Drought can increase peat bog carbon emissions for decades

25 November 2011, by Tom Marshall

Once a peat bog dries out, it starts emitting carbon by giving off carbon dioxide gas (CO₂) and methane into the atmosphere, and by releasing it into rivers and streams in the form of dissolved organic carbon (DOC).

This loss can carry on for a decade or more, and can continue or accelerate even after the bog is submerged again. And the type of bog habitat that's worst-affected accounts for some 60 per cent of the world's peatlands.

'Our findings on re-wetting were a huge surprise,' says Dr Nathalie Fenner of Bangor University, lead author of the paper, which appears in Nature Geoscience. 'There has been a lot of research on the impact of drought on peatlands but this is the first study that shows it can carry on for years after the drought has ended in a range of peats, but more importantly why - it seems to turbo-charge carbon emissions even once the water table has risen again.'

The study combines fieldwork in peatlands in the UK, Finland and Malaysia with experiments in the lab, which give the first detailed insight into the complex chain of chemical and microbial processes that leads from drier weather to carbon loss.

During the initial drought phase, CO₂ emissions from some bogs were unchanged, while others increased by up to 181 per cent. And once they were re-submerged, CO₂ emissions climbed even higher. At the least-affected bog they were 60 per cent higher than before the drought started, while in the worst case they had increased by an extraordinary 551 per cent.

Fenner explains that bogs with comparatively few nutrients tend to undergo much smaller or even no increases in emissions during the dry phase. But the drought effectively unlocks nutrients that then stimulate much greater activity from the microbes involved in breaking down peat once the water table rises again. This means that these bogs often see much greater increases in emissions once the drought ends.

Vicious climate circles

These are worrying findings, because they seems to support the idea that climate change could enter a harmful feedback loop in which global warming causes drought, which causes peat bogs to release more of the carbon they hold, in turn leading to yet more warming. Bogs hold staggering amounts of carbon - estimated at 455 petagrams, around a third of the world's stock of soil organic carbon.

'As our global climate and rainfall patterns change, our peatlands may not have sufficient opportunity to recover between these drought-induced episodes of CO₂ loss,' Fenner explains. 'What we previously perceived as a spike in the rate of carbon loss during drying out now appears far more prolonged, with a potential peak after the initial drought period is over.'

Drought didn't only increase the bogs' CO₂ emissions. In one wetland that was studied, DOC in nearby streams increased by 261 per cent, and emissions of methane - a very potent greenhouse gas, around 21 times stronger than CO₂ - rocketed from a milligram per square metre per day to 40.

It's not yet certain how long this will continue after drought ends before things return to normal. Earlier research at Durham University by Dr Fred Worrall suggested that recovery from severe drought could take a decade or two in a UK peatland. The new findings seem to confirm this; the effects of moderate droughts seem to last three to four years, while the impact of a severe drought in 2006 shows no sign of disappearing yet.

Disquietingly, severe droughts already happen around every 15 years in the UK - not far off the duration of their after-effects. This raises the possibility that some bogs could end up emitting more carbon on a near-permanent basis.

Many peatlands are in areas that are predicted to experience more frequent and severe droughts as the climate changes. Peat locks up so much carbon because it's held in waterlogged conditions where air can't get to it.

Plants grow and absorb carbon; when they die their remains sink and are preserved. Over time, the bog builds up a large store of carbon-rich plant remains. Once it dries out, this accumulated material decays quickly as it comes into contact with oxygen, releasing its carbon content.

'The previous focus of research in this area has been on the drought period, and our own work identified how the release of CO₂ occurs,' explains Professor Chris Freeman, co-author and head of Bangor's Wolfson Peatland Carbon Capture Laboratory.

'We were initially surprised at finding that the effects are so prolonged - we think what's happening is microbial and that this activity has been triggered by the introduction of oxygen into previously waterlogged conditions. Once the water returns, conditions have changed and the microbes are further able to thrive until conditions eventually return to normal.'

If bogs lose more carbon as DOC, this could have serious effects on drinking water supplies. As well as turning water brown, dissolved carbon can interfere with treatment filters and make it more expensive to make water safe to drink.

And if peat bogs become badly degraded, there could also be implications for the risk of flooding. Healthy bogs work like a sponge, soaking up rainfall, slowing its flow off hilly areas and helping prevent flooding downstream. And because these habitats are home to plants and animals that live nowhere else, destruction of peat bogs would be bad for biodiversity too.

Fenner and Freeman are now considering possible ways to prevent or lessen these increases in peatland carbon loss, using the sequence of processes identified in their paper, perhaps by adding more of the phenol chemicals that occur there naturally and work to inhibit microbial activity, but that are destroyed by very dry conditions.

If we could find a practical way to do this over wide areas without too many unwanted side-effects, we might be able to slow the rate of peat loss or even cause bogs to gain carbon. This type of protective geoengineering could be particularly appropriate for bogs that have already been badly degraded.

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