Drought dominates the changes in carbon sources and sinks of vegetation on the Mongolian Plateau

The spatiotemporal fluctuations of vegetation carbon sources and sinks (VCSS) in arid regions are highly sensitive and easily overlooked in the terrestrial carbon cycle. The Mongolian Plateau (MP) spans the arid and semi-arid zones of Eurasia, yet the persistence, environmental controls, and lagged climatic responses of its VCSS remain unclear. Using multi-source remote-sensing and meteorological datasets, this study estimated net ecosystem productivity (NEP) using an ecosystem carbon-balance method incorporating the Carnegie–Ames–Stanford Approach (CASA) model and applied a solar-induced chlorophyll fluorescence (SIF)-based drought indicator to characterize vegetation physiological drought stress. From the perspectives of temporal change, spatial differentiation, and environmental attribution, we evaluated the spatiotemporal variations, trend persistence, environmental controls, and lagged climatic responses of VCSS across the MP during 2001–2020. The results showed that 74.38% of the vegetated area functioned as a carbon sink. During 2001–2020, regional carbon‑sink capacity increased slightly despite pronounced interannual fluctuations, decreasing during 2001–2008 and subsequently increasing rapidly during 2008–2013. VCSS predominantly responded to precipitation within the same month, whereas its response to temperature showed an average lag of approximately 5.6 months. Drought fluorescence monitoring index (DFMI), evapotranspiration, and soil moisture contributed 51.10%, 34.96%, and 13.94%, respectively, indicating that drought-related vegetation stress was the dominant control on VCSS. VCSS in Mongolia was more sensitive to drought variability than that in Inner Mongolia, particularly in typical and desert steppes. These findings support region-specific drought monitoring, grassland management, and ecosystem carbon-sink conservation across the MP.