《Table 2 Inter-batch repeatability (SD) inδ15N andδ18O, the measured difference between N3 and USGS3

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《影响细菌反硝化法测量硝酸盐氧同位素的因素分析（英文）》

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RM:reference material；SD:standard deviation；C:contraction ratio

The measurement accuracies ofδ15N andδ18O were significantly improved using the gas-liquid ratio of 3:10，and theδ18O contraction ratios decreased as nitrate sample sizes decreased（except for 0.05μmol）.These results agree with the observations of Weigand et al.（2016）.When the nitrate sample size was0.05μmol，it was significantly affected by the blank，resulting in a high contraction ratio for bothδ15N andδ18O.Although the optimal nitrate sample sizes reducedδ18O contraction ratios，there was still a slight contraction（6.0%to 6.5%）in the measuredδ18O values（Table 2）.Weigand et al.（2016）suggested that a change in nitrate sample sizes was not the sole cause of theδ18O contraction.Compared with nitrogen isotopes，oxygen isotopes are more difficult to measure due to the exchange of oxygen atoms with water，isotope fractionation，blank，and other factors.