Comparison of Pretreatment Methods for Nitrogen Stable Isotope Abundance Determination in Soil Ammonium Nitrogen

In order to establish more accurately and rapid pretreatment method for the nitrogen stable isotope analysis of soil ammonium nitrogen, diffusion and N2O produced method were compared, and combined with two commercially available equipment’s, i.e., elemental analyzer-isotope ratio mass spectrometer (Flash HT-IRMS) and trace gas preconcentration stable isotope mass spectrometry (PreCon-IRMS), respectively, for the analysis of the nitrogen stable isotope abundance inammonium sulfate reference solutions and soil ammonium nitrogen extracted from different paddy soil samples. The stable isotope measurement results showed that both the diffusion and N₂O produced method coupled IRMS analytical systems were able to provide good analytical data for ammonium sulfate reference solutions in accuracy with precision of superior to 0.5% and 0.2%, respectively. However, there was a nitrogen isotope fractionation accrued in the diffusion-treated process, and the test results needed to be corrected by a standard curve. At the same time, the ¹⁵N stable isotope abundance in ammonium nitrogen extracted from different paddy soils were determined by diffusion and N₂O produced method. The comparison of the analytical data from the two analytical methods indicated that N₂O produced method coupled with PreCon-IRMS provided better nitrogen stable isotope data in reproducibility and accuracy than the data obtained by the other method. In addition, through the auxiliary automatic trace nitrogen conversion N₂O generated device, nitrogen stable isotope abundance of soil ammonium nitrogen could be simultaneously analysis online in batch process, and the pretreatment analysis time was short, the sample consumption was small, only need 10 μg N for one sample (<5 mL soil extract solution). Therefore, for the analysis of nitrogen isotopes in micro-amounts of soil ammonium, the N2O produced method coupled with PreCon-IRMS analysis method has more advantages than the diffusion method coupled Flash HT-IRMS analysis method.