fig. 3), resembling previous measurements at the HBEF (10,17). Nitrate produced by microbial nitrification reflects the δ 15 N of the NH 4 + that these autotrophic bacteria consume, as well as any fractionation that occurs during this process.…”

Section: Methodssupporting

confidence: 83%

“…However, past δ 18 O NO3 measurements show that nitrificationnot precipitation-supplies the vast majority of NO 3 − in streamwater at the HBEF (10,17,18) and other forested catchments (refs. 14 and 19, Table S1).…”

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confidence: 99%

“…− is not replenished or consumed by other processes, denitrification progressively enriches both 18 O and 15 N in the residual NO 3 − , with an O:N fractionation ratio of 0.4-0.7 in the field (14,20,21) and up to 1.0 in laboratory studies (22 Table S1), including the HBEF (10,17), have revealed little if any isotopic evidence of denitrification.…”

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confidence: 99%

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Isotopic signals of summer denitrification in a northern hardwood forested catchment

Wexler

Goodale

McGuire

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

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Despite decades of measurements, the nitrogen balance of temperate forest catchments remains poorly understood. Atmospheric nitrogen deposition often greatly exceeds streamwater nitrogen losses; the fate of the remaining nitrogen is highly uncertain. Gaseous losses of nitrogen to denitrification are especially poorly documented and are often ignored. Here, we provide isotopic evidence (δ 15 N NO3 and δ 18 O NO3 ) from shallow groundwater at the Hubbard Brook Experimental Forest indicating extensive denitrification during midsummer, when transient, perched patches of saturation developed in hillslopes, with poor hydrological connectivity to the stream, while streamwater showed no isotopic evidence of denitrification. During small rain events, precipitation directly contributed up to 34% of streamwater nitrate, which was otherwise produced by nitrification. Together, these measurements reveal the importance of denitrification in hydrologically disconnected patches of shallow groundwater during midsummer as largely overlooked control points for nitrogen loss from temperate forest catchments.M any forested catchments export far less nitrogen (N) in streamwater than they receive in atmospheric deposition (1, 2). The rest of the deposited N may accumulate in vegetation or soil organic matter, or be lost in gaseous form. Losses of N to denitrification, the microbial reduction of aqueous nitrate (NO 3 − ) to nitrous oxide (N 2 O, a greenhouse gas) and N 2 gas, are extremely difficult to measure due to the difficulty in directly measuring N 2 fluxes and due to the high degree of spatiotemporal variability in redox conditions and substrate sources (3). Many past studies using a range of measurements (streamwater nitrate isotopic composition, the acetylene block technique, N 2 O emissions, and mass balance calculations) have concluded that denitrification in temperate forests is highly uncertain or generally unimportant (e.g., refs. 4-8).Nitrogen budgets are particularly perplexing in the northern hardwood forests at the Hubbard Brook Experimental Forest (HBEF) in the White Mountains of New Hampshire, USA, where atmospheric deposition has supplied 6-8 kg N ha −1 ·yr −1 for half a century, a rate ∼5-10 times preindustrial levels (7-10). Accumulation of N in plant biomass ceased in the early 1990s (10, 11), while streamwater inorganic N export from catchments across the HBEF and nearby streams decreased to <1 kg N ha −1 ·yr −1 , for reasons that remain elusive (9,10,12). These N flux measurements imply increasingly important roles for N gas loss or storage in soil organic matter. However, both processes are so difficult to quantify that the fate, drivers, and consequences of the "missing" N remain unknown, at the HBEF and elsewhere (8)(9)(10)12 Table S1).Nitrate isotopic composition reflects not only NO 3 − sources but also fractionation from a range of processes (14, Table S1), including the HBEF (10, 17), have revealed little if any isotopic evidence of denitrification. SignificanceDenitrification is the most poorly under...

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Section: Methodssupporting

confidence: 83%

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confidence: 99%

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Isotopic signals of summer denitrification in a northern hardwood forested catchment

Wexler

Goodale

McGuire

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

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“…In some cases 15 N. These cases include sites either where the deposition has a very distinct isotopic signature and very high magnitude (such as sites severely impacted by NH 3 volatilization from animal husbandry) or where direct canopy uptake represents the dominant part of foliar nutrition. In the northeastern US, studies have shown that most N deposition is cycled biologically within the ecosystem (Burns and Kendall, 2002;Pardo et al, 2004). Microbial cycling of N will alter the 15 N signature of N from deposition.…”

Section: Discussionmentioning

confidence: 99%

Regional patterns in foliar 15N across a gradient of nitrogen deposition in the northeastern US

Pardo

McNulty

Boggs

et al. 2007

Environmental Pollution

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Recent studies have demonstrated that natural abundance 15 N can be a useful tool for assessing nitrogen saturation, because as nitrification and nitrate loss increase, d15 N of foliage and soil also increases. We measured foliar d 15 N at 11 high-elevation spruce-fir stands along an N deposition gradient in 1987e1988 and at seven paired northern hardwood and spruce-fir stands in 1999. In 1999, foliar d 15 N increased from À5.2 to À0.7& with increasing N deposition from Maine to NY. Foliar d 15 N decreased between 1987e1988 and 1999, while foliar %N increased and foliar C:N decreased at most sites. Foliar d 15 N was strongly correlated with N deposition, and was also positively correlated with net nitrification potential and negatively correlated with soil C:N ratio. Although the increase in foliar %N is consistent with a progression towards N saturation, other results of this study suggest that, in 1999, these stands were further from N saturation than in 1987e1988. Published by Elsevier Ltd.

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“…The samples were eluted and then purified to produce silver nitrate (Pardo et al, 2004). The pretreated samples were then processed for stable isotope analysis on a VG 253 isotope-ratio mass spectrometer in the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing.…”

Section: Water Sampling and Analytical Methodsmentioning

confidence: 99%

Effects of land-use patterns on in-stream nitrogen in a highly-polluted river basin in Northeast China

Zhang

Meng

et al. 2016

Science of The Total Environment

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• Land-use types were correlated well with most nitrogen variables over seasons.• Built-up land dominated in predicting nitrogen variables during different seasons.• Shape metrics predicted most nitrogen variables in different seasons.• Nitrogen sources and their contributions were estimated using nitrate isotopes.• Domestic sewage mainly contributed to river nitrogen pollution in residence zone. This study investigated the effects of land-use patterns on nitrogen pollution in the Haicheng River basin in Northeast China during 2010 by conducting statistical and spatial analyses and by analyzing the isotopic composition of nitrate. Correlation and stepwise regressions indicated that land-use types and landscape metrics were correlated well with most river nitrogen variables and significantly predicted them during different sampling seasons. Built-up land use and shape metrics dominated in predicting nitrogen variables over seasons. According to the isotopic compositions of river nitrate in different zones, the nitrogen sources of the river principally originated from synthetic fertilizer, domestic sewage/manure, soil organic matter, and atmospheric deposition. Isotope mixing models indicated that source contributions of river nitrogen significantly varied from forested headwaters to densely populated towns of the river basin. Domestic sewage/manure was a major contributor to river nitrogen with the proportions of 76.4 ± 6.0% and 62.8 ± 2.1% in residence and farmland-residence zones, respectively. This research suggested that regulating built-up land uses and reducing discharges of domestic sewage and industrial wastewater would be effective methods for river nitrogen control. G R A P H I C

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Evaluating the source of streamwater nitrate using δ¹⁵N and δ¹⁸O in nitrate in two watersheds in New Hampshire, USA

Cited by 129 publications

References 65 publications

Isotopic signals of summer denitrification in a northern hardwood forested catchment

Isotopic signals of summer denitrification in a northern hardwood forested catchment

Regional patterns in foliar 15N across a gradient of nitrogen deposition in the northeastern US

Effects of land-use patterns on in-stream nitrogen in a highly-polluted river basin in Northeast China

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Evaluating the source of streamwater nitrate using δ15N and δ18O in nitrate in two watersheds in New Hampshire, USA

Cited by 129 publications

References 65 publications

Isotopic signals of summer denitrification in a northern hardwood forested catchment

Isotopic signals of summer denitrification in a northern hardwood forested catchment

Regional patterns in foliar 15N across a gradient of nitrogen deposition in the northeastern US

Effects of land-use patterns on in-stream nitrogen in a highly-polluted river basin in Northeast China

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Evaluating the source of streamwater nitrate using δ¹⁵N and δ¹⁸O in nitrate in two watersheds in New Hampshire, USA