An Ion-Exchange Method for Nitrogen Isotope Analysis of Nitrate in Soil

Abstract

The N isotopic composition of nitrate in soil extraction samples can deliver critical information regarding the soil N cycle for biogeochemical processes. Here, we present a preparatory ion-exchange resin method for the N isotope analysis of NO3− in saturated CaSO4 soil extracts. This method showed that the NO3− in saturated CaSO4 soil extracts could be effectively separated from SO42− and other anions to avoid interference from the large amounts of coexisting anions in soil extracts. The nitrate recovery rate ranged from 95.54 to 105.04%. The resulting average natural abundance of 15N (δ15N) values of the KNO3 solutions and the KNO3–CaSO4 solutions were +6.04 ± 0.03‰ (n = 6) and +6.03 ± 0.12‰ (n = 6), respectively. The average δ15N value of the KNO3 salt was +6.07 ± 0.06‰ (n = 12). Therefore, we concluded that N isotopic fractionation did not occur throughout the process. In addition, the δ15N-NO3− values of natural soils extracted using saturated CaSO4 were reproducible between separate analytical runs. The standard deviations of the δ15N-NO3− for the three different soils were ± 0.17‰ (n = 4), ± 0.04‰ (n = 3), and ± 0.34‰ (n = 3). Our ion-exchange method was effective at separating NO3− from other anions in the soil extracts and was suitable for measuring the δ15N-NO3− of saturated CaSO4 soil extracts.