Soil acidification in a tailing area of ionic rare earth in Southeast China

Ionic rare earth ores are now commonly mined using the ammonium sulfate in situ leaching method, causing soil acidification in tailings. To evaluate the degree of soil acidification in tailings and the influence of mining activities on acidification, we selected an ionic rare earth tailing and a nearby unmined area in Southeast China. This tailing had been closed for 12 years. We sampled the soil from the surface to the bedrock in layers and determined soil properties related to soil acidification. The results showed that the average soil pH was 5.0 in the unmined area and 4.5 in the mined area (tailing area). Rare earth mining led to a decrease in soil pH of 0.47 units per 10 years, which was 2-5 times higher than that of other land uses. The shallow soil acidification in the mined area is not affected by mining. Deep soils were significantly acidified and the H⁺ concentration in the soil solution was approximately nine times that of the unmined area soil. Deep soil acidification is influenced very little by natural factors. The average soil ammonium‑nitrogen (NH₄⁺-N) and nitrate‑nitrogen contents in the mined area were 58.34 mg kg^-1 and 8.19 mg kg^-1, respectively, 84 times and 21 times that of the unmined area. There were large amounts of NH₄⁺, NO₃^-, and H⁺ in the soil of the mined area, indicating that soil acidification is closely related to exogenous NH₄⁺-N input and nitrogen transformation. Nitrification is the most important driver of soil acidification in mining areas. Continued nitrification of excess NH₄⁺-N will continue to produce H⁺ and migrate with water, which will cause long-term harm to the soil and surrounding environment in the mining area. Therefore, it is necessary to remove the enriched NH₄⁺-N in tailings soil to avoid further soil acidification.