Peatlands are wetlands where permanently waterlogged conditions prevent the complete decomposition of
dead plant material. In peatlands thick layers of carbon rich peat has accumulated over thousands of years. Peatlands contain thus disproportionally much carbon: in the boreal zone seven times, in the tropics even 10 times more per hectare than ecosystems on mineral soil. Peatlands are the most space-effective stocks of organic carbon on the planet: although they cover only 3% of the land, they contain more carbon than the entire forest biomass of the world.
When peatlands are drained, oxygen enters the peat, breaks it down, and the hitherto well-preserved carbon and nitrogen are released as greenhouse gases to
the atmosphere and as nitrate to the surface water. Worldwide only 15% of the peatlands have been drained (= 0.4% of the land area), but these drained peatlands are responsible for 5% of all global anthropogenic Co2 emissions.
Next to emissions, drainage disrupts other peatland ecosystem services, such as water puri cation, ood control and the provision of habitat for specialized biodiversity. One of the largest – but least recognized – problems is subsidence. Drained peatlands lose
1-2 cm of height per year, in the tropics even 5-7 cm. Subsidence leads all over the world to increasing drainage problems, salt intrusion in coastal peatlands and eventually to the loss of productive land. While the sea level is rising, the peatlands are being bogged down.
Land use in peatlands
Not only peatland occurrence and mire types are controlled by climate, but also land use. Human impact on (sub) arctic peatlands is restricted to hunting and gathering, reindeer grazing and infrastructure (roads, pipelines). Historically many boreal mires were mown and grazed for low-intensity agriculture; currently the peatlands are used for forestry and peat extraction. Large areas in the boreal zone, especially in the warmer parts, and in the temperate zone have been drained for arable and grassland agriculture and for peat mining.
Under the wet conditions necessary for peat formation, part of the dead plant material is decomposed in the absence of oxygen, resulting in the emission of methane (CH4). Natural peatlands are thus a major global source of CH4. Methane is a much stronger greenhouse gas than Co2 but has only a short atmospheric residence time (12 years).
When taking a short-term perspective, natural peatlands appear to have hardly any effect on the climate because the climate cooling of sequestered CO2 is annihilated by the climate warming effect of emitted CH4. As, however, the CH4 is rapidly removed from the atmosphere by oxidation, whereas atmospheric Co2 continues to be absorbed, the world’s peatlands effectively cool the climate on the longer run.
Greenhouse gas uxes from peatlands are in uenced by a wide range of interrelated physical, chemical and biological processes, with water table being the single most important factor.
Peatland rewetting as solution
If the water table is restored to pre-drainage levels, greenhouse gas uxes become again similar to
uxes from undrained peatlands. In other words: Co2 emissions decrease, stop or even become negative (peat accumulation), N2o emissions stop and CH4 emissions increase compared to the drained state.
It takes a while for the rewetted peatland to adapt to the new situation. During the rst years after rewetting greenhouse gas uxes tend to deviate from pristine sites, with rewetted nutrient poor sites usually showing lower and rewetted nutrient rich sites higher CH4 emissions than pristine sites.
Because CH4 has a 23 times stronger climate effect than Co2 (the new IPCC Fifth Assessment Report even assumes 28 times), rewetted peatlands do not become ‘positive’ for the climate in an absolute sense (on the 100 year timescale). The immediate bene t of peatland rewetting is in the fact that the net greenhouse gas emissions from rewetted peatlands are signi cantly lower compared to the previous drained situation as is shown by the following table (based on the new IPCC tier 1 values for Co2, CH4, N2o and DoC for drained and rewetted organic soils):
Rewetting of peatlands also leads to a re-instalment
of the other important ecosystem services. Several Nordic-Baltic countries have already practiced large- scale rewetting of peatlands for restoring biodiversity. Does it ring bells, that its time for us to begin too?