Changes in soil quality over time focusing on organic acid content in restoration areas following coal mining

Coal extraction is associated with the creation of waste dumps, in which the soils are extremely depleted owing to dilution of the existing topsoil by mixing with the subsoil; however, the evolution mechanism of such soils re-mains unclear. Here, we investigated the changes in the soil properties over time, focusing on organic acid content in restoration areas following coal mining. The soil of three soil depths (0-10, 10-20, and 20-30 cm) from natural land and three waste dumps with primary succession ages of 5 (R5), 10 (R10), and 20 (R20) years in the Heidaigou open-pit coal mine of Northwest China, were analyzed to examine changes in the soil properties and to highlight relationships between these properties. Results showed soil pH, total nitrogen, alkali -hydrolyzable nitrogen, available phosphorus, soil organic matter, and organic acid increased with increasing restoration age. However, only soil organic matter and organic acid were influenced by soil depth and the interaction of restoration age and soil depth. Moreover, the total phosphorus content did not alter with the restoration age, soil depth, or their interactions. In contrast, the N:P ratio increased with increasing restoration age, indicating that the total phosphorus content might restrict restoration. The alkali-hydrolyzable nitrogen was significantly correlated to available phosphorus, acetic acid, soil organic matter, and pH. The available phos-phorus was significantly correlated to the oxalic acid, while the pH and soil organic matter significantly influ-enced the acetic acid. Our findings provide theoretical support to improve the soil quality and may assist in restoration in coal dam areas.

Automated summary

This study investigated the changes in soil properties in restoration areas following coal mining in Heidaigou open-pit coal mine of Northwest China. The results showed that soil organic matter and organic acid content increased with increasing restoration age, while total phosphorus content remained unchanged. Alkali-hydrolyzable nitrogen was significantly correlated with available phosphorus, acetic acid, soil organic matter, and pH. These findings provide theoretical support for improving soil quality and restoration in coal dam areas.