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1. Drylands, covering c. 45% of the Earth’s terrestrial surface and supporting c. 38% of the global population, play a dominant role in the trend and interannual variability of global land carbon (C) sink. Given that a large proportion of organic C is stored in soils, our knowledge on soil C dynamics in drylands is crucial to evaluate terrestrial C-climate feedback. However, credible understanding on this issue is still greatly limited by the lack of direct observations.

2. Here, based on a regional resampling of historical sites collected during 2002-2004, we explored the soil organic C (SOC) changes in various layers over the past decade across the arid/semi-arid grasslands on the Inner Mongolian Plateau.

3. Our results revealed that the SOC density in this typical dryland increased significantly over the monitoring period, with a mean increase in 50.6g Cm(-2)year(-1) or 0.8% per year in the top 50cm depth. Moreover, soil C dynamics exhibited contrasting spatial patterns between different layers: the rate of C accumulation in surface soils (0-10cm) decreased, whereas that in deep soils (30-50cm) exhibited an increasing trend along the aridity gradient.

4. Collectively, these findings demonstrate that dryland soils function as an important C sink, with the drier region tending to sequester C in deeper soils due to the greater root biomass allocation.