Microbial necromass dominates particulate and mineral‐associated organic carbon accumulation in calcareous soil following afforestation

Abstract Soil organic carbon (SOC) dynamics are a critical regulator of the global carbon cycle and climate feedbacks. Afforestation is widely recognized as a key nature‐based climate solution for enhancing SOC sequestration, but its impact on the dynamics of soil lignin and microbial necromass carbon, and their roles in particulate (POC) and mineral‐associated (MAOC) organic carbon accumulation remains poorly understood. This study selected 14 pairs of maize fields and adjacent plantation forests in a karst region of southwest China, where soils are predominantly calcareous. By combining lignin phenols and amino sugars as biomarkers with multiple biotic and abiotic soil variables, the mechanisms governing soil POC and MAOC accumulation upon afforestation were investigated. Afforestation substantially promoted soil carbon sequestration, resulting in 265% and 136% increases in POC and MAOC, respectively, compared with maize fields. Soil microbial necromass carbon increased by 224% in POC and 96% in MAOC, exceeding lignin increments (100% in POC; 66% in MAOC) upon afforestation. Structural equation modelling demonstrated three synergistic pathways of POC and MAOC following afforestation, that is, enhancing lignin accumulation via increased plant residue input and reduced lignin oxidation; promoting microbial necromass carbon accumulation driven by rising microbial biomass; and strengthening Ca‐mediated mineral protection. However, multiple lines of evidence corroborated that microbial necromass carbon, not lignin, was the dominant driver of both POC and MAOC accumulation upon afforestation. Synthesis and applications . Our study refines the conventional dual‐pathway framework of organic carbon formation by focusing on calcareous soils, and propose that microbial necromass, as a common precursor of soil POC and MAOC, drives their coupled accumulation following afforestation on cropland. This finding emphasizes the necessity of integrating microbial‐mediated pathways for both POC and MAOC sequestration into Earth systems models, conducive to improving the predictive accuracy of SOC dynamics under global changes. Furthermore, afforestation strategies in karst regions should also prioritize tree species traits that promote soil microbial anabolism (e.g. high root exudation) and calcium retention to maximize the regional carbon sink potential.