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Accueil du site → Doctorat → Allemagne → 2008 → Dynamics of soil processes under extreme meteorological boundary conditions - response of below-ground carbon, sulfur, and iron cycling in fen soils

Universität Bayreuth (2008)

Dynamics of soil processes under extreme meteorological boundary conditions - response of below-ground carbon, sulfur, and iron cycling in fen soils

Knorr, Klaus-Holger

Titre : Dynamics of soil processes under extreme meteorological boundary conditions - response of below-ground carbon, sulfur, and iron cycling in fen soils

Auteur : Knorr, Klaus-Holger

Université de soutenance : Universität Bayreuth

Grade : Doktor der Naturwissenschaften (Dr. rer. nat.) 2008

Northern peatlands store approximately 30 % of the global soil carbon stocks. On the other hand, peatlands contribute about 3-10 % to the global methane burden into the atmosphere. Climate predictions foresee not only an increase in the global mean temperature, but also a change in precipitation patterns. As peatlands critically depend on hydrological conditions, a change in precipitation distribution is likely to affect the carbon sink and source function of peatlands. Thus, these ecosystems have become the focus of an increasing number of studies over the past decades. Low water table levels, high temperatures, and a higher nutrient availability were mostly found to increase respiratory activity, but to reduce methane production and –emission. Existing studies, however, investigated changes in average environmental conditions in the long term, while the impact of extreme weather on peatland elemental cycles is still uncertain. Moreover, most studies do not provide a mechanistic understanding of the redox processes underlying the response of peatlands to fluctuations of the water table level. Based on laboratory studies, a thermodynamically constrained competition of the different terminal electron accepting processes for common electron donors was postulated. A detailed validation of this concept under natural or near-natural conditions is, however, still lacking to date. Furthermore, the processes that renew alternative electron accepting capacity during drought are still not yet understood. Fens were also identified to be notable sources or sinks for arsenic. The close association of arsenic with the iron- and sulphur-dynamics – and thus redox dynamics during fluctuations of the water table level in general – is already known. Nevertheless, there exist hardly any study investigating arsenic dynamics and solid phase associations for fens. The main objective of this work was therefore to study the effects of more pronounced drying and rewetting events on redox processes of carbon, iron, and sulphur – and concomitantly arsenic – in an electron acceptor rich fen-ecosystem. In contrast to some existing studies, we could not find a notable effect of the drying/wetting treatment on the overall carbon budgets of the peat. There was an obvious effect of drying/wetting on respiration within the soil, increasing drastically during drought, but the net carbon budget was by far dominated by the autotrophic activity of the vegetation (55-65 %) which was hardly affected by the treatment. Due to the drought event, methanogenesis was effectively suppressed in the unsaturated part of the profile and re-established after rewetting only after a notable time lag of some weeks. This suppression of methanogenic activity – in the laboratory and in the field approach – could successfully be explained by a reoxidation of reduced iron and sulphur compounds, providing alternative electron accepting capacity during and after drought. Only after depletion of alternative electron acceptors, methanogenic conditions could re-establish. Locally, however, in micro-environments especially in the uppermost, intensively rooted layers, methanogenesis re-established even before alternative electron acceptors had been depleted. Based on the obtained data, we propose the high availability of easily degradable organic material, a still high water content, and poor aeration of the peat to responsible for this observation. These factors could support a local depletion of alternative electron acceptors and methanogenesis could thus occur in locally distinct micro-environments. The analysis of the isotopic composition of the dissolved CO2 and the methane produced suggested that the methane was formed via the CO2-reduction pathway with H2 as the electron donor. This pattern was not affected by the drying/wetting treatment. Exceptionally high isotope fractionation factors suggested thermodynamic conditions to be quite unfavourable for methanogens. This coincided with the observation that most of the peat was likely structured in small micro-environments of locally distinct redox conditions and the rapid switches between methanogenic and methanotrophic conditions. The arsenic dynamics under variable redox conditions generally followed the dynamics of ferrous iron, especially in the intensively rooted uppermost soil layers. Coincidingly, a major part of the arsenic was found in the reactive iron-hydroxide fraction, readily available for microbial reduction. Although the total arsenic content in the solid phase was comparably low in the fen under study, concentrations of arsenic exceeded common drinking water standards mostly by far. Methylated arsenic species did not play a noteworthy role in this fen and the immobilization of arsenic in sulfidic phases during reducing conditions was also negligible when compared to mobilization from iron-hydroxide reduction.

Mots Clés : Biogeochemistry , climate change , peatlands , methane , arsenic


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