Soil acidification enhances soil carbon sequestration through increased mineral protection. This study investigated how acid deposition affects soil organic carbon (SOC) and its two fractions: particulate organic carbon (POC) and mineral-associated organic carbon (MAOC). A 11-year field experiment showed that acid addition significantly increased both POC (0–20 cm) and MAOC (10–20 cm). Acid addition reduced the contribution of microbial residue carbon to SOC but increased plant-derived soil carbon in both soil depths. The increase in lignin phenol and suppression of soil organic matter decomposition suggested that soil POC increased with acid addition. Soil acidification enhanced MAOC accumulation through increased lignin and mineral protection by iron-aluminum oxides and cations. The study concluded that increased mineral protection of plant-derived carbon was the dominant driver of increased SOC sequestration under acid addition. This finding identified the dominant pathway for SOC accumulation in a highly acidic subtropical forest and provides new insights into understanding plant-soil-mineral interactions under increasing acid deposition. Acid deposition can affect carbon input to soil from both aboveground and belowground plant biomass. Previous research indicated that acid deposition reduced the biomass production of above-ground plant communities, primarily due to damage to the cuticular structure of plant leaves. However, conflicting findings suggest that fine-root production or necromass accumulation may increase with soil acidity. Soil microbes play a pivotal role in both the decomposition and stabilization of SOC. Acid deposition can influence them by affecting microbial community composition, biomass and activities. Numerous studies demonstrated that acid deposition inhibited soil organic matter decomposition across diverse ecosystems. However, conflicting findings have also been reported, indicating varying responses to acid deposition in different contexts. As important sources of SOC, microbial necromass and root exudates can be stabilized through organomineral interactions and aggregation formation. Consequently, an increase in necromass due to acid deposition will likely increase SOC accumulation. To comprehensively understand the microbial effects on SOC stabilization under acid deposition, it is essential to quantify the contributions of plant-derived and microbial-derived carbon to SOC. SOC comprises POC and MAOC, which differ fundamentally in their formation, persistence, and functioning. POC, composed of partially decomposed plant materials, persists in the soil through inherent biochemical recalcitrance, physical protection in aggregates or microbial inhibition, and some microbial residues. In contrast, MAOC is primarily composed of microbial products that persist in the soil through chemical bonding to minerals or physical protection or direct adsorption of plant-derived carbon. Consequently, POC and MAOC will likely respond differently to acid deposition, collectively influencing the overall response of total SOC.Soil acidification enhances soil carbon sequestration through increased mineral protection. This study investigated how acid deposition affects soil organic carbon (SOC) and its two fractions: particulate organic carbon (POC) and mineral-associated organic carbon (MAOC). A 11-year field experiment showed that acid addition significantly increased both POC (0–20 cm) and MAOC (10–20 cm). Acid addition reduced the contribution of microbial residue carbon to SOC but increased plant-derived soil carbon in both soil depths. The increase in lignin phenol and suppression of soil organic matter decomposition suggested that soil POC increased with acid addition. Soil acidification enhanced MAOC accumulation through increased lignin and mineral protection by iron-aluminum oxides and cations. The study concluded that increased mineral protection of plant-derived carbon was the dominant driver of increased SOC sequestration under acid addition. This finding identified the dominant pathway for SOC accumulation in a highly acidic subtropical forest and provides new insights into understanding plant-soil-mineral interactions under increasing acid deposition. Acid deposition can affect carbon input to soil from both aboveground and belowground plant biomass. Previous research indicated that acid deposition reduced the biomass production of above-ground plant communities, primarily due to damage to the cuticular structure of plant leaves. However, conflicting findings suggest that fine-root production or necromass accumulation may increase with soil acidity. Soil microbes play a pivotal role in both the decomposition and stabilization of SOC. Acid deposition can influence them by affecting microbial community composition, biomass and activities. Numerous studies demonstrated that acid deposition inhibited soil organic matter decomposition across diverse ecosystems. However, conflicting findings have also been reported, indicating varying responses to acid deposition in different contexts. As important sources of SOC, microbial necromass and root exudates can be stabilized through organomineral interactions and aggregation formation. Consequently, an increase in necromass due to acid deposition will likely increase SOC accumulation. To comprehensively understand the microbial effects on SOC stabilization under acid deposition, it is essential to quantify the contributions of plant-derived and microbial-derived carbon to SOC. SOC comprises POC and MAOC, which differ fundamentally in their formation, persistence, and functioning. POC, composed of partially decomposed plant materials, persists in the soil through inherent biochemical recalcitrance, physical protection in aggregates or microbial inhibition, and some microbial residues. In contrast, MAOC is primarily composed of microbial products that persist in the soil through chemical bonding to minerals or physical protection or direct adsorption of plant-derived carbon. Consequently, POC and MAOC will likely respond differently to acid deposition, collectively influencing the overall response of total SOC.