Dead parts of plants submerged in water may be preserved in peatlands as partly decomposed peat for thousands of years. The peatland that accumulates peat acts as an atmospheric carbon sink. At the same time, in oxygen-free conditions, carbon compounds produce methane, a greenhouse gas 25 times more powerful than carbon dioxide.
Lack of oxygen is the precondition for production of methane. Under anoxia, fungi and bacteria are unable to decompose dead plant mass. In those conditions, methane (CH4) is produced from carbon compounds by certain protists: archaea, that only operate in oxygen-free conditions.
Drained and undrained peatlands differ in terms of greenhouse gas balances
After water level drawdown, decomposition that requires oxygen becomes possible in the thicker peat layer that previously was water saturated and was thus an oxygen-free environment. At the same time, the recycling of nutrients from peat to tree roots is improved. The downside is that oxic decomposition increases carbon dioxide emissions especially in nutrient rich peatlands. In drained areas that are rich in trees, the trees bind in considerably more carbon than is emitted into the atmosphere from the decomposing peat over the same period. In less nutritious peatlands, soil can still act as a carbon sink. This is due to growing trees and the ground vegetation storing more carbon below ground in the form of root litter than what is released because of decomposition.
Methane emissions through the surface peat layer of a drained peatland will stop or decrease, since methanotrophs, the microbes living in aerated conditions, use up the methane still produced in deeper, oxygen-free layers. This phenomenon is referred to as methane oxidation. The methanotrophs may even use the methane already in the atmosphere, creating a small methane sink.
However, methane can still be released from ditches if there is water in them. Methane released from ditches originates either from the deeper, oxygen-free peat layers or the ditch itself: Algae biomass living in the ditch water may produce suitable parent material for methane-producing microbes. In some cases, methane emissions from ditches may be of such magnitude that the total methane emissions are in fact not reduced after drainage.
In addition to carbon dioxide and methane, drained peatlands may also emit dinitrogen oxide (N2O), also known as laughing gas or nitrous oxide. In terms of the heating impact, dinitrogen oxide is a much more efficient greenhouse gas than methane. It can only be produced in the peat of peatlands with a high nitrogen content. Cleared peatlands that are turned into fields are examples of such environments, where high CO2 and N2O emissions may sustain long after abandonment of the field. Reforestation may be a way of reducing greenhouse gas emissions from fields with peaty soil.
Research into factors affecting greenhouse gases and carbon sequestration
The Natural Resources Institute Finland carries out research on greenhouse gas emissions and carbon sequestration in natural mires and drained peatlands and in peatlands no longer used for peat extraction. The research also focuses on how the use of peatlands by drainage, various forestry measures, or after-use options such as restoration affect the emissions and carbon sequestration. Another goal is to study the effect of microbial communities and fine roots on emissions and carbon sequestration. Research material is also generated in order to develop methods for assessing the greenhouse gas balance of peatland forests.
Luke is the responsible authority in charge of greenhouse gas inventories in managed lands, including drained peatlands. Luke researchers have participated in planning and preparing the inventory instructions published by the Intergovernmental Panel on Climate Change (IPCC).
Picture on top of the page: Erkki Oksanen, Luke