We have observed a 65% increase in the dissolved organic carbon (DOC) concentration in freshwater draining from upland catchments in the United Kingdom over the past 12 years. Here we show that rising temperatures may drive this process by stimulating the export of DOC from peatlands. Our results indicate that the flux of aged, riverine DOC of terrestrial origin, now recognized as a significant supplier of DOC to oceans1, may increase substantially as a result of global warming.

DOC concentrations have increased significantly (P < 0.05) at 20 of 22 sites in the UK Acid Waters Monitoring Network2, according to the Seasonal Kendall trend analysis (Fig. 1a). These sites span a wide range of acid-deposition levels, soils, topographies, land uses and geographical locations. Annual increases, averaging 5.4%, are proportional to mean DOC concentration (R2 = 0.81, P < 0.001). As freshwater DOC concentrations are linked to storage of carbon in catchment soil3, this indicates that increases are driven by regionally consistent processes within this carbon store, and that they are greatest at sites with large stores of soil carbon, such as peatlands (Fig. 2).

Figure 1: Changing concentrations of dissolved organic carbon (DOC).
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a, Time series of median standardized DOC concentrations determined from quarterly data for 11 lakes (thick line) and monthly data for 11 streams (thin line) in the UK Acid Waters Monitoring Network (standardized concentrations for each site have a mean of zero and a standard deviation of one). b, Laboratory observations of increased concentrations of DOC and phenolic compounds in peat soil in response to rising temperature.

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Figure 2: Peatland bog: northern peatlands remove carbon dioxide from the atmosphere faster than it is released, so they now contain 20–30% of the world's soil carbon stock.
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HOLT STUDIOS/PETER WILSON

But this may be changing in response to warmer conditions.

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Although an inverse relationship has been proposed between mineral acidity and the generation of DOC4, we observed similar proportional increases in DOC at remote, unacidified sites, as well as at those recovering from anthropogenic acidification. Changes in land use or river discharge do not account for the observed increases. However, the Central England Temperature Record5 shows that mean temperatures were 0.66 °C higher in the 1990s than in the three preceding decades, and this factor could have influenced all sites.

The enzyme phenol oxidase has been proposed to regulate carbon storage in peatlands6. We therefore studied the thermal responses of peatland phenol oxidase in relation to the export of DOC. We subjected peat soil to a thermal gradient of 2–20 °C. Phenol oxidase activity was greater at higher temperatures, although this enzyme is known to be highly constrained in these waterlogged soils. An increase of 10 °C led to a 36% increase in activity (Q10 = 1.36). This was accompanied by an equivalent increase in DOC release (Q10 = 1.33) and an even greater increase in release of phenolic compounds (Q10 = 1.72) from the soil matrix. This selective enrichment with phenolic compounds (Fig. 1b) is noteworthy because of the inhibitory character of these compounds7. Under warmer conditions, selective enrichment should impair the metabolism of the remaining DOC8, allowing even more DOC to reach the oceans.

Pre-aged terrestrial sources of carbon are important contributors to the oceanic carbon budget1. Peat-accumulating wetlands have created a significant terrestrial store9 of such highly aged organic matter, despite exporting more organic carbon per unit area than any other significant biogeographical land type in the world10. We have shown that a key terrestrial carbon store could currently be being relocated to the oceans. The rate of movement is likely to increase further if global temperatures increase. Investigation of the fate of that material in the recipient ecosystem will be important.