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

• Main Authors: Wells, Naomi, Clough, Timothy, Johnson-Beebout, SE, Elberling, B, Baisden, WT
• Format: Article
• Language: English

Content Partner: Lincoln University. 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: εdenit), and, 2) how accurately these εdenit values are expressed in overlying aerobic surface waters (the effective enrichment factor: εeff), 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 εdenit based on interactions between benthic denitrification and diffusive transport to surface waters. Under non-limited, anaerobic conditions the εdenit 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’ εdenit values.