Draining of peatlands results in an increase in greenhouse gas emissions

Scientists from Aarhus University are working on pinpointing the magnitude of greenhouse gas emissions from peatland agriculture in Denmark.

Cows burping, cars spewing exhaust fumes, and factories swallowing tons of fossil fuels are not the only sources of greenhouse gases being released to the atmosphere and causing global warming. The Earth’s soil can also be a major source of the three greenhouse gases carbon dioxide, nitrous oxide and methane – especially when it comes to peatlands. But how big is the problem?

Denmark has decided to make use of Article 3.4 of the Kyoto Protocol, which requires that management effects on the greenhouse gas balance of agricultural soils are reported. For peatlands there are currently no available figures. The Danish Ministry of the Environment has therefore established a monitoring programme. Scientists from the Faculty of Agricultural Sciences at Aarhus University are involved in the programme and are currently investigating how the lie of the land is with regard to the greenhouse gas balance.

The determination of carbon fluxes is one of the great challenges since emissions from the soil are partly balanced by plant uptake via photosynthesis.

- Peatlands have huge stocks of carbon, says PhD student Carolyn Schäfer from the Department of Agroecology and Environment at the faculty, and who is deeply involved in the project.

- When peatlands are drained for agriculture, the organic matter is decomposed and greenhouse gases are emitted to the atmosphere in large quantities. According to the organization Wetlands International, these emissions have an estimated global magnitude of 2-3 Gt CO2 equivalents per year.

- If you want to reduce greenhouse gases, then a useful tool would be to look at peatland management. This, however, requires more knowledge of peatlands and there are no emission factors available for cultivated peatlands in Denmark.

Getting to know peatlands

In order to get a clearer picture of management effects on greenhouse gas emissions from peatlands, the scientists working on the project are measuring annual balances of CO2, methane and nitrous oxide emissions from peat soil within three geologically different regions. The PhD study of Carolyn Schäfer has particular focus on the carbon turnover in three permanent grasslands of the monitoring programme.

One area is Store Vildmose in North Jutland, an area in which there used to be many raised bogs (peatlands fed by rain). The second area being monitored is an outwash area in West Jutland one kilometre away from the recently rehabilitated Skjern River. In this area the groundwater has a low pH. The third area is in East Jutland, near Randers. This area is dominated by moraine soils with a high pH in the groundwater.

Each area has a different type of peat because it is developed from different kinds of plant material. The type of plant material affects the peat’s chemical properties, porosity, water-binding capacity and plant decomposition.

For modelling the annual CO2 and methane emissions, Carolyn Schäfer collaborates with colleagues at Hamburg University. Besides modelling, her project focuses on methane producing microorganisms in permanent grasslands. By subsampling from vertical soil profiles she studies where in the peat layers the most methane is being produced, and looks at variations due to season, location and height of the water table.

In addition, soil samples are incubated in the laboratory under conditions which reveal the distribution with relation to depth of methane production potentials. During a stay at the Max Planck Institute for Terrestrial Microbiology in Germany she will also analyze which methane-producing organisms are actually present in the soil, and if the species composition depends on the substrate.

- It will be interesting to see if there are differences in the carbon balances between the regions. Once we have figures from the representative areas, then scientists working on another part of the project will upscale the figures, says Carolyn Schäfer.

- We need to know when and how much CO2 and methane are being released from peat soils in Denmark. If we understand the regulation of these processes, then we have a better chance of managing peatlands. For the carbon balance, the best action would be to stop draining the peatlands - to simply put the anaerobic lid back on and keep the carbon where it is. However, it is important to know the trade-off with methane, which is produced under anaerobic conditions.

The project is supported by the Danish Ministry of Environment.

Facts about peat:

- Peat is formed when plant material does not decay completely due to anaerobic conditions caused by almost permanent water saturation of the soil.

- Normally, the plant material has been under water for millennia.

- When this balance is disturbed, such as when the soil is drained, the plant material decays fast and releases CO2 and nitrous oxide, though less methane.

- For several years after a disturbance, emissions will be elevated to the extent that turnover of organic matter is accelerated.