Natural abundance 15N in soil and litter across a nitrate-output gradient in New Hampshire

Pardo, Linda H.; Hemond, Harold F.; Montoya, Joseph P.; Pett‐Ridge, Jennifer

doi:10.1016/j.foreco.2007.06.047

Cited by 46 publications

(52 citation statements)

References 63 publications

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“…We measured δ 15 N of soil samples collected from nine soil pits in WS3 (37) using a Finnigan MAT DeltaPlus isotope ratio mass spectrometer (Thermo Finnigan) at the Cornell Stable Isotope Lab. These δ 15 N values averaged 1.2 ± 0.4‰ in the surface organic horizons (Oi/Oe) and increased to 5.2 ± 1.1‰ in the Oa horizon and 6.0 ± 1.1‰ in Bh and Bhs horizons at 10-65 cm depth, similar to prior measurements in WS3 (48). Mineralization causes negligible 15 N fractionation (14,49) and so δ 15 N NH4 from mineralization of this soil organic N should also have this range of 15 N values (0.5-7.5‰).…”

Section: Methodssupporting

confidence: 85%

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: 85%

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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“…Species differences in foliar d 15 N values have been reported in the northeastern US (Nadelhoffer et al, 1999;Templer, 2001;Pardo et al, 2006;Pardo et al, 2007). These differences in foliar d…”

Section: Discussionmentioning

confidence: 87%

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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“…Indeed, Högberg and Johannisson (1993) reported a correlation between the fractional loss of added N and an increase in d 15 N over time in tree foliage in a long-term N-loading experiment. Likewise, enrichment in 15 N of tree foliage has been found in field surveys of forests subject to high N loads in both Europe and N. America (e.g., Högberg et al 1996;Emmett et al 1998;Pardo et al 2007). Högberg (1991) and Högberg and Johannisson (1993) also found that trees in plots to which urea, CO(NH 2 ) 2 , had been applied became more enriched in 15 N than did trees in plots to which NH 4 NO 3 had been added.…”

Section: Abstract High Dmentioning

confidence: 89%

Is the high 15N natural abundance of trees in N-loaded forests caused by an internal ecosystem N isotope redistribution or a change in the ecosystem N isotope mass balance?

2013

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High d15 N of tree foliage in forests subject to high N supply has been attributed to 15 N enrichment of plant available soil N pools after losses of N through processes involving N isotope fractionation (ammonia volatilization, nitrification followed by leaching and denitrification, and denitrification in itself). However, in a long-term experiment with high annual additions of NH 4 NO 3 , we found no change in the weighted average d 15 N of the soil, but attributed the high d 15 N of trees to loss of ectomycorrhizal fungi and their function in tree N uptake, which involves redistribution of N isotopes in the ecosystem (Högberg et al. New Phytol 189:515-525, 2011), rather than a loss of isotopically light N. Here, we compare the effects of additions of urea and NH 4 NO 3 on the d 15 N of trees and the soil profile, because we have previously found higher d15 N in tree foliage in trees in the urea plots. Doing this, we found no differences between the NH 4 NO 3 and urea treatments in the concentration of N in the foliage, or the amounts of N in the organic morlayer of the soil. However, the foliage of trees receiving the highest N loads in the urea treatment were more enriched in 15 N than the corresponding NH 4 NO 3 plots, and, importantly, the weighted average d 15 N of the soil showed that N losses had been associated with fractionation against 15 N in the urea plots. Thus, our results in combination with those of Högberg et al. (New Phytol 189:515-525, 2011) show that high d 15 N of the vegetation after high N load may be caused by both an internal redistribution of the N isotopes (as a result of change of the function of ectomycorrhiza) and by losses of isotopically light N through processes fractionating against 15 N (in case of urea ammonia volatilization, nitrification followed by leaching and denitrification).

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Natural abundance 15N in soil and litter across a nitrate-output gradient in New Hampshire

Abstract: Stable isotopes of nitrogen are potentially a valuable tool for regional assessments of nitrogen saturation because they provide an integrated measure of the past nitrogen cycling history of a site.

Cited by 46 publications

References 63 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

Is the high 15N natural abundance of trees in N-loaded forests caused by an internal ecosystem N isotope redistribution or a change in the ecosystem N isotope mass balance?

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