Construction and analysis of a method for grading long-term vegetation carbon sink in waste dumps of an open-pit coal mine

Vegetation carbon sink serves as a crucial indicator for the ecological assessment of an open-pit coal mine. The accurate inversion and grading of vegetation carbon sink play a significant role in exploring the ecological restoration of coal mines. This study proposed a method for grading vegetation carbon sink of an open-pit coal mine based on the particle swarm optimization (PSO) algorithm. Specifically, using a light use efficiency (LUE) model, this study first determined the spatio-temporal distributions of the 2005‒2020 vegetation carbon sink in the waste dumps of the Shengli No.1 open-pit coal mine through inversion based on Landsat images and meteorological data. Then, by developing objective functions and conducting iterative computation, this study ascertained the ideal LUE and optimal temperature and precipitation conditions in the coal mine, followed by the quantification and development of criteria for vegetation carbon sink grading. Finally, this study graded the vegetation carbon sink results of the waste dumps during various periods and analyzed the spatial-temporal changes in the areas of varying grades and their proportions. The key findings are as follows: (1) From 2005 to 2020, the carbon sequestration capacity of vegetation in the study area showed an upward trend, with average annual growth and growth rate of 1.43 gC/(m²·a) and 6.97%, respectively; (2) After ecological restoration of waste dumps in the coal mine, the vegetation carbon sink values increased significantly, and zones with extremely high and high carbon sink values gradually expanded; (3) The ecological restoration effects differed across various waste dumps in the coal mine, proved to be the best in southern and northern waste dumps, followed by the marginal waste dump on the margin, with the inner waste dumps exhibiting the poorest ecological restoration effects. By 2020, in southern and northern waste dumps, zones with extremely high carbon sink values covered areas of 0.94 km² and 0.92 km², respectively, accounting for 92% and 94% of the total areas of the dumps, respectively. In the marginal waste dump, zones with extremely high and high carbon sink values occupied an area of 3.64 km², representing 66% of the total area of the waste dump. In the inner waste dump, the area of zones with extremely high carbon sink values increased by 1.22 km², accounting for up to 31% of the total area. In 2020, there was no zone with extremely low carbon sink values anymore in the southern, northern, and marginal waste dumps. This study will provide both critical data for the ecological assessment of the open-pit coal mine and a scientific basis for developing ecological restoration strategies for the mining area.