Lignin–microbial necromass carbon coupling drives the vertical stratification mechanism of deep soil carbon sequestration in subtropical forests

Abstract Forest restoration and its capacity to enhance soil carbon sequestration constitute critical mechanisms for augmenting global soil carbon pools. However, the distinct regulatory roles of plant‐derived versus microbial‐derived carbon in restoration‐driven carbon sequestration remain poorly understood. Here, we collected whole‐profile soils from over 180 years of forest restoration sequences, including coniferous, mixed, secondary and old‐growth forests. We measured soil microbial necromass carbon (MNC) and lignin phenols to identify the key factors that influence the variation of carbon sequestration in surface (0–20 cm), subsurface (20–60 cm) and deep soil (60–100 cm). We systematically revealed depth‐dependent soil organic carbon (SOC) dynamics: Subsurface and deep SOC accumulation contributed disproportionately to total SOC stock increases during restoration, supplementing the conventional surface‐centric perspective. Through coupled measurements of MNC and lignin phenols, we uncovered divergent transformation pathways—while surface soils exhibited declining plant‐derived carbon proportions, microbial‐derived carbon progressively dominated deeper layers (27.4%–53.4% of SOC in late‐stage restoration). This microbial dominance is driven by depth‐specific conditions (nutrient scarcity, mineral protection) rather than being universal across all soil depths. Crucially, the coupling of lignin degradation and microbial assimilation in deep soils enhanced MNC stabilization, providing empirical evidence for the ‘microbial carbon pump’ theory through vertically resolved lignin–MNC linkages. Mechanistically, lignin distribution correlated with microbial‐mediated liberation and vertical transport, whereas depth‐specific nutrient‐microbe interactions governed MNC accumulation. The proportion of microbial‐derived carbon positively predicted SOC stocks in subsurface and deep soils ( R 2 = 0.63–0.80), contrasting with negative correlations of plant‐derived carbon in surface layers. Synthesis and applications . These findings establish a tripartite framework for precision restoration: (1) prioritizing subsurface carbon sequestration by alleviating nutrient limitations in deep soils, (2) steering plant–microbe‐derived carbon coupling through tree species mixtures that balance lignin inputs and microbial turnover and (3) optimizing vertical carbon allocation by synchronizing root traits with microbial stoichiometric needs. Our work redefines restoration strategies to harness depth‐specific biogeochemical processes, offering a pathway to maximize SOC sequestration across the whole soil profile.