Distinct Roles of Forest Stand Types in Regulating Soil Organic Carbon Stability Across Depths

Soil organic carbon (SOC) is the largest reservoir of terrestrial organic carbon and plays a pivotal role in regulating global climate dynamics. And there are some differences in SOC stocks under different forest stand types. But it is unclear whether this phenomenon is related to SOC stability, especially stable components of SOC. Therefore, coniferous (Pinus tabuliformis), broad-leaved (Quercus aliena), and mixed forests were selected to explore the distributions and chemical structures of SOC components, as well as SOC stabilization mechanisms. Higher SOC contents but lower stability were observed under Quercus aliena forests. Contents of SOC and its components were lowest under Pinus tabuliformis forests. Yet the highest relative abundances of alkyl and aromatic carbon in mineral-associated organic carbon (MAOC) were found at 10–40 cm soil layers, with the highest MAOC/SOC. In contrast, MAOC/SOC was highest under mixed forests at 0–10 cm layer. Total nitrogen (TN), lignin, and silt contents were identified as key drivers of SOC stability. These findings indicated that mixed forest contributes more to enhancing SOC stability in topsoil, whereas coniferous forest promotes greater stability in subsurface layers. These results suggested that the functional complementarity among forest stand types may enhance carbon sequestration and promote the sustainability of forest management.