Response of soil organic carbon fractions to exogenous carbon input in mine reclamation

Studying the effects of exogenous carbon input on soil organic carbon fractions is important for understanding changes in soil organic carbon and fertilization management in the reclamation of mining areas. Based on a long-term experiment in a coal mining subsidence reclamation area in Xiangyuan County, Shanxi Province, China, this paper explored the response characteristics of the carbon fractions of reclaimed soil to the input of different exogenous carbon sources (biochar, compost, biogas residue, cow manure, and straw). Reclamation soil samples (0-20 cm) were collected in 2011 and 2016. The organic carbon, easily oxidized organic carbon, and labile carbon pool Ⅰ/Ⅱ in soil were measured. The results showed that the growth rate and the annual change of soil organic carbon with the biochar treatment increased by 101.80% and 0.56 g·kg^-1·a^-1, respectively, compared with the no-addition control (CK), and the increases with biochar were significantly higher than those with other organic material treatments. Compared with CK, the amounts of carbon sequestration in the biochar and cow manure treatments increased by 100.52% and 91.52% in the 0-20 cm soil layer, respectively; and the amounts of carbon sequestration in the biochar and cow manure treatments were significantly higher than those of other organic material treatments. There were no significant differences among composting, biogas residue, and straw treatments in promoting soil carbon sequestration. The addition of organic materials significantly increased the growth rate and annual change of easily oxidized organic carbon in reclaimed soil; the highest value was observed in the compost treatment, with an increase of 12.37% and 0.16 g·kg^-1·a^-1, respectively, compared with CK. The addition of cow dung significantly increased the growth rate and annual change of the labile carbon pool Ⅰ/Ⅱ, with a greater effect than other organic materials in reclaimed soil. The addition of organic materials also improved the stable organic carbon content of the soil. Compared with CK, the biochar and cow manure treatments showed the greatest improvement in the stable organic carbon content, with a significantly higher contribution than those of other organic material treatments. However, there was no significant difference between cow manure and biochar treatments. The carbon management index of biochar treatment was 36.30%, 52.23%, 41.50%, and 52.02% higher than that of composting, biogas residue, cow manure, and straw treatments, respectively. The application of all the exogenous carbon sources significantly improved the content of carbon fractions and the carbon management index of reclaimed soil. The application of biochar had the best effect. This indicates that biochar can be used as an effective management measure to improve the soil organic carbon of reclaimed mining areas.