Soil plant-derived carbon (PDC) and microbial-derived carbon (MDC) represent the two primary sources of soil organic carbon (SOC). The relative proportions of these carbon (C) sources shape SOC composition, accumulation, stability, and turnover. Dryland ecosystems, while serving as critical C reservoirs, are particularly vulnerable to climate change. However, the influence of aridity on PDC and MDC sequestration remains understudied at the regional scale, which limits our ability to understand and predict soil C dynamics in drylands under the context of global warming. To address this knowledge gap, we measured PDC, MDC, and associated biotic and abiotic variables at 90 sampling sites along a similar to 3000 km transect across the Mongolian Plateau. We examined the biogeographical distribution patterns of PDC and MDC along a natural aridity gradient. Our findings revealed that both PDC and MDC declined concurrently in response to increasing aridity. Based on the contributions of PDC and MDC to SOC, we identified the shifting point marking the transition in the dominance of different pathways of the microbial C pump. This shifting point was further validated using microbial C use efficiency (CUE) and soil extracellular enzyme activity. In subhumid-semiarid region (aridity = 0.37), the contribution of PDC to SOC exceeds that of MDC, coupled with higher extracellular enzyme activity, indicating the dominance of ex vivo modification. Here, PDC formation is primarily driven by the microbial fragmentation of abundant plant litter. High precipitation and nutrient availability in these areas further support the conversion of microbial biomass C into MDC. In contrast, in semiarid region (aridity = 0.78), the contribution of PDC to SOC is lower than that of MDC, coupled with higher microbial CUE, indicating the dominance of in vivo turnover. MDC accumulation is promoted by physical protection mechanisms, such as increased clay and silt content, while PDC formation is constrained by limited root C inputs. Our findings provide new insights into the mechanisms of SOC sequestration in drylands and emphasize the need to consider both PDC and MDC in strategies aimed at preserving the terrestrial C sink under current and future climate change.