Climate change, impacts and vulnerability in Europe ... - MemoFin.fr

Climate impacts on environmental systems3.5.2 Soil organic carbonRelevanceBiomass is generated by photosynthesis bindingCO 2from the atmosphere. If not harvested, thisbiomass becomes incorporated into the soil afterthe death of the plant and through root senescence.The dead plant material is decomposed with thehelp of micro-organisms and CO 2is again releasedinto the atmosphere. Part of the carbon is convertedinto stable (humic) soil organic matter. However,if soil is water-saturated due to poor drainage,the breakdown of carbon is slowed down andonly highly specialised microorganisms are ableto decompose carbon, releasing CO 2and CH 4.Nevertheless, wet soils and peatlands act overall asimportant carbon reservoirs.Low levels of organic carbon in the soil are generallydetrimental to soil fertility, water retention capacityand resistance to soil compaction. Increases insurface water run-off can lead to erosion while lackof cohesion in the soil can increase the risk of erosionby wind. Other effects of lower organic carbonlevels are a reduction in biodiversity (see Box 3.4)and an increased susceptibility to acid or alkalineconditions.Past trendsAround 45 % of the mineral soils in Europe havelow or very low organic carbon content (0–2 %) and45 % have a medium content (2–6 %) (Louwagieet al., 2009). Map 3.20 shows that low levels areparticularly evident in southern Europe where 74 %of the land is covered by soils that have less than 2 %of organic carbon in the topsoil (0–30 cm) (Zdruliet al., 2004). However, areas of low organic carboncan be found almost everywhere, including in someparts of more northern countries such as Belgium,France, Germany, Norway and the United Kingdom.More than 50 % of EU soil organic carbon stocks areto be found in peatlands (Schils et al., 2008).In general, most soils across Europe are likely tobe accumulating carbon. Except under drainageconditions, grassland soils accumulate carbon,although there is a high uncertainty as to the rate.Croplands generally act as a carbon source, althoughexisting estimates are varied. Forest soils generallyaccumulate carbon (estimates range from 17 to39 million tonnes per year (Schils et al., 2008)).However, estimates of European CO 2, CH 4and N 2Ofluxes between 2000 and 2005, using both top-downestimates based on atmospheric observations andbottom-up estimates derived from ground-basedKey messages: 3.5.2 Soil organic carbon• Soil carbon stocks in the EU-27 are around 75 billion tonnes of carbon; around 50 % of which is locatedin Ireland, Finland, Sweden and the United Kingdom (because of the large area of peatlands in thesecountries).• The largest emissions of CO 2from soils are due to conversion (drainage) of organic soils, and amount to20–40 tonnes of CO 2per hectare per year. The most effective option to manage soil carbon in order tomitigate climate change is to preserve existing stocks in soils, and especially the large stocks in peatand other soils with a high content of organic carbon.• On average, soils in Europe are most likely to be accumulating carbon. Soils under grassland andforests are a carbon sink (estimated up to 80 million tonnes of carbon per year) whereas soils underarable land are a smaller carbon source (estimated from 10–40 million tonnes of carbon per year).• The effects of climate change on soil organic carbon and soil respiration are complex, and depend ondistinct climatic and biotic drivers. However, they lack rigorous supporting datasets.• Climate change is expected to have an impact on soil carbon in the long term, but changes in the shortterm will more likely be driven by land management practices and land use change.148 Climate change, impacts and vulnerability in Europe 2012