Responses of soil labile organic carbon fractions and stocks to different vegetation restoration strategies in degraded karst ecosystems of southwest China

The effective restoration of degraded areas, resulting in the sequestration of significant amounts of soil organic carbon (SOC) and labile soil organic carbon fractions (LOCF), is essential for mitigating global climatic changes. It is therefore important to improve our understanding of the responses and magnitudes of SOC and LOCF during restoration processes in degraded land like karst ecosystems. In this study, we selected four different restoration strategies: (1) not subjected to any management practices, mainly bare land (BL); (2) Pinus yunnanensis plantation (PY), native and characterized by slow growth and high drought resistance; (3) Eucalyptus maideni plantation (EM), non-native and characterized by rapid growth and high adaptability; (4) natural regeneration of secondary forest (SF). The SOC concentrations, SOC stocks, sensitive indicators of LOCF (microbial biomass C (MBC), permanganate oxidizable C (KMnO4-C), dissolved organic C (DOC), water soluble organic C (WSOC)), non-labile C (NLC), and the carbon pool management index (CPMI) were investigated. The different reforestation strategies had significant effects (p < 0.05) on SOC stocks; compared with BL, the SOC stocks of SF, EM, and PY increased by 57.7, 38.0, and 26.9%, respectively. Both the LOCF and the NLC stocks were significantly increased by the different restoration strategies relative to the BL, while natural vegetation restoration was more beneficial for C sequestration than afforestation. Compared with BL, in SF, EM, and PY, the CPMI increased by 70.1, 35.8, and 32.6%, respectively. These findings suggest that although afforestation can significantly increase C sequestration, natural regeneration may be a more effective approach in terms of SOC sequestration. Soil C accumulation would be limited by nitrogen and phosphorus during the vegetation restoration in the karst areas. In degraded karst ecosystems, the CPMI could be used as a representative index in evaluating the impacts of vegetation restoration on SOC concentration and soil quality.