1. School of Biological Sciences, University of Wales, Bangor LL57 2UW, UK
2. Centre for Ecology and Hydrology, Lancaster Environment Centre, Lancaster LA1 4AP, UK
3. Centre for Ecology and Hydrology, Bangor LL57 2UP, UK
4. Centre for Ecology and Hydrology, Wallingford OX10 8BB, UK
5. Present address: Department of Environmental Science and Engineering, Ewha University, Seoul 120-750, Korea

Correspondence to: C. Freeman¹ Email: c.freeman@bangor.ac.uk

Abstract

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 destabilize^{2, 3}. Three main factors have been put forward as potential causal mechanisms, but it appears that two alternatives—warming^{2, 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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