Pedosphere 28(6): 943--951, 2018
ISSN 1002-0160/CN 32-1315/P
©2018 Soil Science Society of China
Published by Elsevier B.V. and Science Press
A comparison of different preservation methods for nitrogen isotopes of soil extractable NO3-
LIU Junchi1,2,3, LIU Weiguo1,4, HU Jing1
1State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710075(China);
2Key Laboratory of Vertebrate Evolution and Human Origin of Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing 100044(China);
3University of Chinese Academy of Sciences, Beijing 100049(China);
4School of Human Settlement and Civil Engineering, Xi'an Jiaotong University, Xi'an 710049(China)
      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 NO3-. 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 NO3- 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-.
Key Words:  air-drying,freezing,ion exchange,isotopic fractionation,nitrogen isotope tracing,soil extraction,soil storage
Citation: Liu J C, Liu W G, Hu J. 2018. A comparison of different preservation methods for nitrogen isotopes of soil extractable NO3-. Pedosphere. 28(6):943-951.
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