Background/Question/Methods Soil organic matter is a key determinant of the future of the terrestrial carbon (C) sink but its vulnerability to changing disturbance and management regimes is contentious. We evaluated the impact of changing fire regimes on mineral soil C and the implications for total ecosystem C using a global dataset of 58 sites that experienced altered fire frequencies for multiple decades. We then performed field experiments to identify the mechanisms that lead to cross-site sensitivity in mineral soil C to fire, and performed ecosystem model simulations to scale our results to the globe. Results/Conclusions We found that frequent burning reduced mineral soil C 4.3-fold more in semi-arid than mesic climates, correlated with shifts in plant biomass inputs into soils, and contributed on average 43% of the total ecosystem C changes. Ecosystem model simulations predicted the opposite trends with climate, overestimating the importance of soil C changes in wet regions. Our results present the counterintuitive finding that soils in semi-arid ecosystems, despite being less C dense than forests, could play an important role in the response of terrestrial C storage to changing fire regimes.