1.Institute of Soil Science,Chinese Academy of Sciences;2.University of Chinese Academy of Sciences;3.Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences
Supported by the National Key Research and Development Program of China (No. 2018YFC1801801)
【Objective】Rare earth mining excessively increased the content of leaching agents (e.g. ammonium sulfate) in the soil. The high concentration of ammonium nitrogen (NH4+-N) may be converted into nitrate nitrogen (NO3--N) under active biochemical action, resulting in potential environmental risks, especially nitrate pollution of water bodies around tailings. Therefore, it is necessary to evaluate the content of soil NO3--N, explore the influencing factors and understand the nitrate pollution degree of the rare earth tailings. 【Method】We chose an ionic rare earth tailing after in-situ mining in southern Jiangxi province, which used ammonium sulfate as a leaching agent. Up to sampling, this mine had been closed for 4 years. We set three sampling points regularly from the top to bottom of this mine and collected soil profile samples in different layers from the topsoil to the bedrock. Soil samples were divided into two parts. One part was stored at a low temperature to analyze soil nitrate nitrogen and ammonium nitrogen. The other one was used for analyzing relevant physical and chemical properties after air drying. 【Result】The results showed that the variation range of soil NO3--N content in the tailing area was large (2.80 to 193.99 mg·kg-1), with a mean of 46.30±55.16 mg·kg-1. The average content of topsoil NO3--N was 5.16 mg·kg-1, which was similar to that of natural soil. Also, the average content of soil NO3--N in ore-bearing layers was 48.64 mg·kg-1, which was nearly 10 times that in the natural soil. The soil NO3--N of the ore-bearing layer in the deep profile was higher than that of the top layer. Moreover, the distribution of NO3--N with depth was different from that of the natural soil and was mainly caused by a large number of leaching agents remaining in the ore body. NH4+-N content dominated the generation of NO3--N and determined the upper limit of soil NO3--N accumulation. The accumulation degree of NO3--N in different soil layers and different parts of mountains was controlled by rainfall leaching and the NO3--N migration process. However, the soil""s physical and chemical properties, including water content, cation exchange capacity and particle composition, had no significant correlation with NO3--N content, and were not the determining factors of NO3--N content. 【Conclusion】The soil NO3--N in the tailing mainly originated from nitrification. Four years after mining, a large amount of NH4+-N remained in the tailing, and the NO3--N generated by nitrification was continuously released into the environment. In the long term, the soil NH4+-N enriched in tailing will be transformed into NO3--N and the NO3--N will migrate with water, threatening the ecological environment and human health. This study can provide a theoretical basis and scientific reference for the assessment and treatment of soil and downstream water pollution in rare earth in-situ leaching sites.
XU Zhe, YANG Jinling, ZHAO Yue, ZHANG Ganlin. Study on the Characteristics of Soil Nitrate Nitrogen Accumulation of Rare Earth Tailing in Southern Jiangxi[J]. Acta Pedologica Sinica,2024,61(3).Copy