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...

Indirect nitrous oxide (NO) emissions from rivers are currently derived using poorly constrained default IPCC emission factors (EF) which yield unreliable flux estimates. Here, we demonstrate how hydrogeological conditions can be used to develop more refined regional-scale EF estimates required for compiling accurate national greenhouse gas inventories. Focusing on three UK river catchments with contrasting bedrock and superficial geologies, NO and nitrate (NO) concentrations were analyzed in 651 river water samples collected from 2011 to 2013. Unconfined Cretaceous Chalk bedrock regions yielded the highest median NO-N concentration (3.0 μg L), EF (0.00036), and NO-N flux (10.8 kg ha a). Conversely, regions of bedrock confined by glacial deposits yielded significantly lower median NO-N concentration (0.8 μg L), EF (0.00016), and NO-N flux (2.6 kg ha a), regardless of bedrock type. Bedrock permeability is an important control in regions where groundwater is unconfined, with a high NO yield from high permeability chalk contrasting with significantly lower median NO-N concentration (0.7 μg L), EF (0.00020), and NO-N flux (2.0 kg ha a) on lower permeability unconfined Jurassic mudstone. The evidence presented here demonstrates EF can be differentiated by hydrogeological conditions and thus provide a valuable proxy for generating improved regional-scale NO emission estimates.

A dual-isotope and solute flux mass-balance was used to elucidate the processes that lead to attenuation of nitrogen contamination in an agriculturally impacted river. The River Wensum drains a lowland catchment with an area of 570 km² in East Anglia, eastern England. Analysis of nitrate concentration, δ¹⁵N(NO₃) and δ¹⁸O(NO₃) of samples from the River Wensum collected from upstream locations to the catchment outlet through all seasons and flow conditions showed a consistent pattern of increasing isotope values with decreasing nitrate concentrations downstream. δ¹⁵N(NO₃) and δ¹⁸O(NO₃) of catchment surface water and groundwater samples revealed a dominant influence from microbially cycled and nitrified source-nitrogen, which results in high nitrate concentrations in Chalk groundwater and upstream in the River Wensum. Denitrification of Chalk groundwater-baseflow in the hyporheic zone results in the downstream trend observed in the river. Hyporheic denitrification is estimated to remove 931 kg/day of nitrate-nitrogen by the catchment outlet, representing 31% of the potential riverine nitrate load. The use of dual-isotope and solute flux modeling at the catchment scale is a novel application to quantify denitrification within the river valley, demonstrating the importance of hyporheic zone processes in attenuating the impacts of anthropogenic contamination of hydrologic systems.