Error assessment of nitrogen and oxygen isotope ratios of nitrate as determined via the bacterial denitrification method

摘要

Currently, bacterial denitrification is becoming the accepted method for δ~(15)Nand δ~(18)O-NO~-_3 determination. However, proper correction methods with international references (USGS32, USGS34 and USGS35) are needed. As a consequence, it is important to realize that the corrected isotope values are derived from a combination of several other measurements with associated uncertainties. Therefore, it is necessary to consider the propagated uncertainty on the final isotope value. This study demonstrates how to correctly estimate the uncertainty on corrected δ~(15)Nand δ~(18)O-NO_3~- values using a first-order Taylor series approximation. The bacterial denitrification method errors from 33 batches of 561 surface water samples varied from 0.2 to 2.1‰ for δ~(15)N-NO_3~- and from 0.7 to 2.3‰ for δ~(18)O-NO_3~-, which is slightly wider than the machine error, which varied from 0.2 to 0.6‰ for δ~(15)N-N_2O and from 0.4 to 1.0‰ for δ~(18)O-N_2O. The overall uncertainties, which are composed of the machine error and the method error, for the 33 batches ranged from 0.3 to 2.2‰ for δ~(15)N-NO_3~- and from 0.8 to 2.5‰ for δ~(18)O-NO_3~-. In addition, the mean corrected δ~(15)N and δ~(18)O values of 132 KNO_3-IWS (internal working standard) measurements were computed as 8.4±1.0‰ and 25.1±2.0‰, which is a slight underestimation for δ~(15)N and overestimation for δ~(18)O compared with the accepted values (δ~(15)N=9.9±0.3‰ and δ~(18)O= 24.0±0.3‰). The overall uncertainty of the bacterial denitrification method allows the use of this method for source identification of NO_3~-.