Effects of denitrification and transport on the isotopic composition of nitrate (δ~(18)O, δ~(15)N) in freshwater systems

摘要

Nitrate isotopes (delta N-15-NO3- and delta O-18-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 benthicNO(3)(-) reduction (the kinetic enrichment factor: epsilon(denit)), and, 2) howaccurately these epsilon(denit) values are expressed in overlying aerobic surface waters (the effective enrichment factor: epsilon(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 epsilon(denit) based on interactions between benthic denitrification and diffusive transport to surface waters. Under non-limited, anaerobic conditions the edenit 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 surfacewater environments aremost appropriately evaluated by a range of fractionation values, rather than commonly used single 'site specific' epsilon(denit) values. (C) 2018 Elsevier B.V. All rights reserved.