Effects of denitrification and transport on the isotopic composition of nitrate (δ18O, δ15N) in freshwater systems

Nitrate isotopes (δ¹⁵N-NO₃- and δ¹⁸O-NO₃-) 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 NO₃- 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 NO₃- 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.