Rewetting drained raised bogs will release “substantial” methane into the atmosphere but it would rapidly turn these bogs into carbon sinks, a peer-reviewed study on the rewetting of raised bogs has found.
The study ‘Carbon and climate implications of rewetting a raised bog in Ireland’ was published in the journal Global Change Biology in July 2022.
It was carried out on a 230ha Bord na Móna raised bog site located in Moyarwood, near Woodlawn, in east Co Galway between 2013 and 2018.
The bog had been drained in the 1980s, 40 years before the study started, in preparation for peat extraction.
However, the site was never subsequently developed for peat extraction and vegetation cover remained between the ditches.
Greenhouse gas (GHG) emission measuring points were set up in areas of the bog which remained drained, where domestic peat extraction had taken place, and also on the newly rewet areas.
“This showed that the site was losing close to 1.6t of carbon per hectare per year, including methane, CO2 and dissolved organic carbon.
“Over the course of the five-year study, and taking into account all the gases, the site turned into a carbon sink, sequestering on average 0.78t of carbon per hectare per year,” Dr David Wilson, one of the study’s authors, told the Irish Farmers Journal.
“Rewetting turned the site around immediately. In year one, the site began to flip. Sphagnum mosses were back in and the site recovered. If the water tables are right, vegetation and carbon follows,” he said.
“We found that once we did the calculations and take into account the climate impact, the amount of CO2 the site sequesters is greater than the amount of methane being released,” Dr Wilson said.
“This is better than doing nothing. The impact of CO2 capture is always better than the amount of methane going out, even accounting for the impact of that gas,” he said.
After 2085, the site will have a less warming impact than it would have had without rewetting
The study stated that following rewetting, the site is projected to have a much greater warming impact on the climate for a number of decades compared with pre-rewetting or business as usual.
This is “primarily as a result of substantial [methane] emissions” from parts of the rewet area.
Water table level
In the drained area, the water table level remained between 38cm and 67cm below the peat surface for the duration of the study.
In contrast, the water table level in the rewetted area remained above the peat surface for the majority of the study period, with the exception of short periods during the summers of 2013, 2014 and 2017.
“After 2085, the site will have a less warming impact than it would have had without rewetting,” the study found.
The study also noted that the accurate estimation of net GHG emissions at a national level from these drained areas remains a huge challenge in the absence of high-resolution aerial estimates of peatland land use categories. However, it shows that their inclusion in GHG inventories could potentially add 600,000t carbon per year to reported emissions from the Land Use, Land Use Change and Forestry (LULUCF) sector in Ireland.
This means that including bogs used for domestic turf cutting, an extra 600,000t of carbon emissions could be added to the LULUCF inventory.
In the absence of rewetting, drained peatlands will remain a persistent source of emissions, the study warned.
The study also noted that a prolonged drought period would likely lead to a significant drop in water table levels at rewetted sites, with subsequent impacts on the carbon sink function.
“In contrast, a persistently elevated water table has been shown to not only reduce CO2 emissions from peatlands but can also act as an effective barrier (in combination with the presence of a moss layer) to the spread of wildfires,” it stated.
The authors of the report said that the results from the study are encouraging and confirm that peatland rewetting should be given “high priority in countries where degraded and/or drained peatlands are a key land use category as it achieves restoration objectives and is also an economically sound objective”.