低濃度淡水亜硝酸の陰イオン交換樹脂濃縮およびアジ化ナトリウム還元による同位体分析の改良

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.