A Comparison of Different Preservation Methods for Nitrogen Isotopes of Soil Extractable NO3-

摘要

Nitrogen (N) isotope ratio (δ15N) of soil extractable NO3-plays a pivotal role in the study of N biogeochemical circulation in ecosystems.However,the NO3-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 NO3-.The results demonstrated that no significant changes in the NO3-concentration and δ15N value occurred in the samples stored at-18 ℃.However,the soil NO3-concentration markedly increased,and NO3-δ15N value significantly changed after air-drying storage.Meanwhile,we also found that NO3-and its δ15N were well preserved in the filtered soil extracts after 1 month.In contrast,the NO3-concentration gradually decreased and the 15N in NO~ was gradually enriched in the bactericidal agent-containing soil mixture solution during the storage period.Overall,our results indicated that N isotopes of NO3-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 NO3-.