Towards a Sustainable Use of Cover Soil for Agricultural Reclamation of Coal Mining Waste and Fly Ash Deposits -A case study at the hard coal mining region of Huainan, China

Hard coal mining is the dominating industry in the study area of Huainan (Province Anhui, China). As a consequence, subsidence of the surface takes place. Besides settlement area, flooding of the former terrestrial area mainly affects farmland. As a countermeasure, reclamation is done by depositing coal mining waste (CMW) and fly ash (FA) from the local coal mines and power plants. These substrates are afterwards covered with local natural loamy soil to prepare the sites for agricultural use. To ensure sustainable and economic application of the cover soil, research is done to find out about the optimal range of cover soil thickness for wheat biomass production. Two sites were selected where cover soil was underlain by substrates consisting of CMW (Datong, Xinzhuangzi) and FA (Shangyao A & B), respectively. The size of test sites ranged between 1.3 to 1.9 ha. The weight of wheat biomass was measured after harvesting I m(2) test fields (n = 9-27 test fields per test site). The test fields were placed at different cover soil thicknesses. Push probe and soil profile sampling enabled the description of cover soil thicknesses and horizon specific properties down to 1 m depth. The push probe soil surveys of the test sites resulted in an overall range of cover soil thickness of 25 cm to 90 cm. Four thickness classes were delimited: <= 40 cm (low), 41-55 cm (medium), 56-70 cm (high) and > 70 cm (very high). The wheat biomass ranged between 5 t ha(-1) and 27 t ha(-1) revealing significantly higher wheat biomass at Xinzhuangzi compared with the other test sites. On the contrary, wheat biomass at Shangyao B was significantly the lowest. Generally, wheat biomass increased with increasing cover soil thickness at all test sites. In detail, no benefit was evident if applying more than 60 +/- 5 cm cover soil on the CMW test sites and 45 +/- 5 cm on FA test sites. This finding was derived from significant differences between the low and medium class in all cases. Simultaneously, medium and high cover soil thickness only partly showed significant differences. Further research should include additional test sites to testify the findings. Moreover, investigations on the relevant processes and soil properties are recommended. This will provide further insight into the relevant factors which influence the wheat biomass production. Finally, the findings on the optimal cover soil thickness can be modified.