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A New Twist on the Old Threat of an Imminent Release of Vast Amounts of CO2 from the World's Peatlands
Volume 7, Number 28: 14 July 2004

Earth's peatlands contain a vast amount of sequestered carbon, about as much, in fact, as that contained in the entire atmosphere, or close to one-third of the global soil carbon stock.  Consequently, peatlands represent a vital component of the planet's carbon cycle, and it is important to determine how their carbon balance may change in response to the ongoing rise in the air's CO2 content.

In this regard, it was long believed that higher ground temperatures resulting from CO2-induced global warming would lead to increased decomposition of the organic matter found in the world's peatlands (some of which are currently encased in permafrost and thereby protected from decomposition, but which could thaw and become active in a warmer world).  This phenomenon would obviously have the potential to free up great quantities of carbon, allowing it to either (1) make its way back to the atmosphere as CO2, from whence and in which form it originally came, or (2) exit the land as dissolved organic carbon (DOC) via streams and rivers that drain peatland catchments.

We have previously reviewed a number of studies that refute this hypothesis [see Carbon Sequestration (Peatlands) in our Subject Index]; and the authors of the study that prompts this Editorial (Freeman et al., 2004) agree with us on this point.  In addition, they indicate that neither increased stream and river flow nor reductions in the proportion of annual precipitation arriving in summer significantly stimulate the release of DOC from the world's peatlands, as some have suggested they might (Tranvik and Iansson, 2002; Evans et al., 2002).  Nevertheless, riverine transport of DOC has increased markedly in many places throughout the world over the past few decades (Schindler et al., 1997; Freeman et al., 2001; Worrall et al., 2003); and Freeman et al. claim they have found the reason why.

Their first piece of evidence comes from a three-year study of peat "monoliths" (11-cm diameter x 20-cm deep cores) taken from three Welsh peatlands -- a bog that receives nutrients solely from rainfall, a fen that gains more nutrients from surrounding soils and groundwater, and a riparian peatland that gains even more nutrients from nutrient-laden water transported from other terrestrial ecosystems via drainage streams -- which they exposed to either ambient air or air enriched with an extra 235 ppm of CO2 within a "solardome" facility.  This study revealed that the DOC released by monoliths from the three peatlands was significantly enhanced -- by 14% in the bog, 49% in the fen and 61% in the riparian peatland -- by the additional CO2 to which they were exposed, which is the order of response one would expect from what we know about the stimulation of net primary productivity due to atmospheric CO2 enrichment, i.e., it is low in the face of low soil nutrients, intermediate when soil nutrient concentrations are intermediate, and high when soil nutrients are present in abundance.  Hence, Freeman et al. concluded that the DOC increases they observed "were induced by increased primary production and DOC exudation from plants," which conclusion logically follows from their findings.

Nevertheless, and to further test this hypothesis, they followed the translocation of labeled carbon (13C) through the plant-soil systems of the different peatlands for approximately two weeks after exposing the monoliths of both the ambient-air and CO2-enriched treatments to ~99%-pure 13CO2 for a period of five hours.  This exercise revealed that the plants in the ambient-air and CO2-enriched treatments assimilated 22.9 and 35.8 mg of 13C from the air, respectively, that the amount of DOC that was recovered from the leachate of the CO2-enriched monoliths was 0.6% of that assimilated, or 0.215 mg (35.8 mg x 0.006 = 0.215 mg), and that the proportion of DOC in the soil solution of the CO2-enriched monoliths that was derived from recently assimilated CO2 (the 13C labeled CO2) was ten times higher than that of the control.

This latter observation suggests that the amount of DOC recovered from the leachate of the ambient-air monoliths was only about a tenth as much as that recovered from the leachate of the CO2-enriched monoliths, which puts the former amount at about 0.022 mg.  Hence, what really counts, i.e., the net sequestration of 13C experienced by the peat monoliths over the two-week period (which equals the amount that went into them minus the amount that went out), comes to 22.9 mg minus 0.022 mg = 22.878 mg for the ambient-air monoliths and 35.8 mg minus 0.215 mg = 35.585 mg for the CO2-enriched monoliths, which results are indicative of the fact that even though the CO2-enriched monoliths lost ten times more 13C via root exudation than did the ambient-air monoliths, they still sequestered about 55% more 13C, primarily in the tissues of living plants.

From these observations The Independent's Science Editor somehow concluded that "tests on peat samples taken from three different sites in Britain show that increasing the amount of CO2 in the air around the samples causes the peat itself to emit up to 10 times the amount of carbon it would under normal conditions" and, therefore, that "global warming is set to dramatically worsen because of huge amounts of carbon dioxide being released from the world's peatlands."  Nevertheless, he does not bear all the blame for these wildly ridiculous claims, as he quotes Freeman as saying (in an interview he had with him???) that the store of carbon locked up in peat bogs "appears to have sprung a leak," and that "by 2060 we could see more CO2 being released into the atmosphere [by peat bogs] than is being released by burning fossil fuel."  Quite to the contrary, however, as Freeman et al.'s own data indicate, we will likely see peat bogs of the CO2-enriched world of 2060 extracting CO2 from the atmosphere, and doing it at a far greater rate than they do currently.

Others also got things fabulously wrong in reporting Freeman et al.'s results.  Fred Pearce at NewScientist.com a day earlier (7 July 2004), for example, wrote that "after three years, the proportion of DOC in the CO2-rich soil was 10 times that within the normal soil ? And there was no sign of the increase tailing off."  This 10 times factor actually applies to the two-week study of Freeman et al.; and even in this study the graph of their results indicates that the CO2-enriched treatment's DOC-release was only 10-fold greater than that of the ambient-air treatment during the first day of the two-week period, after which it actually did "tail off," and to something considerably less than a 10-fold enhancement.

What really matters, however, is not what happens over days, but what happens over years and longer; and at the end of their three-year enrichment of peat monoliths with an extra 235 ppm of CO2, the DOC release from the CO2-enriched riparian peatland was enhanced by just 61%, as noted earlier, while that from the fen was increased by only 49% and that from the bog by a mere 14%.  And it must be remembered that these percentages apply to something (root exudation) that is generally far smaller in absolute magnitude than net primary productivity.  In Freeman et al.'s two-week experiment, for example, it was ridiculously small, at a tiny 0.6% of plant-assimilated carbon, the latter and much larger of which entities (plant-assimilated carbon) was enhanced by 56% by the extra CO2, which finding suggests that root exudation is probably enhanced by close to the same percentage that net primary production is enhanced by atmospheric CO2 enrichment.  Also, as an upper limit on the percentage of net primary productivity that can be lost to root exudation, Freeman et al. say that "DOC release from peatlands can account for greater than 15% of photosynthetic carbon capture," but, we would add, probably not much more than that.  Hence, it is clear that over the long haul and averaged over all types of peatlands, the net effect of the ongoing rise in the air's CO2 content should be to enhance the ability of earth's peatlands to sequester carbon, not lose it.

Sherwood, Keith and Craig Idso

References
Evans, C.D., Freeman, C., Monteith, D.T., Reynolds, B. and Fenner, N.  2002.  Climate change - Terrestrial export of organic carbon - Reply.  Nature 415: 862.

Freeman, C., Evans, C.D., Monteith, D.T., Reynolds, B. and Fenner, N.  2002.  Export of organic carbon from peat soils.  Nature 412: 785.

Freeman, C., Fenner, N., Ostle, N.J., Kang, H., Dowrick, D.J., Reynolds, B., Lock, M.A., Sleep, D., Hughes, S. and Hudson, J.  2004.  Export of dissolved organic carbon from peatlands under elevated carbon dioxide levels.  Nature 430: 195-198.

Schindler, D.W., Curtis, P.J., Bayley, S.E., Parker, B.R., Beaty, K.G. and Stainton, M.P.  1997.  Climate-induced changes in the dissolved organic carbon budgets of boreal lakes.  Biogeochemistry 36: 9-28.

Tranvik, L.J. and Iansson, M.  2002.  Climate change - Terrestrial export of organic carbon.  Nature 415: 861-862.

Worrall, F., Burt, T. and Shedden, R.  2003.  Long term records of riverine dissolved organic matter.  Biogeochemistry 64: 165-178.