Abstract:
Soil acidification has become a significant factor constraining the sustainable development of agricultural soils and the safety of the ecological environment. In the acidic soils of southern China, the dual challenges of soil acidity and fluoride contamination commonly coexist. Although amendments applied to remediate soil acidity and those used to mitigate fluoride contamination share similarities, there is currently a lack of amendments highly effective in simultaneously addressing both issues. Therefore, this study aimed to evaluate and identify effective inorganic amendments capable of concurrently remediating soil acidity and fluoride contamination in acidic soils. Eleven representative inorganic amendments, including minerals, industrial by-products, and commercial materials rich in calcium and magnesium, were selected for this study. Their effectiveness was investigated through indoor incubation experiments assessing their capacities to neutralize soil acidity, reduce soluble fluoride levels, and enhance soil nutrient availability. The soil quality index (SQI) was employed as the quantitative parameter to evaluate their comprehensive efficiency in decreasing soil acidity and soluble fluoride levels, while increasing the mineral nutrient contents. The results indicated these inorganic amendments could reduce soil acidity to varying degrees, with their ameliorative effects primarily determined by their acid neutralizing capacity. Notably, the pH of amendments, serving as an intensity indicator of their acidity or alkalinity, cannot accurately reflect their acid-neutralizing capacity. The primary components contributing to the neutralization of soil acidity mainly included carbonates such as calcite, dolomite, and easily dissolved silicates and phosphates minerals. These amendments not only increased the soil pH, but also significantly reduced the contents of soluble aluminum, exchangeable aluminum, and soluble fluoride. Additionally, using a 0.01M CaCl
2 solution for measuring soil pH eliminated ionic strength effects, offering a more accurate reflection of amendments' actual effectiveness in alleviating soil acidity. The effectiveness of inorganic amendments in mitigating fluoride contamination depended on their ability to increase soil pH and was further influenced by their soluble salt contents. Elevating the soil pH toward neutral facilitated the coprecipitation of soluble fluoride with calcium or magnesium ions supplied by the amendments, thereby effectively reducing fluoride availability. Nevertheless, amendments with soluble sulfates and chlorides, and particularly fluoride, were not effective to decrease soluble fluoride concentrations and even potentially promote the formation of soluble aluminum-fluoride complexes, which would intensify the environmental risk of aluminum toxicity. Furthermore, all amendments increased the availability of soil mineral nutrients, such as potassium, calcium, magnesium, and phosphorus to different degree, and these nutrient increments were dependent on the corresponding nutrient contents in the amendments. A comprehensive evaluation based on the SQI values revealed limestone, weathered coal, wollastonite, silicon-calcium-magnesium conditioners, and dolomite as particularly effective in remediating fluoride-contaminated acidic soils. Conversely, attapulgite, biomass ash, alkali slag, and especially the commonly used calcium magnesium phosphate fertilizer were unsuitable for remediation purposes in fluoride contaminated acidic soils. The findings obtained from this study provide valuable technical guidance for the effective management of multiple constraints—including acidity, aluminum toxicity, and fluoride contamination—in acidic soils, thereby promoting soil quality restoration and sustainable agricultural development.