Salt marsh restoration could bring carbon benefits
March 18, 2013 by Tom Marshall
Allowing farmland that's been reclaimed from the sea
to flood and turn back into salt marsh could make it absorb lots of carbon from
the atmosphere, a new study suggests, though the transformation will take many
years to complete.
Scientists looked at one of the oldest such places in
the UK, Tollesbury in Essex. Originally a salt marsh, the site was claimed for farming in the late
18th century, but eventually relinquished in 1995 when the bank separating it
from the sea was deliberately breached. Since then it's been reverting to its
natural state, though this is very slow process.
'People want quick results, but these things take
time,' says lead author Annette Burden, a wetland biogeochemist based at the
Centre for Ecology & Hydrology in Bangor. 'You can't expect a piece of land that's been
farmed for a century to turn overnight into something like a saltmarsh that has
been there for thousands of years. But the evidence is that this will
eventually happen, and this study suggests that the land starts absorbing
carbon very quickly after its flood defences are breached.'
In 2010 the researchers looked at how carbon moves
between soil and air at the site, and at the total amount of carbon accumulated
in the soil. They compared the results to nearby natural saltmarsh and
farmland.
Their results show that the Tollesbury site exchanges
carbon with the atmosphere at a similar rate to the natural marsh, absorbing
around 0.92 tonnes of carbon per hectare per year. But even after 15 years its
total carbon content is far smaller and its general biological functioning is
still very different – the authors think it could take a century or more to
catch up.
Known as coastal managed realignment, the process of
turning farmland back into salt marsh involves breaching sea walls and letting
the land revert naturally to how it once was. It's done for a variety of
reasons. At present the main one is to comply with the EU Habitats Directive,
which obliges the UK to replace salt marsh that's lost to development with new 'biologically
equivalent' habitat elsewhere.
But managed realignment offers many other benefits.
It can improve biodiversity, since salt marsh hosts many rare and valuable
plant and animal species. It can help protect coastlines from flooding by
creating a buffer zone between the sea and infrastructure or homes. It can let
government bodies save cash on maintaining costly flood defences. And now this
study, published in Coastal and Estuarine Science, suggests another possible
benefit – absorbing copious carbon dioxide from the atmosphere and locking it
up in the soil. This could help limit the impact of our carbon emissions on the
climate.
Similar initiatives are afoot around peat bogs,
another enormous store of carbon in the UK landscape. Scientists and conservationists are
experimenting with blocking drains and other measures aimed at restoring
peatlands that were drained in a misguided twentieth-century effort at turning
them into farmland.
This is where Burden's background lies. 'I wanted to
take what we've learned about peat bogs and apply it to salt marsh,' she says.
'Peatland has had much more attention from scientists, but people are thinking
that salt marsh could be just as important as a store of soil carbon.'
She thinks it's possible that per hectare of land,
restoring salt marshes could even be more valuable from a carbon-management
perspective than restoring peatlands. Re-wetting a peat bog initially makes it
emit methane, itself a greenhouse gas; recreating salt marshes has no such
drawbacks. But there is much more peatland that could be restored in Britain than salt marsh, so their overall effect is
potentially more significant.
Other studies, some of which we've covered on Planet
Earth Online (see links to the right) have shown that it takes a very long time
for the mix of plants found in a natural salt marsh to return to a deliberately
recreated one. The same seems to be true of the soil's carbon content.
She's now carrying out further research looking at a
range of sites that stopped being farmed at different times due to breaches in
sea defences during storms, some as long as 100 years ago, to get a better
sense of how they develop over time. 'The Essex coast is brilliant for this kind of work, because you have so many
places that were returned to saltmarsh at different times - it's like a natural
experiment,' she says.
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