Depth-dependent mechanisms underpinning soil organic carbon accumulation following afforestation

Abstract Afforestation is crucial for soil organic carbon (SOC) sequestration, yet the depth-specific mechanisms, particularly the roles of lignin and microbial necromass carbon (MNC) in SOC accumulation, are not well elucidated. Understanding these processes is vital for optimizing carbon management strategies. To address this, we conducted a paired-site study in a subtropical karst region of southwest China, comparing 14 maize fields with adjacent ∼20-year-old plantation forests. Soil lignin, MNC, SOC, and related edaphic variables were quantified in topsoil (0–15 cm) and subsoil (30–45 cm) to determine the impacts of afforestation. Results showed that afforestation significantly increased SOC, lignin, and MNC in both soil layers. Topsoil lignin and MNC contents rose by 74.5% and 64.3% (p < 0.001), respectively. While subsoil SOC also increased, the relative importance of lignin and MNC to SOC accumulation was lower than in topsoil. Our analysis suggests that mineral protection, particularly by exchangeable calcium and magnesium, was the dominant SOC stabilization pathway, responsible for 45% of SOC variance in the subsoil. Our findings reveal distinct, depth-dependent pathways for SOC accumulation following afforestation: in topsoil, accumulation was primarily driven by biotic inputs, especially microbial necromass, while mineral protection, through mechanisms like cation bridging and aggregation, was crucial in subsoil. These insights advocate for depth-differentiated management, such as selecting tree species that enhance topsoil microbial activity and prioritizing sites with high mineral content for subsoil carbon stabilization, to maximize SOC sequestration following afforestation in the karst regions.