[Objective] Rare earths are resources of great strategic significance for national economy. With rapid development of the science and technology, the demand for rare earths is increasing dramatically. However, the environmental pollution caused by mining of rare earth ores has aroused concerns of related countries. In South China, in mining ionic rare earth ores, the technology of in-situ leaching with a large volume of (NH₄)₂SO₄ is commonly used, causing severe chemical pollution of the soil and water environments in the mining areas, especially the pollution of ammonium nitrogen residue in ore tailings. However, the mechanism of ammonium nitrogen changing along with the depth in soil profiles (below 200 cm) of the soil in the rare earth mining areas is still unknown.[Method] Here in this study, a total of 157 soil samples were collected systematically from the upper and down layers of the soil in a hill of ore tailings typical of the Zudong mining area, in South Jiangxi. The mining operation started in 2003 and ended in 2007. The soil samples were analyzed for soil mass moisture content, pH, and ammonium nitrogen content.[Result] Results show:1) serious soil acidification and high concentration of ammonium nitrogen residue were the main environmental pollution problems stemming from the mining activities; 2) after the mining had been closed for 12 years, the mean soil pH followed a decreasing order of raw ore (5.73±0.17) > lower reaches (4.87±0.26) > washing tailing (4.63±0.16) > tailing (3.87±0.32); and the content of soil ammonium nitrogen was 60-204.3 mg·kg^-1, about 12-40 times the background value of the virgin soil; 3)The high concentration of soil ammonium nitrogen has posed great environmental risk; and 4)as soil pH and soil water content are two main factors affecting adsorption and desorption of ammonium nitrogen, in-situ washing is an effective method to remove residual ammonium nitrogen. The higher the soil pH and the more the water flowing through the tailings, the more ammonium nitrogen desorbed from the soil.[Conclusion] All the findings in this study are expected to be able to serve as technical support for exploring mechanism of ammonium nitrogen migration in tailings and management of the pollution. However, up to now, little has been reported about soil conditions affecting ammonium nitrogen migration and transformation in the literature. Further study should be done on mechanisms of interactions between soil texture, particle size gradation, porosity, and permeability in the tailing soil.