Poster, International Symposium Soil processes under extreme meteorological conditions, Bayreuth: 25.02.2007 - 28.02.2007
Abstract:
Extreme meteorological conditions like droughts and heavy rainfalls are known to increase due to global climate change. Peatlands are natural sinks for organic carbon but also significant sources of relevant green house gases like methane. H2 appears to be the dominant precursor for methanogenesis in many slightly acidic peatlands. Minerotrophic fens located in northern Bavaria, Germany receive Fe(II) from anoxic groundwater flow. Thus, considerable amounts of Fe(III) are available as alternative electron acceptor for the oxidation of organic matter. Fe(III)-reducing bacteria may suppress methanogenesis by competing for H2 or acetate. In addition, some methanogens may divert electrons to Fe(III). An increase of drying and rewetting events might not just stimulate soil respiration but also renewal and enhance the pool of reducible Fe(III) in deeper peat layers. The aim of this project was to compare Fe(III)-reducing and methanogenic activities during a fen drying/rewetting experiment. Drying of the fen (Histosol) was experimentally initiated by roof construction and drainage in August 2006 followed by controlled rewetting with artificial rain water of three replicate sites. Three non-manipulated sites served as control. Suction cups and dialysis chambers were used to study biogeochemical parameters like Fe(II), sulfate, nitrate, pH and methane in depth profiles. In the soil solution nitrate declined rapidly with increasing depth followed by an increase of Fe(II), a decrease of sulfate, and the formation of methane. FeS probes demonstrated the stability of anoxic conditions down to depth of 66 cm. Anoxic incubation experiments demonstrated that the upper (0-10 cm) zone was the most active layer. The duration of Fe(III)-reduction and starting point of methane formation depended on the stage of peat oxidation at all different sampling times. The reduction of Fe(III) occurred with a rate of approximately 4 µmol Fe(II) * g (fresh weight soil)1 * d-1, CO2 formation was in the range from 0.5 to 1.5 µmol * g (fresh weight soil)-1 * d-1, whereas methane formation approximated 25 to 50 nmol * g (fresh weight soil)-1 * d-1. In the presence of methyl fluoride, a selective inhibitor of acetoclastic methanogenesis, the formation of methane was inhibited by more than 50% indicating that acetate might be also an important precursor for the formation of methane in this fen. Although relative high amounts of Fe(II) were present in deeper zones, there was no on-going Fe(III)-reducing activity prior to the onset of the drying experiment. Numbers of acetate-, ethanol-, or lactate utilizing Fe(III)-reducing bacteria cultured at pH 5.5 approximated 10^5 to 10^6 cells * g (fresh weight soil)-1 and decreased only slightly with soil depth. Numbers of fermentative glucose-utilizing Fe(III)-reducers were most abundant indicating that fermenting bacteria might contribute substantially to the reduction of Fe(III) which appeared to be an important alternative electron acceptor for the oxidation of organic matter in upper zones of this acidic fen.