Peatlands represent a vast store of global carbon1. Observations of rapidly rising dissolved organic carbon concentrations in rivers draining peatlands have created concerns that those stores are beginning to destabilize2, 3. Three main factors have been put forward as potential causal mechanisms, but it appears that two alternatives—warming2, 4 and increased river discharge3—cannot offer satisfactory explanations5. Here we show that the third proposed mechanism, namely shifting trends in the proportion of annual rainfall arriving in summer6, 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
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