Nonlinear temperature change responses shape soil organic carbon loss-gain transitions in global Mollisol croplands

Mollisol croplands are important for global food production and soil carbon storage, yet it remains unclear how climate warming affects transitions between soil organic carbon loss and gain. Here we show that surface soil organic carbon across global Mollisol croplands declined by 5.59% from 1990 to 2020, although the rate of loss slowed over time. Using 5,535 soil samples and satellite observations, we identified a nonlinear relationship between soil carbon change and the rate of warming, with a threshold at 0.0309 °C/yr above which net soil carbon change shifted from loss toward gain. This threshold increased toward lower latitudes, indicating that warmer regions require stronger warming to reach a similar transition. If recent climate-soil relationships persist, about 65% of Mollisol cropland may exceed this threshold by 2050 under a moderate emissions scenario. These results highlight the importance of nonlinear threshold responses in predicting soil carbon dynamics under climate change.