A comparison of different preservation methods for nitrogen isotopes of soil extractable NO3-

Nitrogen (N) isotope ratio (δ¹⁵N) of soil extractable NO^?₃ plays a pivotal role in the study of N biogeochemical circulation in ecosystems. However, the NO^?₃ concentration and its isotope composition of soil samples are unstable, making sample storage critical for preserving the N isotope composition of extracted soil NO^?₃. Nevertheless, studies on the appropriate selection of storage methods after soil sampling are scarce. In this study, we compared two commonly used methods for storing soil samples and investigated the stability of N isotopes of soil NO^?₃. The results demonstrated that no significant changes in the NO^?₃ concentration and δ¹⁵N value occurred in the samples stored at –18 °C. However, the soil NO^?₃ concentration markedly increased, and NO^?₃δ¹⁵N value significantly changed after air-drying storage. Meanwhile, we also found that NO^?₃ and its δ¹⁵N were well preserved in the filtered soil extracts after 1 month. In contrast, the NO^?₃ concentration gradually decreased and the ¹⁵N in NO^?₃ was gradually enriched in the bactericidal agent-containing soil mixture solution during the storage period. Overall, our results indicated that N isotopes of NO^?₃ could be effectively preserved in frozen-stored soil samples or filtered soil extracts. For field investigations conducted in remote areas and continued for a long-time period (and lacking a refrigerant supply), soil extraction/filtration using a CaSO₄-saturated solution may be a superior preparation and storage method for analyzing N isotopes of soil NO^?₃.