Opening Opportunities for High-Resolution Isotope Analysis - Quantification of δ¹⁵N_NO3 and δ¹⁸O_NO3 in Diffusive...

The fate of nitrate transported across groundwater-surface water interfaces has been intensively studied in recent decades. The interfaces between aquifers and rivers or lakes have been identified as biogeochemical hotspots with steep redox gradients. However, a detailed understanding of the spatial heterogeneity and potential temporal variability of these hotspots, and the consequences for nitrogen processing, is still hindered by a paucity of adequate measurement techniques. A novel methodology is presented here, using Diffusive Equilibrium in Thin-film (DET) gels as high-spatial-resolution passive-samplers of δ¹⁵N_NO3 and δ¹⁸O_NO3 to investigate nitrogen cycling. Fractionation of δ¹⁵N_NO3 and δ¹⁸O_NO3 during diffusion of nitrate through the DET gel was determined using varying equilibrium times and nitrate concentrations. This demonstrated that nitrate isotopes of δ¹⁵N_NO3 and δ¹⁸O_NO3 do not fractionate when sampled with a DET gel. δ¹⁵N_NO3 values from the DET gels ranged between 2.3 ± 0.2 and 2.7 ± 0.3‰ for a NO₃^- stock solution value of 2.7 ± 0.4‰, and δ¹⁸O_NO3 values ranged between 18.3 ± 1.0 and 21.5 ± 0.8‰ for a NO₃^- stock solution of 19.7 ± 0.9‰. Nitrate recovery and isotope values were independent of equilibrium time and nitrate concentration. Additionally, an in situ study showed that nitrate concentration and isotopes provide unique, high-resolution data that enable improved understanding of nitrogen cycling in freshwater sediments.