Important role of Pinus massoniana mixed forests in enhancing soil carbon stocks in degraded forests in southern China

Forest degradation, including low-function forest (LFF), native forest destruction or transformation (NFDT), and subsequent soil erosion (SE), has occurred. Historically, single-species plantations rather than mixed forests have been used to offset forest losses. However, the important role of Pinus massoniana mixed forests (PMFs) at degraded sites in enhancing soil organic carbon (SOC) stocks in southern China remains untested. Our meta-analysis included 1840 paired observations from 136 studies. The results demonstrated that converting P. massoniana pure forests (PPFs) to PMFs at degraded sites increased SOC stocks (0–100 cm) by 46.55%, which was 7.03 times greater than that at nondegraded sites. The PMFs at the SE sites sequestered significantly higher SOC stocks (2.40 and 4.35 times) than did those at the LFF and NFDT sites. At the LFF and SE sites, SOC stock increases were attributed to significant increases in litter biomass, root biomass, tree biomass, and tree OC stock. At the SE sites, converting bare lands to PMFs increased SOC stocks (0–100 cm) by 274.30%. At the NFDT sites, converting natural evergreen broadleaved forests (NEBFs) to PPFs significantly decreased the SOC stocks. However, SOC stocks remained unchanged after the conversion of NEBFs to PMFs. At the NFDT sites, the conversion of PPFs to PMFs increased SOC stocks by significantly increasing the contribution of soil microbial residue accumulation and microbial residue-carbon to SOC. The transformation of NEBFs into PMFs and PPFs significantly reduced soil microbial residue retention and microbial residue-carbon. Nevertheless, only PPFs significantly decreased the proportion of microbial residues in SOC to reduce SOC stocks. Together, transforming PPFs to PMFs in degraded areas significantly enhances SOC stocks but needs plant species richness not exceeding 20. Thus, PMF cannot replace NEBF, and NEBF requires adequate protection.