Isotopic signals of summer denitrification in a northern hardwood forested catchment

Abstract

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...

Urban rivers as hotspots of regional nitrogen pollution

“…4d), revealing the same pollution sources for these two pollutants. C:N ratio was a good indicator for resources that can be used by microbes, such as denitrification bacteria (Fang et al, 2015;Wexler et al, 2014). C:N ratio was sharply reduced with increase of NH þ 4 concentration, for which exceeding 10.0 mg N L À1 reduced the C:N ratio to round 5, although further increase of NH þ 4 concentration did not continue reduce the C:N ratio.…”

A stable isotope approach and its application for identifying nitrate source and transformation process in water

“…The same case also occurs in the northern hardwood forests at the Hubbard Brook Experimental Forest in the White Mountains of New Hampshire, USA. Wexler et al (2014) found that both δ 15 N-NO 3 − and δ 18 O-NO 3 − values of a few samples fell below the denitrification line, which might be attributed to a mix of partially denitrified NO 3 − in groundwater and the NO 3 − in stream water when a hyporheic flow path brought water from the stream to the shallow groundwater in the riparian zone. One possible means of differentiating denitrification from the mixing of source materials is to analyze the relationship between δ 15 N-NO 3 − values (or δ 18 O-NO − 3 ) and NO 3 − concentrations ).…”

“…Denitrification causes both δ 15 N and δ 18 O values of the residual NO 3 − to increase. Sigman et al (2005) and Granger et al (2008)) found that δ 15 N and δ 18 O values of residual NO 3 − increased at a ratio of 2 during denitrification.However, some other surface and groundwater studies found that denitrification caused δ 18 O and δ 15 N to increase at a ratio of more than 1:1(Minet et al 2012; Dale et al 2014a, b) and some closed to 2(Burns et al 2009;Baily et al 2011;Critchley et al 2014;Wexler et al 2014) in agricultural watersheds. However, these lower ratios were not found in urban watersheds(Kaushal et al 2011), presumable because denitrification was limited by less organic carbon(Barnes and Raymond 2010;Wong et al 2015).…”

“…However, these lower ratios were not found in urban watersheds(Kaushal et al 2011), presumable because denitrification was limited by less organic carbon(Barnes and Raymond 2010;Wong et al 2015). The reported ratio of δ 15 N-NO 3 − and δ 18 O-NO 3 − values (1.13 to 2.1)(Vidal- Gavilan et al 2013;Wexler et al 2014) as being indicative of denitrification is widely used in groundwater system and groundwater surface-water interactions. The relationship between enriched δ 15 N and δ 18 O values of the residual NO 3 − is used byCritchley et al (2014) to provide evidence of in situ denitrification after a cross-injection experiment in glacial sand gravel aquifer Izbicki et al (2015).…”

The Forestry Reclamation Approach: guide to successful reforestation of mined landsSite-specific critical acid load estimates for forest soils in the Osborn Creek watershed, Michigan

“…Whitfield et al (2010) more significant in wet soils than dry soils. In areas where soil acidification is a concern, denitrification can be an important mitigating process as it prevents excess NO 3 -from entering groundwater (Wexler et al 2014). However, in many such areas (e.g., well-drained, sandy upland forest soils), denitrification is often negligible, if not absent.…”