wise use of mires and peatlands - Peatland Ecology Research Group

76 VALUES AND FUCTIONS OF MIRES AND PEATLANDSThe dominant effect of peatland drainage foragriculture is that the peat is exposed tooxygen which leads to peat mineralisation.This causes a decrease in the peat carbonstore and an increased emission of carbondioxide, especially in summer.Under tillage, peat mineralisation isaccelerated as compared to grassland due tomore intensive aeration. Carbon dioxideemissions in arable fens are higher than inbogs.Methane emissions from drained peatlandsare generally very low though someemissions have been observed in bogs.Drained fens emit less methane than bogsand function more frequently as net sinks foratmospheric methane.Nitrous oxide emissions from bogs are lowdue to the low pH and low total nitrogencontent. In the more nutrient-rich fenssomewhat higher nitrous oxide emissionshave been observed. Nitrous oxide emissionsdepend on the available nitrogen andtherefore on nitrogen fertilisation. It isassumed that 1% of the nitrogen applied asfertiliser is emitted as nitrous oxide.Figure 3/2 gives an overview of the globalwarming potential of drained peatlands underdifferent forms of agricultural use. Carbondioxide is by far the most relevant gas,contributing between 85 and 98% of thecumulative global warming potential of allgreenhouse gases. Intensively used boggrasslands have a similar warming potentialto that of tilled bogs. Fertilisation and limingof grasslands strongly increases peatmineralisation.The role of drainage - forestry: The effectof peatland drainage for forestry 180 is morecomplicated than that of agricultural drainage,as various processes with contrasting effectsoccur simultaneously and the integratedeffects differ considerably over different timescales.Figure 3/2: Rough estimates of the global warming potential of fens and bogs (in kg CO 2-Cequivalentsha -1 y -1 ) under different types of land use (compiled by Heinrich Höper 2000).