Effect of Vegetation Succession on Organic Carbon, Carbon of Humus Acids and Dissolved Organic Carbon in Soils of Copper Mine Tailings Sites

Carbon of humus acids (HSAC) and dissolved organic carbon (DOC) are the most active forms of soil organic carbon (SOC) and play an important role in global carbon recycling. We investigated the concentrations of HSAC, water-soluble organic carbon (WSOC), hot water-extractable organic carbon (HWOC) and SOC in soils under different vegetation types of four copper mine tailings sites with differing vegetation succession time periods in Tongling, China. The concentrations of HSAC, WSOC, HWOC and SOC increased with vegetation succession. WSOC concentration increased with the accumulation of SOC in the tailings, and a linearly positive correlation existed between the concentrations of HSAC and SOC in the tailings. However, the percentages of HSAC and DOC in the SOC decreased during vegetation succession. The rate of SOC accumulation was higher when the succession time was longer than 20 years, whereas the speeds of soil organic matter (SOM) decomposition and humification were slow, and the concentrations of HSAC and DOC increased slowly in the tailings. The percentage of carbon of humic acid (HAC) in HSAC increased with vegetation succession, and the values of humification index (HI), HAC/carbon of fulvic acid, also increased with the accumulation of HSAC and SOC in soils of the tailings sites. However, the HI value in the each of the tailings was less than 0.50. The humification rate of SOM was lower than the accumulation rate of SOM, and the level of soil fertility was still very low in the tailings even after 40 years of natural restoration.