The stable carbon isotopes in soil organic carbon (SOC) undergo fractionation during the enrichment and decomposition processes. While several biochemical processes play a role in SOC enrichment and decomposition, their impact on the fractionation of stable carbon isotopes, particularly during initial soil formation, remains poorly understood. The sand dunes in the southern margin of the Tengger Desert in north-central China have recently stabilized due to vegetation restoration projects, creating the ideal conditions to study stable carbon isotopic fractionation during biological soil crust (BSC) formation. Samples of plant litterfall, BSCs, and subsurface sandy soils were collected from dunes restored at different years, and their SOC content and stable carbon isotope composition were analyzed. The results showed that the SOC content of BSCs and subsurface soils increased steadily since restoration and that there was significant fractionation of the stable carbon isotopes. The δ¹³C of BSCs rapidly increased with a large variation in the beginning, likely due to microbial activities dominated by cyanobacteria, and were substantially higher than that of subsurface soils. However, after only a few years of restoration, the carbon isotopic composition of BSCs and subsurface soils gradually decreased and approached that of soils under native vegetation. This highlights the critical role of microbial activities in carbon turnover during early soil formation, though the ultimate equilibrium state of SOC content and its carbon isotopic composition is determined by climate conditions.
 

drylands; microbial activity; stable carbon isotope; sandy soils; soil organic carbon; BSCs