Browsing by Subject "Phragmites"

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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.