FRONTLINE

Peatland

and the

BY DAVID WILSON

Since the end of the last Ice Age, peatlands have played a major role in the regulation and maintenance of the global climate. Natural (undamaged) peatlands act as long-term sinks for carbon dioxide but are also sources of methane (a strong greenhouse gas), and it is this interplay in gas exchange over millennia that has led to the storage of vast amounts of carbon within peatlands and organic soils during this time. The removal and storage of carbon by peatlands has had a cooling effect on the climate and recent estimates suggest that approximately 600 billion tonnes of carbon are locked up in peatlands globally, with 2.2 billion tonnes held in Irish peatlands alone 1 .

Key to this role is the position of the water table within the peat, with the optimum level from a carbon perspective likely to occur when the water table is close to the peat surface. This wet environment prevents the easy decomposition of leaves, stems and roots of peatland plants, which then accumulate over time and form the basis of peat. On a dry matter basis, peat contains approximately 40–50% carbon, and can extend to over 10 metres depth in some of our raised bog remnants.

Drainage results in a radical transformation in peatland carbon exchange; former carbon sinks can rapidly switch to large sources as the peat is rapidly decomposed by oxygen-loving microbes, resulting in a warming effect on the climate. In Ireland, around 85% of peatlands have been drained for agriculture, forestry or peat extraction, and only a small proportion of peatland sites can be considered as active carbon sinks.

REWETTING CLIMATE

A case study from Moyarwood bog, Co. Galway

 Measuring gas exchange on the bog

The premise that rewetting of drained or degraded peatlands can assist in climate change mitigation has gained considerable traction in recent years. Rewetting aims to reverse the hydrological changes brought about by drainage and to establish the requisite conditions for the (re) establishment of typical peatland plant communities. In some cases, rewetting may involve the blocking of drains only, while in others, more extensive management actions may be required.

To-date, research has indicated a strongly site-specific response to peatland rewetting, although very few studies have examined the long-term impacts of peatland rewetting on carbon exchange with the atmosphere. In our recently published paper in Global Change Biology2 , we measured and modelled the movement (fluxes) of carbon dioxide, methane and nitrous oxide (all greenhouse gases) over a 5-year period in Moyarwood bog, Co. Galway. The site had been drained by Bord

 Large emerald

na Móna in the 1980s with the intention to extract peat. However, site preparation did not proceed beyond these actions, and a vegetation cover remained in situ.

In 2012/2013, Bord na Móna installed peat dams in the drainage ditches across the 230-hectare site and in spring 2013, we commenced gas measurements in tandem with the monitoring of water table levels, soil temperature, phenology etc. We set up two study areas in the bog to determine the impact of rewetting: one area in the drained margin and the second in a newly rewetted area. Gas fluxes were measured with static chambers (polycarbonate “boxes” placed on permanent steel collars inserted into the peat before the start of the study). Over a 5-year period, gas measurements were taken in all four seasons with the aim to record the full range of light, soil temperatures and water table values that Moyarwood is likely to experience. Such long-term monitoring is essential to provide robust estimates of how an ecosystem will react to changes in weather and climate.

Our results showed that the drained marginal area was a large source of carbon dioxide emissions and a very small source of methane emissions throughout the 5-year study period. In contrast, the rewetted area quickly switched from acting as a carbon dioxide source to functioning as a very strong sink, in the process preventing substantial emissions to the atmosphere. However, methane

 xxxxxx  xxxx "THE PREMISE THAT REWETTING OF DRAINED OR DEGRADED PEATLANDS CAN ASSIST IN CLIMATE CHANGE MITIGATION HAS GAINED CONSIDERABLE TRACTION IN RECENT YEARS"

emissions surged strongly in the rewetted area, which may be due to inundation of the vegetation caused by drain blocking. We then examined the impact of rewetting this peatland on the climate by modelling the data for the next 200 years. Our model projections suggest that peatland rewetting at Moyarwood has already resulted in a reduced warming effect on the climate, compared to leaving the site drained. Moreover, our modelling indicates that the site will begin to have a cooling effect (similar to natural peatlands) on the climate after 2085.

Under the Paris Agreement, the goal is to limit the global temperature increase to below 1.5oC. Peatland rewetting represents an important requirement for meeting the Paris Agreement commitments, although compliance will require that 50 million hectares of drained peatlands be rewetted globally by 2070. In Ireland, impetus for peatland rewetting has gained considerable momentum in recent years, and we hope that studies such as at Moyarwood will provide robust findings for landowners and policy makers.

David is an environmental consultant based in north Donegal. He can be reached at david.wilson@earthymatters.ie or on Twitter (@peatyGHG).