Vegetation restoration enhances soil organic carbon accumulation in Southwest China’s karst region: The role of aggregation, calcium, and microbes

Vegetation restoration promotes soil organic carbon (SOC) accumulation through physical aggregate protection, mineral chemical stabilization, and microbial-driven mechanisms. However, the quantitative contributions of these three factors to SOC stocks, including their interactions and influences, remain poorly understood. This study investigated the influence of soil aggregate fractions: large macroaggregates (>2 mm), small macroaggregates (2–0.25 mm), microaggregates (0.25–0.053 mm), and silt + clay fractions (<0.053 mm), aggregate-associated organic carbon (OC), minerals, and microbial variables on SOC stocks and their relative contributions following cropland restoration. Soil samples were collected from depths of 0–10 cm across the croplands and two cropland restoration systems: a secondary forest and a plantation forest. The four aggregate-associated OC fractions significantly contributed to SOC stocks (p < 0.05). Variations in each aggregate-associated OC fraction were linked to changes in the soil aggregate fractions, exchangeable calcium (Ca), bacterial abundance, and community compositions. In addition, aggregates, minerals, and microbes explained 90.1 % of the variance in the SOC stocks. The SOC stock was primarily influenced by the interactive effects of aggregates, microbes, and minerals (34.2 %), aggregates alone (27.2 %), and aggregate–microbial (22.3 %) interactions. Our findings underscore the critical role of soil bacterial variables in influencing aggregate-associated OC and stabilizing SOC, whereas soil-exchangeable Ca plays a significant and indispensable role in mineral protection and the regulation of SOC stocks. These results highlight that the SOC protection and stabilization mechanisms following vegetation restoration arise from the interplay and interdependence of physical aggregate protection, soil mineral binding, and microbial metabolic processes in this karst region.