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Stable isotope ratios of nitrogen and oxygen ( 15 N/ 14 N and 18 O/ 16 O) of nitrate (NO 3 - ) are excellent tracers for developing systematic understanding of sources, conversions, and deposition of reactive atmospheric nitrogen (N r ) in the environment. Despite recent analytical advances, standardized sampling of NO 3 - ) isotopes in precipitation is still lacking. To advance atmospheric studies on N r species, we propose best-practice guidelines for accurate and precise sampling and analysis of NO 3 - isotopes in precipitation based on the experience obtained from an international research project coordinated by the International Atomic Energy Agency (IAEA). The precipitation sampling and preservation strategies yielded a good agreement between the NO 3 - concentrations measured at the laboratories of 16 countries and at the IAEA. Compared to conventional methods (e.g., bacterial denitrification), we confirmed the accurate performance of the lower cost Ti(III) reduction method for isotope analyses ( 15 N and 18 O) of NO 3 - in precipitation samples. These isotopic data depict different origins and oxidation pathways of inorganic nitrogen. This work emphasized the capability of NO 3 - isotopes to assess the origin and atmospheric oxidation of N r and outlined a pathway to improve laboratory capability and expertise at a global scale. The incorporation of other isotopes like 17 O in N r is recommended in future studies.

Nitrate isotopes (δ 15 N-NO 3 - and δ 18 O-NO 3 - ) are a potentially powerful tool for tracking the biological removal of reactive nitrogen (N) as it is transported from land to sea. However, uncertainties about, 1) the variability of the strength of biological isotopic fractionation during anaerobic benthic NO 3 - reduction (the kinetic enrichment factor: ε denit ), and, 2) how accurately these ε denit values are expressed in overlying aerobic surface waters (the effective enrichment factor: ε eff ), currently limit their use in freshwater systems. Here we used a combination of incubation experiments and numerical modelling to construct a simple framework for defining freshwater ε denit based on interactions between benthic denitrification and diffusive transport to surface waters. Under non-limited, anaerobic conditions the ε denit values produced in submerged soils (n = 3) and sediments (n = 4) with denitrification rates between 10 and 600 mg N m -2  d -1 ranged from -3‰ to -28‰. Critically, model results indicated that diffusive transport would homogenise this to an effective fractionation range of -6 ± 4‰. Evidence for biological and hydrological variability of NO 3 - isotope fractionation means that values measured in aerobic surface water environments are most appropriately evaluated by a range of fractionation values, rather than commonly used single 'site specific' ε denit values., (Copyright © 2018 Elsevier B.V. All rights reserved.)