Response of belowground carbon, sulphur and iron cycling in fen soils
Final Report Abstract
The project aimed at elucidating the impact of extreme drought and rewetting on the coupled redox dynamics of carbon, sulfur and iron and further redox sensitive compounds. These dynamics is scientifically important because northern peatlands represent major global sinks of carbon and sources of methane. The specific objective was to identify and quantify the impact of drying and rewetting on the production and consumption of inorganic and organic electron acceptors and to determine how the redox dynamics affects methanogenesis and respiration rates, as well as corresponding emissions on the annual scale. The investigation utilized three different methodologies: (I) Incubation experiments to identify controlling factors such as temperature, intensity and duration of drying, (II) mesocosm experiments to quantify redox processes and carbon cycling under controlled irrigation scenarios, (III) ecosystem manipulation experiments to characterize the response of soil solution, gas phase, and redox sensitive solid phases to drying, rewetting and flooding. Process rates were determined through mass balances, stable and radioactive isotope analyses. Enhanced drying and rewetting only had a limited impact on CO2 fluxes in mesocosms and in the field because respiration concentrated on the uppermost soil layers that were little influenced by hydrologic boundary conditions. Potential aerobic respiration itself was also little affected by drying and rewetting duration and intensity in laboratory experiments, whereas potential methane production was delayed to a varying degree. Soil temperate was thus probably the most effective control on soil respiration at the investigated peatland. In contrast, drying and rewetting induced a strong redox dynamics in all soil layers that often led to sustained suppression of methanogenesis on the annual scale through the well-known thermodynamic inhibition mechanism. This inhibition was only eased by sustained flooding and in the uppermost reactive and dense peat layers. Pathways of methanogenesis appeared not be influenced by this dynamics. As in other studies before some CO2 production could not be explained by consumption of known electron acceptors in electron flow balances suggesting activity of organic electron acceptors. This could not be demonstrated though, due to methodological limitations. Commonly used concepts assuming the water table as redox cline and redox sequences based on standard Gibbs free energies proved to be too simplistic to understand the occurrence of particular processes. The redox dynamics in the peat could rather be conceived as a continuum of oxidation and reduction gradually shifting by changes in AFP on a weekly to monthly scale. Oxygen concentration as function of air filled porosity (AFP) and bulk density was of central importance for controlling the redox dynamics. A surprising result was a production of methane in the unsaturated zone that has hardly been described before. Whether this is a general phenomenon remains to be investigated in future work.
Publications
- (2008) Groundwater derived arsenic in high carbonate wetland soils: Sources, sinks, and mobility. Science of the Total Environment 401, 109-120
Bauer, M., Fulda, B. and Blodau, C.
- (2008): Arsenic speciation and turnover in intact organic soils mesososms during experimental drought and rewetting. Geochimica et Cosmochimica Acta 72, 3991-4007
Blodau, C. Fulda, B., Bauer, M., Knorr, K.-H.
- (2008): Experimental drought alters rates of soil respiration and methanogenesis but not carbon exchange in soil of a temperate fen. Soil Biology and Biochemistry 40, 1781-1791
Knorr, K.H., Osterwoud, M., Blodau, C.
- (2008): Fluxes and 13 C isotopic composition of dissolved carbon and pathways of methanogenesis in a fen soil exposed to experimental drought. Biogeosciences 5, 1457– 1473
Knorr, K.H., Glaser, B., Blodau, C.
- (2009): Dynamics of redox processes in a minerotrophic fen exposed to a water table manipulation. Geoderma 153, 379-392
Knorr, K.H., Lischeid, G., Blodau, C.
- (2009): Impact of experimental drought and rewetting on redox transformations and methanogenesis in mesocosms of a northern fen soil. Soil Biology and Biochemistry 41, 1187- 1198
Knorr, K.-H., Blodau, C.
- (2010): Impact of altering the water table height on an acidic fen on N2O and NO fluxes and soil concentrations. Global Change Biology 16, 220-233
Goldberg, S. D., Knorr, K.-H., Blodau, C., Lischeid, G., Gebauer, G.
