ObjectiveRare 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.MethodWe 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.ResultThe 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.ConclusionThe 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.