From: Tom Marshall, Planet Earth Online
Published March 16, 2013 07:40 AM

Salt Marshes are great Carbon Sinks

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.

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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.

Salt marsh photo via Shutterstock.

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