The isotopic composition (δ15N and δ18O) of nitrite (NO2-) provides valuable insights into nitrogen cycling in aquatic environments. However, its typically low concentrations as a transient redox intermediate in natural waters often limit isotopic analysis. In this study, a method for δ15N and δ18O analysis of low-level NO2- in freshwater has been developed by coupling anion-exchange resin preconcentration with the azide reduction method. This approach enables the measurement of δ15N and δ18O with low-concentrations (0.02 μmol L-1), which significantly expands the applicability of NO2- isotope analysis to low-nutrient environments. Experimental evaluation of the azide reduction step demonstrated that oxygen exchange between NO2- and water was consistently suppressed to below 10% when the NaCl concentration exceeded 0.41 mol L-1, and a 1:1 mixture of 90% acetic acid and 4 mol L-1 sodium azide was added at dosages greater than 28 μL mL-1. Field application in Lake Biwa yielded reasonable and interpretable NO2- concentration and isotope profiles, with nearly uniform vertical distributions during full mixing and clear contrasts across the thermocline during stratification. Our isotope data further suggest that NO2- derived from partial nitrate assimilation likely makes a non-negligible contribution to the NO2- pool in the upper layer of Lake Biwa. The robustness of this method in capturing NO2- dynamics in oligotrophic systems, as demonstrated here, adds a valuable tool to the toolbox for studying nitrogen cycling in freshwater systems.
Jiang et al. (2026) studied this question.