Physical and chemical characteristics of substrate material for plant growth on coal wastes in rangelands of Kiasar, Sari, Mazandaran province

Mining activities lead to the degradation of rangeland ecosystem structures and functions. Coal mining in rangelands generates substantial quantities of waste dumps, which serve as the primary environmental pollutants in the area and its surrounding ecosystems. Vegetation restoration in these mining areas can help reduce soil pollution and contribute to the overall landscape restoration. The aim of this study was to assess the physical and chemical properties of abandoned coal wastes in the rangelands of Kiasar, Mazandaran province, Iran. This information can be utilized to promote vegetation restoration on coal mine wastes by identifying the factors that limit plant growth and establishment on such substrates.

This study was conducted at the Kiasar coal mine in Sari, Mazandaran province, Iran. The mine site was established in 1983. The area is located at an elevation of 1030 m above sea level, with an average rainfall of 520 mm and a mean temperature of 13.2°C. A total of 10 coal waste dumps were abandoned in the area over the last three decades. Six age classes were identified based on the time since abandonment of these waste dumps in the surrounding area of the mine site. The identification process involved the assistance of experienced engineers and local workers. The age classes were categorized as follows: 4-6, 8-12, 15-18, 19-21, 22-24, and 25-27 years since abandonment. The control site consisted of natural rangelands around the mine, which are currently used for livestock grazing. Waste and soil samples were collected using a random-systematic method from a depth of 0-20 cm. The samples were then placed in plastic bags for subsequent chemical analysis. The soil samples were air-dried at room temperature, and various parameters, including texture, moisture, pH, EC, OC, CaCO3, N, P, K, Cu, Fe, Cd, Ni, and Pb, were measured in the laboratory. Data were analyzed using one-way ANOVA, and means were compared using Duncan multiple tests.

The analysis of variance indicated that all measured parameters of soil and coal, except nitrogen and potassium, exhibited statistically significant differences among the coal wastes and the nearby rangelands. The coal wastes had lower percentages of silt, clay, moisture, and lime compared to those in the surrounding rangelands. However, these properties did not show significant differences among the various abandoned coal wastes. Most of the abandoned wastes were characterized as neutral or alkaline, with an electrical conductivity (EC) less than 1.5 dS/m. An exception was found in the waste that was abandoned 22-24 years ago, which exhibited acidic pH

In conclusion, our study revealed that moisture availability is a crucial limiting factor for plant growth on coal wastes. This limitation can be attributed to the sandy and sandy-loamy texture, unstable slope of coal dumps, and high porosity of the substrate. Moreover, coal waste undergoing erosion and leaching exhibited high electrical conductivity (EC) and low pH levels. The levels of organic matter and available phosphorus were found to be insufficient for supporting optimal plant growth. However, the concentrations of heavy metals remained within the normal range, even after a significant period since the abandonment of coal wastes. We recommend the implementation of remediation treatments in similar restoration projects to enhance plant growth and establishment.