Dissolved organic matter composition predicts the carbon sink in a northern peatland warming experiment

Northern peatlands are major carbon sinks, but their response to warming is difficult to predict because carbon uptake depends on complex interactions between climate, vegetation and drainage. We tested whether the composition of dissolved organic matter (DOM) in peat pore water reflects the biogeochemical functioning of the system and, therefore, can predict the carbon sink response to increased temperature. Using 16 peatland mesocosms subjected to contrasting climatic and hydrological conditions, we measured noon-time CO₂ exchange and analyzed DOM optical properties. Interaction forest models were used to predict carbon balance from peatland characteristics combined with DOM composition data. The CO₂ sink strengthened with warming in mesocosms with low DOM aromaticity, marked by high protein-like and microbially derived fluorescence, but remained weak when DOM was more aromatic. These results show that DOM composition is a sensitive indicator of peatland carbon balance under warming and can improve predictions of future carbon sink behavior.