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The Rhône River originates in the high Alps and drains an intensely cultivated and industrialised catchment before it discharges to the Gulf of Lion. We investigated the interaction of catchment geomorphology with nitrate sources (atmosphere, agriculture, and nitrification of soil organic matter) and removal processes in large and diverse watersheds on the basis of dual nitrate isotope signatures in river water.In March 2015, we took surface water samples along the Rhône River, including its main tributaries, and measured nutrient concentrations and the stable isotopic composition of nitrate (δ 15 N, δ 18 O and Δ 17 O), and water (δ 18 O-H 2 O).Results show that high altitude regions are dominated by nitrate from nitrification in pristine soils and atmospheric deposition, while nitrate in the downstream Rhône River originates mainly from nitrification of agricultural/urban sources. Parallel increases in δ 15 N and δ 18 O reflect the influence of primary production. Previous studies suggested robust correlations between land use and [Formula: see text]. Based on our observation that nitrate δ 15 N values at higher altitudes are lower than expected, we assume that lower nitrate δ 15 N values likely reflect limited nitrate consumption and lower soil nitrogen turnover rates. We propose that correlation between land use and nitrate δ 15 N is sensitive to slope and geomorphology.

Natural abundance nitrogen and oxygen isotopes of nitrate (δ 15 N NO3 and δ 18 O NO3 ) provide an important tool for evaluating sources and transformations of natural and contaminant nitrate (NO 3 - ) in the environment. Nevertheless, conventional interpretations of NO 3 - isotope distributions appear at odds with patterns emerging from studies of nitrifying and denitrifying bacterial cultures. To resolve this conundrum, we present results from a numerical model of NO 3 - isotope dynamics, demonstrating that deviations in δ 18 O NO3 vs. δ 15 N NO3 from a trajectory of 1 expected for denitrification are explained by isotopic over-printing from coincident NO 3 - production by nitrification and/or anammox. The analysis highlights two driving parameters: (i) the δ 18 O of ambient water and (ii) the relative flux of NO 3 - production under net denitrifying conditions, whether catalyzed aerobically or anaerobically. In agreement with existing analyses, dual isotopic trajectories >1, characteristic of marine denitrifying systems, arise predominantly under elevated rates of NO 2 - reoxidation relative to NO 3 - reduction (>50%) and in association with the elevated δ 18 O of seawater. This result specifically implicates aerobic nitrification as the dominant NO 3 - producing term in marine denitrifying systems, as stoichiometric constraints indicate anammox-based NO 3 - production cannot account for trajectories >1. In contrast, trajectories <1 comprise the majority of model solutions, with those representative of aquifer conditions requiring lower NO 2 - reoxidation fluxes (<15%) and the influence of the lower δ 18 O of freshwater. Accordingly, we suggest that widely observed δ 18 O NO3 vs. δ 15 N NO3 trends in freshwater systems (<1) must result from concurrent NO 3 - production by anammox in anoxic aquifers, a process that has been largely overlooked., Competing Interests: The authors declare no conflict of interest.

Global inputs of NO(x) are dominated by fossil fuel combustion from both stationary and vehicular sources and far exceed natural NO(x) sources. However, elucidating NO(x) sources to any given location remains a difficult challenge, despite the need for this information to develop sound regulatory and mitigation strategies. We present results from a regional-scale study of nitrogen isotopes (delta15N) in wet nitrate deposition across 33 sites in the midwestern and northeastern U.S. We demonstrate that spatial variations in delta15N are strongly correlated with NO(x) emissions from surrounding stationary sources and additionally that delta15N is more strongly correlated with surrounding stationary source NO(x) emissions than pH, SO4(2-), or NO3- concentrations. Although emission inventories indicate that vehicle emissions are the dominant NO(x) source in the eastern U.S., our results suggest that wet NO3- deposition at sites in this study is strongly associated with NO(x) emissions from stationary sources. This suggests that large areas of the landscape potentially receive atmospheric NO(y) deposition inputs in excess of what one would infer from existing monitoring data alone. Moreover, we determined that spatial patterns in delta15N values are a robust indicator of stationary NO(x) contributions to wet NO3- deposition and hence a valuable complement to existing tools for assessing relationships between NO3- deposition, regional emission inventories, and for evaluating progress toward NO(x) reduction goals.

Sewage effluent, storm runoff, discharge from polluted rivers, and inputs of groundwater have all been suggested as potential sources of land derived nutrients into Hanalei Bay, Kauai. We determined the nitrogen isotopic signatures (delta(15)N) of different nitrate sources to Hanalei Bay along with the isotopic signature recorded by 11 species of macroalgal collected in the Bay. The macroalgae integrate the isotopic signatures of the nitrate sources over time, thus these data along with the nitrate to dissolved inorganic phosphate molar ratios (N:P) of the macroalgae were used to determine the major nitrate source to the bay ecosystem and which of the macro-nutrients is limiting algae growth, respectively. Relatively low delta(15)N values (average -0.5% per hundred) were observed in all algae collected throughout the Bay; implicating fertilizer, rather than domestic sewage, as an important external source of nitrogen to the coastal water around Hanalei. The N:P ratio in the algae compared to the ratio in the Bay waters imply that the Hanalei Bay coastal ecosystem is nitrogen limited and thus, increased nitrogen input may potentially impact this coastal ecosystem and specifically the coral reefs in the Bay. Identifying the major source of nutrient loading to the Bay is important for risk assessment and potential remediation plans.