Long-term effects of thinning on soil organic carbon fractions and carbon pool management indices in secondary forests of heavily burned areas.

Despite decades of recovery, soil carbon in heavily burned areas has failed to reach pre-fire levels. It is unclear whether stand management practices can promote soil organic carbon accumulation at such sites. This study evaluated the changes in soil labile organic carbon (LOC) fractions (including dissolved organic carbon (DOC), microbial biomass carbon (MBC), and easily oxidizable organic carbon (EOC)) and the carbon pool management index (CPMI) after the thinning of a heavily burned area in the Daxing'an Mountains and selected sample plots. This study compared thinned birch secondary forests (17 years (17a-S), 14 years (14a-U), 2 years (2a-S) post-thinning, where 17a-S and 2a-S were strip thinned and 14a-U was uniform thinned) with unthinned control (CK) plots. The contents of soil LOC and CPMI at a depth of 0-10 cm were found to increase with thinning, indicating that thinning promoted the accumulation of soil organic carbon in secondary forests in heavily burned areas. The two-way ANOVA showed that the differences in C fractions and CPMI at different times after thinning were significant, whereas the differences between thinning methods were not significant. In comparison to CK, only the DOC content was found to be significantly elevated at 2a-S. However, at both 14a-U and 17a-S, the elevation of the LOC fraction content reached a significant level. Among them, 14a-U demonstrated the most pronounced improvement (DOC (+11.37%), MBC (+42.80%), and EOC (+19.51%)). The CPMI at the 0-10 cm depth also increased significantly (18.20% ∼ 27.77%) at 14a-U. The study revealed that soil bulk density and understorey vegetation biomass were the main influences on the changes in soil LOC fractions and CPMI post-thinning. This finding also indicates that greater attention should be given not only to the soil itself but also to the understorey vegetation during forest soil carbon restoration under conservation management.