Comparison of the silver nitrate and bacterial denitrification methods for the determination of nitrogen and oxygen isotope ratios of nitrate in surface water

Nitrogen (N) and oxygen (O) isotope ratios of NO ― 3 are often used to trace dominant NO ― 3 pollution sources in water. Both the silver nitrate (AgNO 3 ) method and the bacterial denitrification method are frequently used analytical techniques to determine δ 15 N- and δ 18 O-NO ― 3 in aqueous samples. The AgN0 3 method is applicable for freshwater and requires a concentration of 100-200 μmol of NO ― 3 for isotope determination. The bacterial denitrification method is applicable for seawater and freshwater and for KCl extracts of soils with a NO ― 3 concentration as low as 1 μmol. We have carried out a thorough method comparison using 42 real surface water samples having a wide range of δ 15 N- and δ 18 O-NO ― 3 values and NO ― 3 concentrations. Various correction pairs using three international references and blanks were used to correct raw δ 15 N- and δ 18 O-NO ― 3 values. No significant difference between the corrected data was observed when using various correction pairs for each analytical method. Both methods also showed excellent repeatability with high intraclass correlation coefficients (ICC). The ICC of the AgN0 3 method was 0.992 for δ 15 N and 0.970 for δ 18 O. The ICC of the bacterial denitrification method was 0.995 for δ 15 N and 0.954 for δ 18 O. Moreover, a positive linear relationship with a high correlation coefficient (r≥0.88) between the two methods was found for δ 15 N- and δ 18 O-NO ― 3 . The comparability of the methods was assessed by the Bland-Altman technique using 95% limits of agreement. The average difference between results obtained by the bacterial denitrification and the AgNO 3 method for δ 15 N was ―1.5‰ with 95% limits of agreement ―3.6 and +0.5‰. For δ 18 O this was +2.0‰, with 95% limits of agreement ―3.3 and +7.3‰. We found that for δ 15 N and for δ 18 O, 97% of the differences fell within these 95% limits of agreement. In conclusion, the AgN0 3 and the bacterial denitrification methods are highly correlated and statistically interchangeable.