Background and aim As global climate change intensifies, the frequency and duration of extreme droughts are predicted to increase, resulting in extended periods of reduced soil water availability across ecosystems. The allocation of carbon (C) to above- and below-ground plant biomass is a fundamental ecosystem property that varies spatially and temporally with water availability. Yet, how extreme drought affects biomass pools along an aridity gradient remains poorly understood. Methods To elucidate the effects of extreme drought on above- and belowground carbon storage, we conducted a 3-year (2015-2017) precipitation manipulation experiment (66% reduction in growing season precipitation) to simulate a multi-year extreme drought across six grasslands spanning an aridity gradient from desert steppe, typical steppe, and meadow steppe in northern China. Results Extreme drought significantly decreased aboveground biomass carbon (AGBC) and litter carbon (LC), but did not affect belowground biomass carbon (BGBC) across the six grasslands. As a result, grassland total carbon (TC) in plant biomass declined overall by similar to 10%. Across the aridity gradient, drought-induced reductions in AGBC and LC increased with increasing aridity (i.e., reductions were greater in desert steppe than meadow steppe). In contrast, extreme drought increased BGBC in the drier grass-lands (desert steppe and typical steppe), but decreased BGBC in the more mesic meadow steppe sites. Conclusion We found that extreme drought elicited contrasting responses of plant above- and below-ground carbon across an aridity gradient, and that regionally extreme drought will result in a loss of total plant carbon in grasslands dominated by above-ground plant carbon pools.