Conversion of monoculture plantation to two-aged mixed plantation enhances soil organic carbon via increased microbial residue carbon accrual

Mixed plantations are more effective at storing soil organic carbon (SOC) than monoculture plantations. However, the accrual and stabilization of SOC are contingent upon its composition and source. Further, research on the impact of converting monoculture plantations to mixed plantations on SOC fractions and sources remains limited. We aimed to investigate the variations in SOC fractions (mineral-associated organic C [MAOC]; particulate organic C [POC]) and sources (lignin phenols and amino sugars as biomarkers), and associated biotic and abiotic factors in a monoculture coniferous plantation (used as a control) and three two-aged mixed plantations (4-, 7-, and 11-year-old stands of broad-leaved tree species planted in monoculture coniferous plantations). The conversion from monoculture to two-aged mixed plantations resulted in a greater accumulation of POC than MAOC, as well as higher accumulation rates of amino sugars (representing microbial residue) than lignin phenols (representing plant residues). The influence of soil microbial traits on the accumulation of lignin phenols and amino sugars was more pronounced than that of plant traits (root biomass) and soil nutrients (N and P availability). The presence of greater quantities of plant material inputs has been observed to stimulate microbial activity (e.g., β-1,4-glucosidase) and growth (e.g., saprotrophic fungi), which has been shown to increase microbial biomass and residue production and enhance recalcitrant lignin accumulation. Furthermore, the contribution of microbial-derived carbon to SOC in the mixed plantation (46 %) greatly increased compared with that in the monoculture plantation (28 %). These results demonstrate that the conversion of monoculture to two-aged mixed plantations greatly affects the composition and sources of SOC, leading to enhance accumulation and stabilization of SOC. To conclude, two-aged mixed plantations may serve as an effective silvicultural model for the promotion of SOC sequestration in forest ecosystems.