Effects of recultivation on soil organic carbon sequestration in abandoned coal mining sites: a meta-analysis

Opencast coal mining results in high loss of soil organic carbon (SOC), which may be restored via recultivation. Common strategies include liming, topsoil application, and phytoremediation. It remains unclear, however, which parameters determine the effectiveness of these varying recultivation strategies especially regarding SOC sequestration. This meta-analysis analyses the effect of varying recultivation strategies on SOC sequestration under different climate and soil conditions (pH, texture, depth) as well as in relation to time, based on 404 data entries from 51 studies. All included climatic regions recorded increases in SOC stocks, with tropical soils showing the highest potential for relative gains at up to 637%. We demonstrate that loamy soils sequester twice as much newly introduced SOC than sand. Strategy-wise, the highest mean rate of SOC sequestration is achieved by forest after topsoil application (3.9 Mg ha^-1 a^-1), agriculture after topsoil application (2.3 Mg ha^-1 a^-1), and agriculture with topsoil and fertiliser application (1.9 Mg ha^-1 a^-1) with a response ratio of 304%, 281%, and 218%, respectively. Soils analysed to less then 40 cm depth show higher SOC sequestration rates (< 10 cm: 0.6 Mg ha^-1 a^-1, < 20 cm: 1.0 Mg ha^-1 a^-1, and 20-40 cm: 0.4 Mg ha^-1 a^-1; response ratio of 123%, 68%, and 73%, respectively) than those analysed to a depth of 41-80 cm (0.1 Mg ha^-1 a^-1; response ratio of 6%). In terms of pH, strongly acidic soils (pH < 4.5) and alkaline conditions (pH > 7) offer the most beneficial environment for SOC sequestration at 0.4 Mg ha^-1 a^-1 and 0.8 Mg ha^-1 a^-1, respectively (185% and 273% response). Given comparable SOC sequestration potentials of forest after topsoil application, agriculture without amendments, and forest without amendments, we recommend to weigh these strategies against each other. Potentially decisive aspects are short- vs. long-term economic gains, food security concerns, and-in case of agriculture-the risk of overintensification leading to losses in SOC. Our data suggests that amendments exert considerable influence on SOC sequestration and need to be introduced under careful consideration.