Microbial-derived carbon dominates shrub encroachment carbon loss in Inner Mongolia grasslands

Shrub encroachment in semiarid regions significantly impacts the global carbon budget. However, how shrub encroachment affects soil organic carbon (SOC) and its components, including microbial-derived C (MDC) and plant-derived C (PDC), within different soil horizons remains unclear. A space-for-time substitution approach and two biomarkers (amino sugars and lignin phenols) were employed to examine the effects of shrub encroachment on SOC and its components, along with their driving factors, in the semiarid grasslands of northern China. The analysis revealed that shrub encroachment led to a substantial reduction in SOC stocks, with reductions ranging from 44.2% to 82.7% in the topsoil and 15.3% to 62.1% in the subsoil compared to grassland. This decline was driven primarily by reductions in MDC and PDC, with MDC alone explaining 48.43-59.17% (topsoil) and >65% (subsoil) of the total decrease. Moreover, mineral protection emerged as the primary factor influencing MDC and PDC in topsoil and subsoil. This study highlighted the important role of MDC in SOC vulnerability and emphasized the necessity of preventing shrub encroachment to safeguard SOC stocks in semiarid grasslands.