Browsing by Subject "Fen"

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Die Rolle des Porenraums im Kohlenstoffhaushalt anthropogen beeinflusster Niedermoore des Donaurieds
(2007) Höll, Bettina; Stahr, Karl
The use of peatlands in Central Europe for hundreds of years has led to their degradation (loss of organic matter) due to intensive mineralisation. Re-wetting of formerly drained peat aereas has been a popular method of retaining existing peatlands. The effect of re-wetting of degraded fens on their C-pools and C-fluxes is unknown. The protection of these natural resources combined with the creation of biological C-sinks might render the protection and conservation of peatland ecosystems more attractive. Water-logging leads to the accumulation of water in previously air-filled soil pores, something that might increase the C-pool of the soil. It is unknown whether the pore space, which possibly accounts for up to 90% of peatlands, contains carbon components that are similar to those found in the solid soil substance. It is also unknown how much the utilisation of peatlands affects the composition of C-components of the pore space. The major objectives of the present study were (1) to assess the temporal and spatial variability of the C-components in the pore space in fens undergoing different anthropogenic use (drainage, re-wetting) and (2) to assess the role of the pore space in the C-budget. In a Southern German area known as the Schwäbisches Donaumoos, carbon components of the gaseous phase (CO2, CH4) and the liquid phase (CO2/DIC, CH4, DOC, POC) were collected at different depths (5, 10, 20, 40, 60, 80 cm) from different drained (deep, moderately) fen sites and from a long-term re-wetted fen site. Sampling was done at weekly intervals between April 2004 and April 2006. The samples of the water phase and gas phase were collected at the respective sites using slotted PVC tubes and soil-air probes. Gas was analysed using a gas chromatograph and dissolved organic carbon was analysed using a TC water analyser. The fen sites were characterised by selected static parameters of the solid substance and dynamic parameters such as redox potentials, temperature, water level, soil-moisture tension and pH value. The specific use of the fens, which is closely related to the water budget of the area, was a decisive determinant of the amounts of carbon in pore space. Although the solid soil substance in fen sites accounted for less of 10% of the total substance (solid + pores), it still contained a higher amount of carbon (60 -152 kg C m-3) than the pore space. Furthermore the amount of time that the carbon remains is eventually longer in the solid soil substance than in the pore water. Assuming the pore water works only as a short time reservoir. Filling of the pore space with either air or water had a decisive effect on the amount of C. The investigations showed that the amount of C in the air-filled pore space contained an annual average of 15 g C m-3 (deep-drained area), whereas the water-filled pore space contained on average 263 g C m-3 (re-wetted area). The variable anthropogenic effects on fens led to area-specific situations (e.g. groundwater level) that not only affected the amount of C but also had a significant effect on the composition of C components. Dissolved inorganic carbon (DIC), with an average proportion of 55-72%, accounted for the largest proportion of dissolved carbon. Particulate organic carbon (POC) had similar concentrations to dissolved organic carbon (DOC), whereas dissolved methane (CH4) only accounted for a minor proportion (< 0.1%) of the entire carbon of the liquid phase. The DIC concentration was highest in the water from the pores of re-wetted fen. Independent from the use of the fens, different DIC isotope signatures of the ground, karst and spring waters (-11.7‰ to -14.3‰) in comparison to the pore waters (-16.7‰ to -18.4‰) were observed. The further differentiation into the 13C ratios of CO2 contained in the gaseous phase (-23.0‰ to -26.6‰) suggests that DIC ‘accumulated’ in the pore water by way of biogenic CO2. DOC concentrations were lowest in the re-wetted fen. The temporal variability of DOC was related to changes in the bioavailability of DOC. This was also observed in the moderately drained area. The low degree of aromatisation (= higher bioavailability) associated with higher DOC concentrations led to significantly lower values in the re-wetted area compared to the moderately drained area. The microbially easily available DOC proportion was not only temporally but also spatially limited and had a significant effect on the CO2 and CH4 concentrations. At similar depths, CO2 values 10- to 1000-fold higher than CH4 levels could be measured in the gaseous phase (2.7-67 mg CO2-C l-1 vs. < 5.3 mg CH4-C l-1). The highest concentrations were measured in the re-wetted fen. The CO2-C/CH4-C ratios rarely achieved ratios of below 100:1. Due to the higher concentrations of CO2, it can be assumed that the carbon dioxide could compensate for the effect of methane on the climate, on the condition that comparable CO2-C/CH4-C ratios are found in the emissions. The protection of fens as natural resources could be related to carbon uptake (results of the gas exchange to the atmosphere) and higher carbon amounts in the pore space. The amount of time that the carbon remains in the pore waters is correlated to carbon turnover and hydrological conditions. The latter are also important when assessing the indirect emissions, playing an important role in drained fens and rounding out carbon balances.
The role of Phragmites australis in carbon, water and energy fluxes from a fen in southwest Germany
(2019) van den Berg, Merit; Streck, Thilo
The global carbon emission from peat soils adds up to 0.1 Gt-C per year. Under anaerobic conditions, organic material is decomposed to methane (CH4). Over a 100-year cycle, methane is a 28 times stronger greenhouse gas than carbon dioxide and is an important factor for climate change. Therefore, there is a great interest to get a better understanding of the carbon flows in peatlands. Phragmites peatlands are particularly interesting due to the global abundance of this wetland plant (Phragmites australis, common reed) and the highly efficient internal gas transport mechanism. This is a humidity-induced convective flow (HIC) to transport oxygen (O2) to the roots and rhizomes, with the effect that simultaneously soil gases (CH4 and CO2) can be transported to the atmosphere via the plant. Thereby, Phragmites is expected to have a high evapotranspiration (ET) rate due to the large leaf area, open water habitat and high aerodynamic roughness. This ET could highly influence the hydrology of the system. Because he accumulation of organic material occurs because of limiting oxygen levels, hydrological processes are fundamental in the development of peatlands. The research aims were: 1) to clarify the effect of plant-mediated gas transport on CH4 emission, 2) to find out whether Phragmites peatlands are a net source or sink of greenhouse gases, and 3) to evaluate ET in perspective of surface energy partitioning and compare results with FAO’s Penman-Monteith equation. CO2, CH4 and latent and sensible energy fluxes were measured with the eddy covariance (EC) technique within a Phragmites-dominated fen in southwest Germany in 2013, 2014 and 2016. In 2016, a field experiment was set up to quantify the contribution of plant-mediated CH4 transport to the overall CH4 flux and how it influences ebullition. One year of EC flux data (March 2013–February 2014) shows very clear diurnal and seasonal patterns for both CO2 and CH4. The diurnal pattern of CH4 fluxes was only visible when living green reed was present. This diurnal cycle had the highest correlation with global radiation, which suggests a high influence of HIC on CH4 emission. But if the cause were HIC, relative humidity should correlate stronger with CH4 flux. Therefore, we conclude that in addition to HIC at least one other mechanism must have been involved in the creation of the convective flow within the Phragmites plants. We quantified the influence of pressurized flow within Phragmites on total CH4 emission in a field experiment (see chapter 3) and found between 23% and 45% lower total CH4 flux when pressurized flow was excluded (by cutting or cutting and sealing the reed). The gas transport pathways from the soil to the atmosphere changed as well. Relative contribution of ebullition to the total flux increased from 2% in intact Phragmites to 24-37% in cut vegetation. This increase in ebullition in cut vegetation, obviously, did not compensate the excluded pathway via the pressurized air flow at our site. It also means that the effect of CH4 bypassing the oxic water layer by plant transport on CH4 emission is much larger than the effect of O2 transport through the plants on CH4 oxidation and production in the rhizosphere. Overall, the fen was a sink for carbon and greenhouse gases in the measured year, with a total carbon uptake of 221 g C m-2 yr-1 (26% of the total assimilated carbon). The net uptake of greenhouse gases was 52 g CO2 eq.m-2 yr-1, which is obtained from an uptake of CO2 of 894 g CO2 m-2 yr-1 and a release of CH4 of 842 g CO2 eq.m-2 yr-1. Compared to the long term uptake of carbon by northern peatlands (20–50 g C m-2 yr-1) 212 g C yr-1 is therefore very high. One year of measurements is not enough to draw hard conclusions about the climate change impact of this peatland. The measured ET at our site was lower than other Phragmites wetlands in temperate regions. ET was half the amount of precipitation (see chapter 4). Therefore, the risk of the wetland to dry out is not realistic. ET was especially low when there was little plant activity (May and October). Then, the dominant turbulent energy flux was sensible heat not latent heat. This can be explained by the high density of dead reed in these months. the reed heats up causing a high sensible heat flux. Evaporation was low due to the shading of the water layer below the canopy and low wind velocities near the surface. FAO’s Penman-Monteith equation was a good estimator of measured ET with crop factors from the regression model of Zhou and Zhou (2009) (see chapter 4). Especially the day-to-day variation was modeled very well. Their model had air temperature, relative humidity and net radiation as input variables. This is likely related to stomatal resistance, which depends on the same variables. Therefore, the model of Zhou and Zhou (2009) is an interesting tool for calculating daily crop factors and it is probably robust enough to be used also in different regions.