In Ireland, peat soils cover approximately 1.46 million ha or 21% of the land surface and store approximately 2.3 billion tonnes of carbon. Undrained peatlands are net carbon sinks, as the amount of CO2 sequestered from the atmosphere by the peatland is greater than the amount of methane emitted by the peatland, and the amount of carbon released into streams and rivers. However, the vast majority of peatlands in Ireland have been drained, either for turf cutting, energy production, horticulture, agriculture or forestry. In each case, the fundamental goal of drainage is to lower the water level within the soil to facilitate the movement of machinery across the peat, for the grazing of livestock, and the growth of plant species, such as agricultural grasses and trees, which do not thrive under water-saturated conditions. Drainage results in considerable changes in many aspects of the peatland. For carbon dynamics, the lowering of the water level increases the oxygen content of the peat, which stimulates microbial decomposition of the organic matter and the release of the carbon contained therein. Studies in Ireland and abroad have shown that drained peatlands are a net CO2 source, emit less methane than undrained sites and release more carbon to adjacent water bodies than undrained sites.
In Ireland’s national inventory report, NIR, 2020, greenhouse gas emissions from the estimated 330,000 ha of drained grassland on organic peat soils under the land use, land use change and forestry, LULUCF, sector are reported at 8.3 million tonnes of CO2 equivalent. Recent work by Teagasc, however, indicates that the area of grassland on drained peat soils could be 450,000 ha, which would suggest that emissions from this land-use category may be currently underestimated by 3 million tonnes of CO2 equivalent.
Industrial peat extraction areas are a persistent source of CO2 for as long as the drains or water pumping, or both, remain in operation. Work in Ireland and the UK indicates that drained, cutaway and cutover sites emit approximately 6 tonnes of CO2 per hectare every year. Moreover, while methane emissions may decrease from the main peat extraction areas, carbon loading on adjacent water bodies is significantly increased. In NIR 2020, emissions from peat extraction areas are reported at 1.15 million tonnes of CO2 equivalent. However, this is likely to be a gross underestimation as the areas impacted by domestic turf cutting are nominally estimated in the report at only 400 ha, due to the absence of high-resolution land use data.
In NIR 2020, forestry on organic soils under LULUCF is reported as a sink of 1.88 million tonnes on 450,000 ha. Recent data, however, suggest that the default soil emission factor used in the models may be unrealistic and that soil emissions from the forest stand may be much higher, in which case forestry on peat could switch from acting as a net CO2 sink to a net CO2 source.
In addition to having a deleterious impact on climate forcing, peatland drainage also results in the release of aquatic organic carbon, nitrates, ammonium, loss of flood control and water storage capacity, and an increased risk of fires. Recent research has also highlighted the presence of trihalomethanes, potential carcinogens, in the waters located in peat catchments.
Rewetting of peat soils has been suggested as an important climate change mitigation tool to reduce greenhouse gas emissions, create suitable conditions for carbon sequestration, stimulate biodiversity and improve water quality. How do we define rewetting? The Intergovernmental Panel on Climate Change's wetlands supplement describes it as "the deliberate action of raising the water table on drained soils to re-establish water saturated conditions, for example, by blocking drainage ditches or disabling pumping facilities, and managing the water table so that it remains close to the surface”.
Clearly, in this regard, rewetting does not equate to flooding. Rewetting is achieved through a suite of management actions that are tailored to the site in question. The principal method is to block the drains, which involves stopping or slowing the flow of water along the drain. Alternative dam materials include plastic sheets and wood, while coconut fibre logs have been used on some peatland sites to reduce the overground lateral flow of the water. On some sites, a berm is installed at the edge of the site to prevent water from leaving, while on others a compartmental approach is employed to hold rainfall on the site.
Peatland rewetting can have several objectives, such as restoration, which aims to establish a functioning peatland ecosystem, although rewetting can also allow other management practices, such as paludiculture, or wet agriculture, to take place on the saturated peat soils. In Ireland, peatland rewetting has focused mainly on cutaway and cutover sites, with a smaller quantity of former forest lands also rewetted under various EU-funded projects. From a carbon and climate point of view, the aim is to establish a water level where greenhouse gas emissions are reduced, CO2 sequestration resumes and methane emissions are minimised.
Studies have shown the optimal water level to be between 10 cm and 15 cm below the soil surface, although it must be maintained at that level as much as possible throughout the year, and particularly during the growing season, to minimise soil emissions. On sites where greenhouse gases have been monitored, CO2 emissions are significantly reduced following rewetting, and in many instances the rewetted site can quickly become a CO2 sink once more, in tandem with the re-establishment of suitable plant communities. However, a wide range of greenhouse gas values have been reported for rewetted peatlands, largely driven by differences between climate zones and peatland nutrient status.
My experience of working on Bord na Móna rewetted cutaway bogs over the past 20 years has been that for the majority of these sites, the flooding of adjacent lands has not been an issue, an experience in agreement with the community living around Abbeyleix bog, for example. Issues might exist, however, for those areas around the River Shannon where industrial extraction was previously facilitated by the active pumping of drainage water and where flooding, as opposed to rewetting, may be a problem when the pumps are switched off. For cutover sites, rewetting of drained special area of conservation, SAC, sites has been carried out mainly by the National Parks and Wildlife Service, NPWS, which has rewetted, or is in the process of rewetting, 50,000 ha of raised bogs.
The NPWS places considerable emphasis on developing site-specific drainage management plans that utilise aerial imagery, ground surveys and hydrological modelling and monitoring. It actively engages with landowners and farmers and provides compensation when required. To ensure that the water remains on the target area on all rewetted raised bog sites, the marginal drain is generally retained, although it can be blocked if the adjacent landowner is in agreement. Technical implementations include the construction of berms or bunds to hold back water, while on some sites the installation of outfalls and weirs even permit a degree of control of the water level.
The momentum for peatland rewetting is clearly increasing. The peatlands climate action scheme, launched in November 2020, initially targets 33,000 ha for rewetting on more than 80 Bord na Móna bogs. Under the Department of Agriculture, Food and the Marine's Ag Climatise roadmap, released in December 2020, at least 40,000 ha of drained grassland on peat soils has been targeted for rewetting. Rewetting of these soils is interesting as it has not been widely implemented in this country, although it has been carried out in other jurisdictions. In contrast to cutover and cutaway sites, where there is an emphasis on restoration to a functioning peatland ecosystem, with a desire to bring back characteristic peatland flora and fauna, rewetting of drained grassland sites offers the opportunity to target solely a reduction in greenhouse gas emissions. To date, only one study in Ireland has evaluated the potential for greenhouse gas emission reductions in this land use category and showed that carbon dioxide sequestration and minimal methane emissions could be achieved even at water levels down to 25 cm in depth.
The Ag Climatise roadmap also identifies the requirement to identify the drainage status of this land use category to reduce carbon losses through water table management. This is critical to determine accurately the area of drained soils; the differentiation between deep drained and shallow drained soils; nutrient status, as greenhouse gas emissions from drained nutrient-rich fen peats are considerably higher than emissions from drained nutrient-poor peats; and the area of rewetted soils, to ensure that Ireland can avail of greenhouse gas emission reductions in future national inventory reporting.
A new Teagasc project will address some of the uncertainties associated with the rewetting of grasslands and enable Ireland to benefit from the 2018 EU effort sharing regulation. The experience of farmers, landowners and organisations such as the community wetlands forum that will participate in the new European innovation partnerships announced by the Minister of State, Senator Hackett, on 12 February 2021, will be highly informative and build on the outstanding work carried out by ongoing farmer-orientated projects, such as the Burren Life, pearl mussel and hen harrier projects. These will provide an important platform to evaluate the potential of transitioning from current conventional practices to new carbon farming models.