<p>Soils can be either a source or sink of atmospheric CO₂ depending on how soil organic carbon (SOC) responds to climate warming and changes in plant productivity. Whereas warming typically accelerates SOC decomposition, the effect of plant productivity changes remains unclear. Here we use a space-for-change substitution approach to analyse a global dataset of SOC measurements down to 1 metre. We find that warming-induced SOC reduction in the 0–0.3-m topsoil is gradually offset by increasing plant productivity but exacerbated in the 0.3–1-m subsoil until plant productivity increase crosses a threshold of 30%. Consequently, entirely offsetting warming-induced SOC reduction in the top metre of soil requires an unrealistically high increase in plant productivity, albeit with substantial variance across ecosystems. Soil carbon-to-nitrogen ratio is the dominant predictor of the variance in SOC response, as this ratio determines whether nitrogen released during carbon loss meets the requirement for additional plant growth or whether additional nitrogen must be mined from soil organic matter. Such mining accelerates SOC losses, particularly in the subsoil, where the soil carbon-to-nitrogen ratio is lower than in topsoils. We conclude that globally, increased plant productivity may exacerbate SOC losses under climate warming, particularly in the subsoil.</p>