Effects of denitrification and transport on the isotopic composition of nitrate (δO, δN) in freshwater systems.

Nitrate isotopes (δN-NO and δO-NO) 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 reduction (the kinetic enrichment factor: ε), and, 2) how accurately these ε values are expressed in overlying aerobic surface waters (the effective enrichment factor: ε), 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 based on interactions between benthic denitrification and diffusive transport to surface waters. Under non-limited, anaerobic conditions the ε values produced in submerged soils (n = 3) and sediments (n = 4) with denitrification rates between 10 and 600 mg N m d 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 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 values.