High soil solution carbon and nitrogen concentrations in a drained Atlantic bog are reduced to natural levels by 10 years of rewetting

Anthropogenic drainage of peatlands releases additional greenhouse gases to the atmosphere, and dissolved carbon (C) and nutrients to downstream ecosystems. Rewetting drained peatlands offers a possibility to reduce nitrogen (N) and C losses. In this study, we investigate the impact of drainage and rewetting on the cycling of dissolved C and N as well as on dissolved gases, over a period of 1 year and a period of 4 months. We chose four sites within one Atlantic bog complex: a near-natural site, two drained grasslands with different mean groundwater levels and a former peat cutting area rewetted 10 years ago. Our results clearly indicate that long-term drainage has increased the concentrations of dissolved organic carbon (DOC), ammonium, nitrate and dissolved organic nitrogen (DON) compared to the near-natural site. DON and ammonium contributed the most to the total dissolved nitrogen. Nitrate concentrations below the mean groundwater table were negligible. The concentrations of DOC and N species increased with drainage depth. In the deeply-drained grassland, with a mean annual water table of 45 cm below surface, DOC concentrations were twice as high as in the partially rewetted grassland with a mean annual water table of 28 cm below surface. The deeply drained grassland had some of the highest-ever observed DOC concentrations of 195.8 ± 77.3 mg L−1 with maximum values of >400 mg L−1. In general, dissolved organic matter (DOM) at the drained sites was enriched in aromatic moieties and showed a higher degradation status (lower DOC to DON ratio) compared to the near-natural site. At the drained sites, the C to N ratios of the uppermost peat layer were the same as of DOM in the peat profile. This suggests that the uppermost degraded peat layer is the main source of DOM. Nearly constant DOM quality through the profile furthermore indicated that DOM moving downwards through the drained sites remained largely biogeochemically unchanged. Unlike DOM concentration, DOM quality and dissolved N species distribution were similar in the two grasslands and thus unaffected by the drainage depth. Methane production during the winter months at the drained sites was limited to the subsoil, which was quasi-permanently water saturated. The recovery of the water table in the winter months led to the production of nitrous oxide around mean water table depth at the drained sites. The rewetted and the near-natural site had comparable DOM quantity and quality (DOC to DON ratio and aromaticity). 10 years after rewetting quasi-pristine biogeochemical conditions have been re-established under continuously water logged conditions in the former peat cut area. Only the elevated dissolved methane and ammonium concentrations reflected the former disturbance by drainage and peat extraction. Rewetting via polder technique seems to be an appropriate way to revitalize peatlands on longer timescales and to improve the water quality of downstream water bodies

Preconditioning effects of intermittent stream flow on leaf litter decomposition

Autumnal input of leaf litter is a pivotal energy source in most headwater streams. In temporary streams, however, water stress may lead to a seasonal shift in leaf abscission. Leaves accumulate at the surface of the dry streambed or in residual pools and are subject to physicochemical preconditioning before decomposition starts after flow recovery. In this study, we experimentally tested the effect of photodegradation on sunlit streambeds and anaerobic fermentation in anoxic pools on leaf decomposition during the subsequent flowing phase. To mimic field preconditioning, we exposed Populus tremula leaves to UV-VIS irradiation and wet-anoxic conditions in the laboratory. Subsequently, we quantified leaf mass loss of preconditioned leaves and the associated decomposer community in five low-order temporary streams using coarse and fine mesh litter bags. On average, mass loss after approximately 45 days was 4 and 7% lower when leaves were preconditioned by irradiation and anoxic conditions, respectively. We found a lower chemical quality and lower ergosterol content (a proxy for living fungal biomass) in leaves from the anoxic preconditioning, but no effects on macroinvertebrate assemblages were detected for any preconditioning treatment. Overall, results from this study suggest a reduced processing efficiency of organic matter in temporary streams due to preconditioning during intermittence of flow leading to reduced substrate quality and repressed decomposer activity. These preconditioning effects may become more relevant in the future given the expected worldwide increase in the geographical extent of intermittent flow as a consequence of global change. © 2011 Springer Basel AG

Organic sediment formed during inundation of a degraded fen grassland emits large fluxes of CH₄ and CO₂

Peatland restoration by inundation of drained areas can alter local greenhouse gas emissions as CO(2) and CH(4). 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 CO(2) and CH(4) 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 CO(2) and CH(4) production and emission. The position in the soil profile of the pre-existing drained peat substrate affected initial rates of anaerobic CO(2) production subsequent to flooding, with the uppermost peat layer producing the greatest specific rates of CO(2) evolution. CH(4) 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 CO(2) (maximum of 412 mg Cd(-1) kg(-1) C) and CH(4) production (788 mg Cd(-1) kg(-1) C) were observed in a new sediment layer that accumulated over the 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 CO(2) (max. 28 mg Cd(-1) kg(-1) C) and CH(4) (max. 34 mg Cd(-1) kg(-1) C) production that were 10-20 times lower than for the new sediment layer and were comparable to those of a newly flooded upper peat layer. Lowest rates of anaerobic CO(2) and CH(4) production (range of 4-8 mg Cd(-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 CH(4) production, whereas peat without any fresh plant-derived material is relatively inert. Significant anaerobic CO(2) and CH(4) production in peat only occurs when some labile organic matter is available, e.g. from remaining roots or root exudates

Changes of the CO₂ and CH₄ production potential of rewetted fens in the perspective of temporal vegetation shifts

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 (<i>Phalaris arundinacea</i>, <i>Ceratophyllum demersum</i>, <i>Typha latifolia</i>, <i>Phragmites australis</i> and <i>Carex riparia</i>) 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% (<i>P. australis</i>) to 64% (<i>C. demersum</i>). Substantial differences were found for the CH₄ production with highest values from decomposing <i>C. demersum</i> (0.4 g CH₄ kg⁻¹ dry mass day) that were about 70 times higher than CH₄ production from <i>C. riparia</i>. Thus, we found a strong divergence between mass loss of the litter and methane production during decomposition. If <i>C. demersum</i> as a hydrophyte is included in the statistical analysis solely nutrient contents (nitrogen and phosphorus) explain varying greenhouse gas 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

Top soil removal reduces water pollution from phosphorus and dissolved organic matter and lowers methane emissions from rewetted peatlands

1. A valid strategy to mitigate the eutrophication of water bodies due to non‐point source phosphorus (P) pollution and to reduce the emissions of greenhouse gases is the rewetting of degraded peatlands. However, long‐term drainage and intensive agricultural use make it unlikely that the original sink functions for nutrients and carbon (C) as well as low‐nutrient conditions can be re‐established within a human time perspective. 2. We hypothesized that the removal of the upper degraded peat layer can be a suitable measure to avoid the negative implications of excess mobilization of P and C after rewetting. To evaluate the effect of top soil removal (TSR) we performed lab and field experiments in six inundated peatlands in northern Germany without TSR compared to six inundated sites with TSR. In addition, we included data from a rewetted peatland where the degraded peat had been removed from about half of the area and groundwater level was just beneath the soil surface. 3. The results emphasized that following inundation newly formed detritus mud layers overlying the former peat surface are the dominating source for P and methane in particular in sites without TSR but also in sites with TSR, although at significantly lower rates. Although highly decomposed peat released more or less no methane, dissolved organic matter mobilization was highest in this substrate while less decomposed peat was characterized in general by lowest rates of mobilization. 4. Synthesis and applications. Top soil removal prior to rewetting can be a suitable method to avoid the negative consequences of the excess release of phosphorus (P) and carbon post‐rewetting. We developed a simple decision support schematic to assist the peatland restoration process and to better understand the implications of top soil removal. Despite the potential benefits, top soil removal should not be declared as a universal method, as it requires detailed consideration prior to application. However, this and other research demonstrate that it is inevitable that without any further management interventions high mobilization of P, dissolved organic matter and methane may persist for centuries following rewetting of peatlands