Dominik Zak

Phosphorus mobilization in rewetted fens: the effect of altered peat properties and implications for their restoration

Rewetting of drained fens is necessary to stop further soil degradation and to reestablish important ecological functions. However, substantial changes of peat characteristics in the upper soil layers, due to drainage and land use, could counteract their recovery as nutrient-poor systems for an unknown period. We assessed the importance of altered peat properties, such as the degree of peat decomposition and the amount of redox-sensitive phosphorus (P) compounds, for P mobilization in different degraded fens. An experimental design involving 63 intact peat cores from fens with varying drainage and land-use histories was developed to quantify the mobilization of P, as well as that of iron (Fe), ammonium, carbon dioxide, and methane, all indicators of organic-matter decomposition and/or P-releasing processes. We found that net P release rates in peat cores with highly decomposed peat (range: 0.1-52.3 mg P x m(-2) x d(-1)) were significantly correlated to the amount of P bound to redox-sensitive compounds and the molar Fe:P as well as Al:P ratios of peat. We conclude that the following general rules apply for P mobilization in rewetted fens: (1) elevated levels of P release rates and P concentrations in pore water up to three orders of magnitude larger than under natural reference conditions can only be expected for rewetted fens whose surface soil layers consist of highly decomposed peat; (2) peat characteristics, such as the amount of P bound to redox-sensitive Fe(III) compounds (positive correlation) and molar ratios of Fe:P or Al:P (negative correlations), explain the high range of P release rates; and (3) a critical P export to adjacent lakes or rivers can only be expected if molar Fe:P ratios of highly decomposed peat are less than 10.

Comparison of organic matter composition in agricultural versus forest affected headwaters with special emphasis on organic nitrogen.

Agricultural management practices promote organic matter (OM) turnover and thus alter both the processing of dissolved organic matter (DOM) in soils and presumably also the export of DOM to headwater streams, which intimately connect the terrestrial with the aquatic environment. Size-exclusion chromatography, in combination with absorbance and emission matrix fluorometry, was applied to assess how agricultural land use alters the amount and composition of DOM, as well as dissolved organic nitrogen (DON) forms in headwater streams, including temporal variations, in a temperate region of NE Germany. By comparing six agriculturally and six forest-impacted headwater streams, we demonstrated that agriculture promotes increased DOC and DON concentrations, entailing an even more pronounced effect on DON. The major part of DOC and DON in agricultural and forest reference streams is exported in the form of humic-like material with high molecular weight, which indicates terrestrial, i.e., allochthonous sources. As an obvious difference in agricultural streams, the contribution of DOC and particularly DON occurring in the form of nonhumic high-molecular-weight, presumably proteinous material is clearly elevated. Altogether, DOM in agricultural headwaters is mainly complex-soil-derived and aromatic material with a low C:N ratio, which is more microbial processed than its counterpart from forest reference catchments. Our results emphasize the importance of agricultural land use on DOM loss from soils and identify agricultural soils as important DOC and particularly DON sources to headwater streams.

Preface: Restoration, biogeochemistry and ecological services of wetlands

Natural wetlands, including fens, shallow lakes and floodplains, provide many benefits to society (Costanza et al., 1997). In particular, these wetlands deliver important functions as long-term sinks for nutrients and carbon, as hydrological buffers, and as habitats for many endangered plant and animal species (Millennium Ecosystem Assessment, 2005). In the face of eutrophication of watercourses, decreasing biodiversity, and expected climate changes, the importance of these multi-functional wetlands is being recognized more and more (McInnes, 2011). Currently, there are international efforts to protect wetlands or to repair their functions in the landscape through restoration measures (Erwin, 2009). However, their ecological rehabilitation has long been based on trial and error. Increasingly knowledge of the biogeochemical drivers of natural and biodiverse wetlands, and how these pertain to successful restoration, is vital in order to prevent costly operations with an unpredictable outcome (Klotzli & Grootjans, 2001). With relevant knowledge the restoration outcomes can be forecasted and choices should, therefore, be based on the actual restoration potential of an area rather than on the reconstruction of the prior ecosystem (van Diggelen et al., 2001). This approach requires information on causal relations and therefore experiments and investigations at different scales are of pivotal importance. Successful restoration of wetlands calls for interdisciplinary experimental research in which ecology, hydrology, microbiology, and geochemistry merge into a system-ecological approach. Furthermore, different scientific disciplines must be integrated with wetland managers, stakeholders, and crosssectoral government agencies throughout all stages of wetland restoration (from objective setting to postrestoration monitoring), in order to optimize restoration measures (Trepel, 2007). Embracing this approach, the 4th Annual Meeting of the European Chapter of the Society of Wetland Scientists (SWS) was held between 20th and 24th of May 2009 in Erkner, near Berlin, both scientists and practitioners came together to share their knowledge. The SWS is a 3500? member organisation that promotes scientific understanding, scientificallybased management and sustainable use of wetlands. Under the theme ‘‘Progress and problems in wetland science—with a particular focus upon wetland restoration in Europe’’, the 4th Annual Meeting attracted Guest editors: Dominik Zak, Robert McInnes, Jorg Gelbrecht / Restoration, biogeochemistry and ecological services of wetlands

Soil Iron Content as a Predictor of Carbon and Nutrient Mobilization in Rewetted Fens

Rewetted, previously drained fens often remain sources rather than sinks for carbon and nutrients. To date, it is poorly understood which soil characteristics stimulate carbon and nutrient mobilization upon rewetting. Here, we assess the hypothesis that a large pool of iron in the soil negatively affects fen restoration success, as flooding-induced iron reduction (Fe3+ to Fe2+) causes a disproportionate breakdown of organic matter that is coupled with a release of inorganic compounds. We collected intact soil cores in two iron-poor and two iron-rich drained fens, half of which were subjected to a rewetting treatment while the other half was kept drained. Prolonged drainage led to the mobilization of nitrate (NO3-, > 1 mmol L-1) in all cores, regardless of soil iron content. In the rewetted iron-rich cores, a sharp increase in pore water iron (Fe) concentrations correlated with concentrations of inorganic carbon (TIC, > 13 mmol L-1) and dissolved organic carbon (DOC, > 16 mmol L-1). Additionally, ammonium (NH4+) accumulated up to phytotoxic concentrations of 1 mmol L-1 in the pore water of the rewetted iron-rich cores. Disproportionate mobilization of Fe, TIC, DOC and NH4+ was absent in the rewetted iron-poor cores, indicating a strong interaction between waterlogging and iron-mediated breakdown of organic matter. Concentrations of dissolved phosphorus (P) rose slightly in all cores upon rewetting, but remained low throughout the experiment. Our results suggest that large pools of iron in the top soil of drained fens can hamper the restoration of the fen’s sink-service for ammonium and carbon upon rewetting. We argue that negative effects of iron should be most apparent in fens with fluctuating water levels, as temporary oxygenation allows frequent regeneration of Fe3+. We conclude that rewetting of iron-poor fens may be more feasible for restoration.

Mitigation of Sulfate Pollution by Rewetting of Fens — A Conflict with Restoring Their Phosphorus Sink Function?

Sulfate pollution of lakes and rivers is recognized as a serious problem in many regions of Central Europe, thus we evaluated the role of rewetted fens in mitigating sulfate pollution and tested if high sulfate concentrations in fen-feeding water counteract the re-establishment of their function as sinks for phosphorus (P). A long-term incubation experiment was conducted with highly decomposed peat from upper soil layers of fens that have been rewetted for 1 to 15 years. Periodic sulfate pulses to inundated peat mesocosms, equating to an annual loading of 50 g S m−2, induced significant changes of sulfate consumption and phosphorus mobilization. Sulfate consumption of highly decomposed peat from all sampling sites was related to sulfate concentrations in overlying water (linear regression, p<0.01). Sulfate additions also led to significant increases of P concentrations or P mobilization in peat porewater (t test, p<0.05) and P concentrations in the overlying water were 2–3 times higher than in non-treated controls. In conclusion, rewetting of fens is an important tool to mitigate sulfate pollution of adjacent lakes and rivers. However, an elevated sulfate concentration in waters feeding the fen impairs P retention and increases P losses to adjacent surface waters.

Going with the flow: Planktonic processing of dissolved organic carbon in streams

A large part of the organic carbon in streams is transported by pulses of terrestrial dissolved organic carbon (tDOC) during hydrological events, which is more pronounced in agricultural catchments due to their hydrological flashiness. The majority of the literature considers stationary benthic biofilms and hyporheic biofilms to dominate uptake and processing of tDOC. Here, we argue for expanding this viewpoint to planktonic bacteria, which are transported downstream together with tDOC pulses, and thus perceive them as a less variable resource relative to stationary benthic bacteria. We show that pulse DOC can contribute significantly to the annual DOC export of streams and that planktonic bacteria take up considerable labile tDOC from such pulses in a short time frame, with the DOC uptake being as high as that of benthic biofilm bacteria. Furthermore, we show that planktonic bacteria efficiently take up labile tDOC which strongly increases planktonic bacterial production and abundance. We found that the response of planktonic bacteria to tDOC pulses was stronger in smaller streams than in larger streams, which may be related to bacterial metacommunity dynamics. Furthermore, the response of planktonic bacterial abundance was influenced by soluble reactive phosphorus concentration, pointing to phosphorus limitation. Our data suggest that planktonic bacteria can efficiently utilize tDOC pulses and likely determine tDOC fate during downstream transport, influencing aquatic food webs and related biochemical cycles.

Direct Analysis of Lignin Phenols in Freshwater Dissolved Organic Matter

A novel approach for the analysis of dissolved lignin in freshwaters is presented. Lignin concentrations in natural water samples are low, and a lignin extraction is usually required to obtain sufficient material for analysis. In this study, the alkaline CuO oxidation, which liberates a set of lignin-derived phenols, is performed directly on 15 mL of water sample in a microwave digestion system, hence reducing the required sample amount and preparation time considerably. These features make the method particularly suitable to study diagenetic changes of dissolved lignin in small-scale laboratory or field experiments. Phenol separation and quantification by gas chromatography tandem mass spectrometry lead to method detection limits between 22.7 and 1260 ng/L for single phenols, which corresponds to minimum lignin concentrations in the range of 8.5 μg/L (Σ8), offering applications for wetland, river, and lake waters with high terrestrial dissolved organic matter inputs. As a general method improvement, we present the addition of EDTA during phenol workup. EDTA binds remaining copper, thereby speeding up sample flow through the solid phase sorbent during phenol extraction and, furthermore, prevents substantial phenol losses, which occur if a water sample contains nitrate. Three natural water samples, a fresh leaf leachate and two humic-rich lake waters, were analyzed by the direct method presented here and in comparison with the established C18 extraction approach. Results show a similar reproducibility of both methods but reveal lower absolute lignin phenol yields in the humic-rich lake water samples upon C18 extraction.

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

Summary 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 hypothesised that the removal of the upper degraded peat layer can be a suitable measure to avoid the negative implications of excess mobilisation 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 (DOM) mobilisation was highest in this substrate while less decomposed peat was characterized in general by lowest rates of mobilisation. 4.Synthesis and applications. In conclusion, top soil removal (TSR) 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 understand better the implications of TSR. Despite the potential benefits TSR 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 mobilisation of P, dissolved organic matter and methane may persist for centuries following rewetting of peatlands. This article is protected by copyright. All rights reserved.

Nitrogen and Phosphorus Removal from Agricultural Runoff in Integrated Buffer Zones

Integrated buffer zones (IBZs) represent a novel form of edge-of-field technology in Northwest Europe. Contrary to the common riparian buffer strips, IBZs collect tile drainage water from agricultural fields by combining a ditch-like pond (POND), where soil particles can settle, and a flow-through filter bed (FILTERBED) planted with Alnus glutinosa (L.), a European alder (black alder). The first experimental IBZ facility was constructed and thoroughly tested in Denmark for its capability to retain various nitrogen (N) and phosphorus (P) species within the first three years after construction. We calculated the water and nutrient budget for the total IBZ and for the two compartments, POND and FILTERBED, separately. Furthermore, a tracer experiment using sodium bromide was conducted in order to trace the water flow and estimate the hydraulic residence time in the FILTERBEDs. The monthly average removal efficiency amounted to 10-67% for total N and 31-69% for total P, with performance being highest during the warm season. Accordingly, we suggest that IBZs may be a valuable modification of dry buffer strips in order to mitigate the adverse impacts of high nutrient loading from agricultural fields on the aquatic environment.