Biogeosciences Research Spotlight

Headwater Streams May Export More Carbon Than Previously Thought

New research sheds light on the streams that carry carbon away from peatlands with the hope that the data will better inform climate models.

Source: Journal of Geophysical Research: Biogeosciences

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Peatlands—bogs, swamps, and mires—act as vital carbon sinks, keeping billions of metric tons of carbon dioxide out of the atmosphere. However, streams carry carbon—mainly as dissolved organic carbon and carbon dioxide—away from those peatlands, eventually releasing the greenhouse gas back into the atmosphere. Previous research has shown that as much as 30% to 50% of the carbon taken up from the atmosphere by peatlands can be lost through runoff, prompting scientists to pay closer attention to carbon export via streams to predict how climate change will affect not just these shallow waters but the global carbon cycle as a whole.

atmospheric-carbon-absorbed-by-peatlands-exported-by-streams
A peatland in northern Sweden. Credit: Michal Gazovic ([email protected])

Here Leach et al. used a 12-year record of carbon measurements to study a particular 3-square-kilometer catchment located in a mire in northern Sweden. In this catchment,70% is peatland, and 30% is coniferous forest.

The new data suggest that streams might have an even larger role to play in removing carbon than previously thought. The team found that during exceptionally dry summers as much as 90% of total annual carbon emitted from peatlands is likely lost from runoff—significantly higher than the previous estimates of 50%. Further, the rate of carbon release from mires is mainly influenced by high water flow; carbon export, for example, increases substantially when rains arrive in the fall and in response to melting snow during spring. As water levels drop during the summer, little carbon moves out of the peatlands in streams.

Climate change projections predict increases in precipitation and air temperature for northern Europe, with the greatest temperature increase during winter and the greatest precipitation increase from April to September. This means that streamflow—and therefore carbon export—will increase in both the winter and summer months. It might, however, decrease in spring since snowmelt will be less substantial. Additionally, the results suggest that omitting dissolved carbon dioxide from future calculations might result in underestimating stream carbon export by up to 30%. (Journal of Geophysical Research: Biogeosciences, doi:10.1002/2016JG003357, 2016)

—Shannon Hall, Freelance Writer

Citation: Hall, S. (2016), Headwater streams may export more carbon than previously thought, Eos, 97, doi:10.1029/2016EO057411. Published on 17 August 2016.
© 2016. The authors. CC BY-NC-ND 3.0
  • Brad Sherman

    I suspect that what this means is the terrestrial C sink has been overestimated in global C models with the consequence that cuts to emissions become even more important going forward in order to stabilise atmospheric CO2 and CH4 concentrations. As recently as 4 or 5 years ago, many of the soil C models did not allow for lateral transport of DOC out of the root zone thereby potentially overestimating C ‘burial’. I don’t know what the current state of these models is today but I’m guessing many of us need to update our thinking.

    It seems that the more data that are acquired by aquatic scientists the closer the estimated aquatic system C flux approaches net (terrestrial) ecosystem production. To me, this implies that nature is pretty much in balance with itself (let’s call ‘nature’ carbon < 6000 years old or from the C horizon to the top of the atmosphere and including the oceans), which shouldn't come as much of a surprise given 300K years of stable atmospheric C in ice core data. The implication is that reducing the introduction of 100m+ year old carbon is more important than ever to allow the atmosphere to achieve whatever new equilibrium condition is in store for us sooner rather than later. Many nations, such as Australia, rely too much on manipulating biological C sinks as a strategy to stabilise atmospheric chemistry. This research suggests to me that mankind needs to do relatively more of the heavy lifting.

  • davidlaing

    This study is interesting, especially in light of the fact that average annual stream loading by HCO3- is about 47 percent of dissolved load, representing the largest single component of stream chemistry, much of which is thought to be due to such weathering reactions as CaAl2Si2O8 + 2CO2 + 3H2O → Al2Si2O5(OH)4 + Ca+ + 2HCO3- (Anorthite feldspar plus two carbon dioxide plus three water yields kaolinite clay plus calcium ion plus two bicarbonate). In the oceans, by contrast, HCO3- is the least common constituent, constituting only 0.0021 percent of the dissolved substances present, and showing how aggressively carbon dioxide is removed from the marine environment by life processes. Regardless of its ultimate source, it is clear that life heavily regulates the fate of CO2 in the Earth system.

    It should be borne in mind that the theory of “greenhouse warming” while long-established, actually lacks hard, experimental, or even observational, proof. Most climate scientists tend to assume its factual basis in spite of this rather glaring omission, because the case has been so well argued. We should be careful here, though. Theory, no matter how sophisticated, or how generally accepted, it may be, is no match for hard data, and therefore we should be open to the possibility that other, more logical explanations for warming, such as ozone depletion by chlorine emissions from non-explosive (basaltic) volcanoes, actually make more sense than greenhouse warming by carbon dioxide.