|Paul, S; Küsel, K; Alewell, C: Reduction processes in forest wetlands: Tracking down heterogeneity of source/sink functions with a combination of methods, Soil Biology Biochemistry, 38, 1028-1039 (2006), doi:10.1016/j.soilbio.2005.09.001|
Wetlands are considered to be the biggest unknowns regarding the influence of global climate change on element dynamics. Knowledge of processes and conditions controlling sink and source function of redox processes is crucial. The aim of this study was to investigate the sink/source function of nitrate, iron, sulfate reduction and methanogenesis of an upland and a lowland fen within a boreal spruce catchment (Lehstenbach, Fichtelgebirge, Southern Germany). Methods used were (i) suction cups and anaerobic dialysis chambers for soil solution sampling, (ii) FeS probes for the determination of sulfur oxidation potential and stability of anoxic conditions and (iii) analysis of the soil solid phase (contents of carbon, sulfur and iron species). Results indicated that both fens had high rates of nitrate reduction and potentially high rates of methane production. The upper few centimetres of all investigated profiles were oxic with low methane concentrations. Even though the latter might suggest low methane emission rates from the soil, methane emissions by vascular plants cannot be excluded. Both sites differed significantly regarding sulfate and iron reduction processes. The upland fen is characterized by relatively stable anoxic conditions, low iron contents but high contents of organic sulfur and relatively low C/S ratios. We concluded that the upland fen can be considered an effective sink for sulfate with long term sulfur storage. In contrast, the lowland fen was characterized by alternating oxidation–reduction cycles with high iron contents, relatively lower contents of organic sulfur and higher C/S ratios. Thus, even though low sulfate and high iron concentrations in soil solutions indicated high reduction rates in the lowland fen, long term storage of sulfur is not likely in this fen. Differences between sites are most likely not induced hydrologically as water table data is fairly comparable. Vegetation at the two fens is distinctly different suggesting a possibly significant role of vascular plants in regulating biogeochemical processes in these forest wetland soils.
Keywords: Wetland soils; Reduction processes; Sulfate retention; Methane emissions; Sequential reduction chain