By Mike Peacock. 9th September 2011.
Northern peatlands are typically nutrient-poor, acidic ecosystems. They store one third of global soil carbon, and one tenth of global freshwater. However, over the past century they have been damaged in numerous ways; they have been burnt for agricultural management, drained for forestry and agriculture, and harvested as a fuel source. In the UK the major change in peatland management has been through the digging of drainage ditches to lower the water table. These ditches vary, but are typically half a metre wide and a metre deep, often in dense networks across large areas.
Decades of research into drained peatlands has created a large knowledge base on the effects of altered water tables. Most studies report an increase in carbon dioxide emissions from respiration. Methane emissions, on the other hand, decrease. This is due to an ingress of oxygen into the peat leading to increased methanotrophy due to a larger and more continuous oxic zone Most studies agree that taken as a whole the biogeochemical changes following drainage lead to a decreased carbon store in the peat, and a net increase in greenhouse gas emissions. With current concerns for climate change and carbon, this is clearly a hot topic.
Now, peatland restoration is in vogue. In the UK the favoured method is to block the man-made drainage ditches, thus restoring the water table to approximately its original level. A research project involving Bangor University, Leeds University, the Open University and the Centre for Ecology and Hydrology is based on one such example. Following the collection of baseline data, hundreds of miles of ditches were blocked on the Migneint, a large blanket bog in north Wales. The aim of the project is to examine the effects of two types of ditch blocking on greenhouse gas fluxes, as well as changes in hydrology and water chemistry. As blocking took place in February 2011, the experimental phase is now well underway.
In addition to being drained, the study site is near the crest of a hill and so was originally moderately dry. Methane fluxes were therefore found to be generally low from both the drained landscape, and have so far remained low from the intact blanket bog areas between the blocked ditches. However, both types of blocking being trialled use peat dams to restrict water flow down the ditches. Pools of varying sizes have formed behind these dams, and large methane fluxes have been recorded from their surfaces. It appears that the removal of the oxic zone has limited the niche for aerobic methanotrophs, thus allowing more methane to reach the atmosphere. Although these fluxes are generally fairly steady over short periods, intense pulses have also been observed, indicating methane ebullition (bubble emissions) from the peat.
Further to this, preliminary CEH/Bangor University work by Mark Cooper at an adjacent site found that blocked ditches were rapidly recolonised by Eriophorum. These plants were large methane hotspots as they act as ‘chimneys’ to allow methane to bypass methanotrophs and enter the atmosphere. It seems probable that the newly blocked ditches will also be colonised by Eriophorum, as this has been seen at numerous peatlands.
Hopefully the study will be able to untangle the complex effects of vegetation and hydrology on gas fluxes and water chemistry, to elucidate whether expensive ditch blocking is a cost-effective method for reducing greenhouse gas emissions, in the UK and elsewhere.