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Export of dissolved organic carbon from peatlands under elevated carbon dioxide levels


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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Figure 1: The field-based drought simulation.
Figure 2: Comparison of DOC mobilization from control and drought-treated wetlands.
Figure 3: Effects of elevated CO2 on DOC release.
Figure 4: 13C pulse labelling of released DOC.


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We are grateful to the Royal Society, the Welsh Assembly Government and the Natural Environment Research Council, UK, for funding this research.

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Correspondence to C. Freeman.

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Competing interests

C.F. has obtained support from United Utilities for an NERC-CASE PhD studentship. C.F. has accepted a Royal Society industry fellowship that would involve work (part-time) at Welsh Water from October 2004 to October 2008.

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Freeman, C., Fenner, N., Ostle, N. et al. Export of dissolved organic carbon from peatlands under elevated carbon dioxide levels. Nature 430, 195–198 (2004).

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