Vegetation restoration drives a shift from plant-derived to microbial-derived soil organic carbon accumulation in a semi-arid sandy land in China

Vegetation restoration in desertified regions enhances soil organic carbon (SOC) sequestration, but the relative contributions of plant-derived (PDC) and microbial-derived carbon (MDC) to this accrual remain largely unquantified in sandy lands. We used lignin phenol and amino sugar biomarkers to investigate SOC component dynamics across a five-stage restoration gradient from mobile dunes to sparse forest-grassland (aboriginal vegetation) in Horqin Sandy Land of northeastern China. Analysis showed that vegetation restoration significantly promoted SOC, PDC, and MDC accumulation. Crucially, SOC composition shifted dramatically; the relative contribution of PDC decreased from 58% to 14%, while that of MDC increased from 3.85% to 33%, identifying MDC gains as the primary driver of SOC accrual in later successional stages. Although the microbial community shifted towards bacterial dominance, a lower fungal-to-bacterial (F:B) phospholipid fatty acid (PLFA) ratio, fungal necromass consistently outweighed bacterial necromass, underscoring the dominant role of fungi in forming persistent necromass carbon. Random Forest model identified total PLFAs, soil texture (silt and clay content), and aboveground biomass as key predictors of SOC composition. Furthermore, structural equation modeling revealed that the PDC contribution was directly governed by the microbial F:B ratio, whereas the MDC contribution was controlled by the soil C:N ratio, with plant biomass and soil texture as indirect drivers. Our findings reveal a functional shift towards an MDC-driven SOC accrual pathway during sandy land restoration. Thus, SOC sequestration strategies must move beyond maximizing plant inputs to foster soil conditions promoting the microbial conversion of plant C into stable necromass, particularly persistent fungal carbon.