Elevated CO₂ mitigated warming-enhanced carbon loss from northern peatland

Peatlands store one-third of the global soil carbon (C) on only ∼3% of Earth's land surface but risk becoming C sources to accelerate climate change due to warming-induced C loss. Elevated CO₂ (eCO₂) may mitigate this loss by boosting plant C uptake. However, how eCO₂ and climate warming interact to influence peatland C dynamics remains unclear. Here, we investigated peatland C fate at five warming levels (+0°, +2.25°, +4.5°, +6.75°, and + 9°C), each under ambient CO₂ (aCO₂) and eCO₂ (+500 parts per million), by model-guided synthesis of 18 measured variables for each of the 10 treatments from an experiment. Our results showed that the peatland C loss increased with warming at a rate of 62.6 g C m^-2 year^-1°C^-1, reaching 476.5 ± 104.7 g C m^-2 year^-1 at +9°C under aCO₂, primarily because of increased Sphagnum mortality and accelerated decomposition. eCO₂ reduced the loss rate to 17.3 g C m^-2 year^-1°C^-1, nearly halving the C loss at +9°C (269.4 ± 80.5 g C m^-2 year^-1), mainly due to enhanced tree and shrub growth. Methane emissions increased with warming, similarly under aCO₂ and eCO₂. Our findings suggest that northern peatlands likely become major C sources under climate warming, although rising atmospheric CO₂ concentration may partially slow down the C loss. Climate-driven shifts in plant community structure, rather than peat decomposition alone, play a critical role in regulating peatland C fate and may threaten the stability of peatland ecosystem functions and services in the future.