What is loss of organic matter in peat soils?
A decline of soil organic matter in peats soil across Europe is mainly due to mineralisation (biochemical decomposition). Drainage to reclaim peatlands results in subsidence and decomposition of the peat. Peatlands are one of the major stocks of carbon (C) in the world and the loss of organic matter in peat soils, turn them into a major source of CO2 and N2O.
|Drained peatland for dairy farming in the Netherlands. (Photo: J.J.H van den Akker)||Drained agricultural peatland (carrots) in Sweden. (Photo: Kerstin Berglund)|| Forestry on drained peatland in Finland.
(Photo: Björn Klöve)
Where does it occur?
It is estimated that the EU (27) had in 2008 about 229 000 km2 peat soils with a conservative estimated C stock of 18 700 Mton of C. CO2 emission of drained peat soils of the EU (27) is estimated at 173 Mton CO2 per year, which means that the European Union is, after Indonesia and before the Russian Federation, the world’s second largest peatland emission hotspot.
|Relative cover (%) of peat and peat-topped (0 – 30 cm) soils in the SMUs of the European Soil Database3|
More than 50% of this peat area is in just a few northern European countries (Norway, Finland, Sweden, United Kingdom) and the remainder mainly in Ireland, the Netherlands, Germany, Poland and the Baltic states. Of this area, approximately 50% has already been drained, while most of the undrained areas are in Finland and Sweden. It is estimated that the decline of organic matter (OM) in drained agricultural peat soils due to mineralisation is about 10-20 tonnes OM per hectare per year.
What causes it?
A major reason for the decline in organic matter in peat is reclamation and drainage of peat soils for forestry and agricultural land and feed and food production. Fen peat soils are particularly suited for agricultural use and practices, such asthe drainage, cultivation, liming and fertilizer use, has causesd rapid mineralisation of organic matter.
It is predicted that climate change will have a major impact on peat soil degradation and increase CO2 emissions, partly due to the increase of decomposition rate by the temperature rise, and mainly by the more often occurrence of long periods with extreme drought. Climatic conditions in natural peatland areas can gradually change from favourable for peat growth into peat degrading conditions.
How can it be measured or assessed?
|Soil threat||Soil threat indicators|
|Decline in OM
in peat soils
The table below lists key indicators, purpose of the indicator, methods and corresponding references for measuring peat stocks.
|Peat stocks and peat stocks reduction||Measure amount of C in peat soils||PS = PA *PD * 10-4 * Db where PStock is peat stock in Mt; PArea is peat area in km2; PDepth is peat depth in m; Db is bulk density in t m-3 (t m-3)|
|Measure subsidence||Peat stock reduction can be calculated from the subsidence over a period. The annual loss of OM per hectare per mm subsidence is about 1000 – 1100 kg. This equals a CO2 emission of about 600 kg CO2-C.|
|Measure/estimate direct CO2 emissions||Closed gas chamber|
|Micro-meteorological measurements using eddy-covariance
|Identify vegetation type||Mapping of vegetation types characterized by the presence and absence of species groups indicative for specific water level classes and GHG emissions.2|
|Estimate loss of OM and GHG emissions||Simulate peatland emissions of CO2, CH4 and N2O, soil subsidence and nutrient loading of surface waters with models.3|
How can it be prevented or remediated?
Much of the decline in organic matter caused by peatland drainage can be reversed by raising water tables to the land surface, a process known as rewetting. This will exclude agricultural use and to a large extent also forestry. There is no universal strategy for rewetting a drained peatland, however, malpractices can result in a boost of GHG emissions and severe pollution of surface waters. Other major constraints are the costs and the lack of water now or in the future due to climate change. Specifically, rewetting of agricultural peat soils can be very costly and de facto impossible due to socio-economic and cultural, historic reasons. There can be various causes for the drained conditions, and the rewetting options vary widely depending on climate, water availability and topography.
|Decreasing water losses from the peatland||Increasing water supply to the peatland||Enlarging water storage in the peatland|
|Damming or infilling of drainage canals and ditches||Decreasing groundwater extraction and/or increasing groundwater recharge in the catchment area||Installing bunds (elongated dams) to
increase water storage over the peat surface
|Raising overflow heights of weirs and sluices||Diverting water into the site||Creating paddy fieldlike cascades to rewet sloping peatlands|
Raising groundwater level
|Irrigating by pumping into the site||Maintaining or creating hollows (e.g. dammed canals) to increase depression storage|
|Establishing and allowing obstructions in water courses (e.g trees, rocks)
||Perforating stagnating surface peat soil horizons to restore discharge of groundwater|
|Removing subsurface drainage pipes by excavation or destruction|
|Reducing evapotranspiration from tree growth in the peatland|
|Establishing hydrological buffer zones with higher water levels|
An alternative experimental measure is cropping and afforesting on wet and rewetted peatlands with crops adapted to the wet soil conditions, known as Paludiculture, which may maintain and add organic matter to peatlands.
|Conservation of peat soils in use as grassland by raising groundwater levels by infiltration via submerged drains, one of the case studies in RECARE (Photos: J.J.H van den Akker)|
Case Study Experiments
How does it interact with other soil threats?
Natural peat soils are hotspots of biodiversity. As the source of energy underpinning food-webs, a decline in OM may lead to reduced biodiversity. Natural peatlands and even not completely degraded peat lands store water and act as a sponge. They absorb and retain water during periods with a surplus of precipitation and slowly release water in times of water deficit. In this way, peat lands slow down peak discharge and reduce incidences of flooding and prevent water erosion. Degraded peat soils in arable agriculture or in overgrazed grassland are vulnerable to water and wind erosion. Water erosion is especially a problem in overgrazed blanket peats. Wind erosion is a serious problem on peat soils in arable agriculture.
How does it affect soil functions?
- Biomass production - Oxidation of peat soils results not only in emissions of CO2, but also in mineralization of N, which makes the degradation of peat soils an important supply of nutrients and therefore can increases food and biomass production considerably. However, on-going oxidation and loss of peat results in time in the total loss of the peat layer. The level of biomass production then depends on the fertility and soil physical properties of the soil underneath the original peat layer, many of which are acid.
- Storage/filtering/transforming - Peat soils have a high storage, filtering, buffering and transformation capacity. Loss of peat results in loss of these capacities, especially the storage of C.
- Gene pool (biodiversity) Drainage of natural peat soils results in a significant change in biodiversity and more so in peatland meadows than peat soils in arable agriculture.
- Cultural heritage - Peat soils are by nature historical archives and can store artefacts of ancient cultures and human bodies. Drainage and oxidation of peat results in a total loss of this historical archive. On the other hand in e.g. the Netherlands the historic drained peat meadow landscape is also considered part of the cultural heritage.
1 Montanarella, L., Jones, R.J.A. and Hiederer, R., 2006. The distribution of peat land in Europe. Mires and Peat, 1, 1-10. http://mires-and-peat.net/pages/volumes.php
2 Couwenberg, J., J. Augustin, D. Michaelis and H. Joosten, 2008. Emission reductions from rewetting of peat lands - Towards a field guide for the assessment of greenhouse gas emissions from Central European peat lands. Duene /Greifswald University, Report RSPB, Bedfordshire, 28 pp
3 Hendriks, R.F.A., Wolleswinkel, R.J. and Van den Akker, J.J.H., 2008. Predicting greenhouse gas emission in peat soil depending on water management with the SWAP-ANIMO model. Proceedings 13th International Peat Congress, Tullamore, Ireland, International Peat Society.
- FAO UN - The importance of soil organic matter: Key to drought-resistant soil and sustained food production (2005)
- European Communities - Organic matter decline (2009)
- University of Minnesota Extension - Organic matter management
- US Department of agriculture: Soils and Men, Yearbook of Agriculture; Loss of Soil Organic Matter and Its Restoration By William A. Albrecht (1938)