Peatland restoration by inundation of drained areas can alter local greenhouse gas emissions by altering rates of CO2 and CH4 emissions. Factors that can influence these emissions include the quality and amount of substrates available for anaerobic degradation processes and the sources and availability of electron acceptors. In order to learn about possible sources of high CO2 and CH4 emissions from a rewetted degraded fen grassland, we performed incubation experiments that tested the effects of fresh plant litter in the flooded peats on pore water chemistry and CO2 and CH4 production and emission.
The position in the soil profile of the pre-existing drained peat substrate affected initial rates of anaerobic CO2 production subsequent to flooding, with the uppermost peat layer producing the greatest specific rates of CO2 evolution. CH4 production rates depended on the availability of electron acceptors and was significant only when sulfate concentrations were reduced in the pore waters. Very high specific rates of both CO2 (maximum of 412 mg C d−1 kg−1 C) and CH4 production (788 mg C d−1 kg−1 C) were observed in a new sediment layer that accumulated over 2.5 years since the site was flooded. This new sediment layer was characterized by overall low C content, but represented a mixture of sand and relatively easily decomposable plant litter from reed canary grass killed by flooding. Samples that excluded this new sediment layer but included intact roots remaining from flooded grasses had specific rates of CO2 (max. 28 mg C d−1 kg−1 C) and CH4 (max. 34 mg C d−1 kg−1 C) production that were 10–20 times lower, and were comparable to those of newly flooded upper peat layer. Lowest rates of anaerobic CO2 and CH4 production (range of 4–8 mg C d−1 kg−1 C and <1 mg C d−1 kg−1 C) were observed when all fresh organic matter sources (plant litter and roots) were excluded. In conclusion, the presence of fresh organic substrates such as plant and root litter originating from plants killed by inundation has a high potential for CH4 production, whereas peat without any fresh plant-derived material is relatively inert. Anaerobic CO2 and CH4 production in peat only occurs when some labile organic matter is available, e.g. from remaining roots or root exudates
The hydrogen-dependent and methylotrophic order Methanomassiliicoccales consists of the families Methanomethylophilaceae and Methanomassiliicoccaceae. While Methanomethylophilaceae are comparatively well studied, there is a lack of knowledge on Methanomassiliicoccaceae. In this 16S rRNA gene amplicon sequencing based study we investigated the temporal and spatial dynamics of the Methanomassiliicoccales in drained and rewetted sites of three common temperate fen peatlands. A 2.5-year-monitoring of the fen microbiome composition in three peat depths revealed a dynamic methanogen and Methanomassiliicoccales composition across space and time. Four Methanomassiliicoccales phylotypes were found and they were differentially distributed between the fen types. The wetland cluster phylotype was omnipresent and dominant in abundance in all sites along all depths. The Methanomassiliicoccales phylotype was highly abundant in topsoil while the AB364942 phylotype was exclusively found in deeper regions of the rewetted percolation fen. The phylotype affiliated with Methanomassiliicoccales strain U3.2.1 was only detected in the rewetted percolation fen. We discussed the distribution of the four phylotypes with implications to their ecophysiology, where oxygen tolerance and substrate spectrum might play major roles. In conclusion, the Methanomassiliicoccales are widespread and do account for a significant proportion of methanogens, which might suggest their importance for methane emissions from peatlands.
The ongoing climate warming is likely to increase the frequency of freeze-thaw cycles (FTCs) in cold-temperate peatland regions. Despite the importance of soil hydro-physical properties in water and carbon cycling in peatlands, the impacts of FTCs on peat properties as well as carbon sequestration and release remain poorly understood. In this study, we collected undisturbed topsoil samples from two drained lowland fen peatlands to investigate the impact of FTCs on hydro-physical properties as well as dissolved organic carbon (DOC) fluxes from peat. The soil samples were subject to five freeze-thaw treatments, including a zero, one, three, five, ten cycles (FTC0, FTC1, FTC3, FTC5, and FTC10, respectively). Each FTC was composed of 24 h of freezing (−5°C) and 24 h of thawing (5°C) and the soil moisture content during the freeze-thaw experiment was adjusted to field capacity. The results showed that the FTCs substantially altered the saturated hydraulic conductivity (Ks) of peat. For peat samples with low initial Ks values (e.g., < 0.2 × 10−5 m s−1), Ks increased after FTCs. In contrast, the Ks of peat decreased after freeze-thaw, if the initial Ks was comparably high (e.g., > 0.8 × 10−5 m s−1). Overall, the average Ks values of peatlands decreased after FTCs. The reduction in Ks values can be explained by the changes in macroporosity. The DOC experiment results revealed that the FTCs could increase DOC concentrations in leachate, but the DOC fluxes decreased mainly because of a reduction in water flow rate as well as Ks. In conclusion, soil hydraulic properties of peat (e.g., Ks) are affected by freezing and thawing. The dynamics of soil hydraulic properties need to be explicitly addressed in the quantification and modelling of the water flux and DOC release from peatlands.
Abstract. Rewetting of long-term drained fens often results in the formation of eutrophic shallow lakes with an average water depth of less than 1 m. This is accompanied by a fast vegetation shift from cultivated grasses via submerged hydrophytes to helophytes. As a result of rapid plant dying and decomposition, these systems are highly-dynamic wetlands characterised by a high mobilisation of nutrients and elevated emissions of CO2 and CH4. However, the impact of specific plant species on these phenomena is not clear. Therefore we investigated the CO2 and CH4 production due to the subaqueous decomposition of shoot biomass of five selected plant species which represent different rewetting stages (Phalaris arundinacea, Ceratophyllum demersum, Typha latifolia, Phragmites australis, and Carex riparia) during a 154 day mesocosm study. Beside continuous gas flux measurements, we performed bulk chemical analysis of plant tissue, including carbon, nitrogen, phosphorus, and plant polymer dynamics. Plant specific mass losses after 154 days ranged from 25 (P. australis) to 64% (C. demersum). Substantial differences were found for the CH4 production with highest values from decomposing C. demersum (0.4 g CH4 kg−1 dry mass day) that were about 70 times higher than CH4 production from C. riparia. Thus, we found a strong divergence between mass loss of the litter and methane production during decomposition. If C. demersum as a hydrophyte is included in the statistical analysis solely nutrient contents (nitrogen and phosphorus) explain varying GHG production of the different plant species while lignin and polyphenols demonstrate no significant impact at all. Taking data of annual biomass production as important carbon source for methanogens into account, high CH4 emissions can be expected to last several decades as long as inundated and nutrient-rich conditions prevail. Different restoration measures like water level control, biomass extraction and top soil removal are discussed in the context of mitigation of CH4 emissions from rewetted fens.
<p>ReWet is currently establishing four observatories on drained peatlands in Denmark. These observatories will serve as platforms for ecosystem monitoring, experimental research, technological development and demonstration. The objective of ReWet is to facilitate climate smart management and land use change related to agriculture and forestry on peat soils &#160;The ReWet observatories will focus on measurements of fluxes of greenhouse gases (GHG), energy, water and matter (including major nutrients and dissolved carbon) in the interface of the upper groundwater, soil, vegetation and the atmosphere under different rewetting strategies and land use combinations. The ecosystem monitoring will include biodiversity namely vegetation composition and soil microbial communities. The vertical movement of the peat surface will be monitored using dynamic radar reflectors. The monitoring and research carried out at the observatories will, in combination with nationwide &#160;soil databases, enable development of science based national strategies for rewetting of temperate wetlands like in Denmark to achieve substantially lower GHG emissions at national scale, less nutrient pollution of aquatic ecosystems and increased landscape biodiversity.</p>
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