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 explanationss. 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.
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Acknowledgements This work was supported by the US Department of Energy. Some high-resolution and analytical electron microscopy was performed at the National Center for Electron Microscopy (NCEM) with the help of E. C. Nelson and some low-resolution electron microscopy was performed at the Electron Microscope Laboratory at UCB with the help of M. Casula. For the theoretical calculations we used the National Energy Research Scientific Computing Center.
Acknowledgements 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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