Conversion of SIC to SOC enhances soil carbon sequestration and soil structural stability in alpine ecosystems of the Qinghai-Tibet Plateau

Soil organic carbon (SOC) and soil inorganic carbon (SIC) possess an underlying dynamic interrelationship driven by abiotic and microbial factors with important effects on soil structure and carbon sequestration. However, these dynamics remain poorly understood across different aggregate scales. This study investigated the spatial distribution of SOC, SIC, microbial biomass, enzymes, and community composition within aggregates in different vegetation types, slope positions, and slope aspects in the alpine terrestrial ecosystem of the Qinghai-Tibet Plateau. Small aggregate size classes (mainly <0.053 mm) generally showed the lowest microbial activity, while dissolved organic carbon (DOC), SIC, and pH exhibited the opposite pattern. Bacterial diversity was greater than fungal diversity and less sensitive to environmental factors, while dominant phyla abundances were mostly influenced by slope position. The effect of aggregate size on community structure was unevenly distributed. The organic carbon turnover efficiency in the alpine mountains followed as alpine meadow on a sunny slope (MS) > alpine wetlands on the riverside (WR) > alpine shrubs on a shady slope (SS), and macroaggregates > microaggregates > silt + clay fraction. The pH was the major abiotic regulator of the microbially-driven organic–inorganic carbon dynamics within soil aggregates, facilitating the shift from SIC to SOC by inhibiting enzyme activity, reducing bacterial community abundance and diversity, and decreasing fungal community abundance, but increasing the fungal community diversity. This led to increased total carbon stored in soil aggregates, thereby promoting macroaggregation and stability of the soil structure.