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Peatlands represent a vast store of global carbon. Observations of rapidly rising dissolved organic carbon concentrations in rivers draining peatlands have created concerns that those stores are beginning to destabilize. Three main factors have been put forward as potential causal mechanisms, but it appears that two alternatives--warming and increased river discharge--cannot offer satisfactory explanations. Here we show that the third proposed mechanism, namely shifting trends in the proportion of annual rainfall arriving in summer, is similarly unable to account for the trend. Instead we infer that a previously unrecognized mechanism--carbon dioxide mediated stimulation of primary productivity--is responsible. Under elevated carbon dioxide levels, the proportion of dissolved organic carbon derived from recently assimilated carbon dioxide was ten times higher than that of the control cases. Concentrations of dissolved organic carbon appear far more sensitive to environmental drivers that affect net primary productivity than those affecting decomposition alone.

Peatlands export more dissolved organic carbon (DOC) than any other biome, contributing 20% of all terrestrial DOC exported to the oceans. Both warming and elevated atmospheric CO2 (eCO2) can increase DOC exports, but their interaction is poorly understood. Peat monoliths were, therefore, exposed to eCO2, warming and eCO2 + warming (combined). The combined treatment produced a synergistic (i.e., significant interaction) rise in DOC concentrations available for export (119% higher than the control, interaction P < 0.05) and enriched this pool with phenolic compounds (284%). We attribute this to increased plant inputs, coupled with impaired microbial degradation induced by competition with the vegetation for nutrients and inhibitory phenolics. Root biomass showed a synergistic increase (407% relative to the control, P < 0.1 only), while exudate inputs increased additively. Phenol oxidase was suppressed synergistically (58%, interaction P < 0.1 only) and beta-glucosidase (27%) additively, while microbial nutritional stress increased (51%) additively. Such results suggest intensified carbon exports from peatlands, with potentially widespread ramifications for aquatic processes in the receiving waters.

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