Study on the Characteristics of Soil Nitrate Nitrogen Accumulation of Rare Earth Tailing in Southern Jiangxi
[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 NO 3--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 NO 3--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 content of topsoil NO 3--N was 2.89-6.75 mg·kg-1,which was similar to that of natural soil.Also,the content of NO3--N in the soil below surface layer was significantly higher than that in the natural soil.The soil NO 3--N of the ore-bearing layer in the deep profile was higher than that of the top layer.Moreover,the distribution of NO 3--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.NH 4+-N content dominated the generation of NO 3--N and determined the upper limit of soil NO 3--N accumulation.The accumulation degree of NO 3--N in different soil layers and different parts of mountains was controlled by rainfall leaching and the NO 3--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 NO 3--N content,and were not the determining factors of NO 3--N content.[Conclusion]The soil NO 3--N in the tailing mainly originated from nitrification.Four years after mining,a large amount of NH 4+-N remained in the tailing,and the NO 3--N generated by nitrification was continuously released into the environment.In the long term,the soil NH 4+-N enriched in tailing will be transformed into NO 3--N and the NO 3--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.
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