No abstract

Currently, more than 10,000 ha of fens have been rewetted to re-establish their function as nutrient sinks in NE Germany. However, field investigations reveal that porewater concentrations of P, dissolved organic carbon (DOC) and ammonium in rewetted fens are orders of magnitude larger than under pristine conditions. Hence, the objective of this study was to investigate the reasons behind enhanced P, organic carbon (OC) and ammonium mobilisation due to rewetting by means of a long-term incubation experiment. Highly, moderately and slightly decomposed peat of a drained fen (polder Zarnekow) was incubated under waterlogged conditions. A time course of concentrations of P, DOC, ammonium, sulphate and other dissolved substances was investigated by means of permanently installed dialysis samplers during 54 weeks of incubation. Simultaneously, the concentrations of these dissolved substances were investigated after rewetting of the field site. Before, and at the end of the incubation study, the amounts of bicarbonate-dithionite (BD) and NaOH soluble P and OC of incubated peat samples were determined by a sequential extraction procedure. The highest mobilisation of P, OC and ammonium occurred in the highly decomposed peat. Final concentrations of P, DOC and ammonium reached about 143 mM, 46 and 1.9 mM, respectively. The initial sulphate concentrations in the rewetting experiment, as well as in the field investigations, were extremely high and ranged between 3 and 13 mM; however, a complete consumption of sulphate was only observed in highly decomposed peat. In conclusion, the reasons for enhanced P, OC and ammonium mobilisation are increased amounts of redox sensitive substances and enhanced availability of decomposable organic matter in the upper highly decomposed peat horizon. These results should be considered in future rewetting management strategies.

It is demanded currently in public discussions to rewet peatlands and re-establish their function as nutrient sinks. But due to high phosphorus (P) concentrations in the pore water of rewetted peatlands (40-420 mM) it is hypothesized that they can act as a surplus P source for adjacent surface waters and consequently support the eutrophication of such waters. Our detailed investigations of processes at the redox interface in four fens with different geochemical character show the dependence of P retention from the chemistry of the pore water. The precipitation of Fe(III) oxyhydroxide led to high retention of phosphorus and other substances such as DOC and sulphate in the eutrophic fens. When molar Fe=P ratios were larger than about 3 the initially high P concentrations in the anaerobic pore water (20-210 mM) decreased to concentrations below 1 mM under aerobic conditions. Thus, after rewetting high pore water concentrations of P do not automatically result in an increased P load to adjacent surface waters compared to pre-rewetting conditions. An enhanced P export to adjacent surface waters from eutrophic fens can be expected when the Fe=P ratio is smaller than 3 in the anaerobic pore water. In our investigations of natural, oligotrophic to mesotrophic fens the precipitation of Fe(III) oxyhydroxide was inhibited by the formation of stable dissolved Fe * humic complexes. P retention in these fens was only related to the DOC concentrations at the redox interface, so that lower DOC concentrations concurred with higher P retention. The P equilibrium concentrations in an aerobic environment can be higher than that of eutrophic fens with Fe=P ratios larger than about 3 in the anaerobic pore water.

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.

Quantifying the catchment water balance and the characterization of its water quality changes are effective tools for establishing the response of catchments to shifting land management practices. Here we assess long-term hydrological partitioning and stream water chemistry over a 30-year period in a rural mixed land use catchment in northern Germany undergoing riparian wetlands and 2 widespread re-colonisation by beavers (Castor fiber) along the river network. We used long-term spatially distributed stream discharge, groundwater levels and surface water quality data with a simple monthly water balance model, changes in the variability in discharge measurements, and statistical analysis of spatio-temporal changes in stream water quality to assess long-term changes. Water balance estimates indicated high proportions of evapotranspiration loss (~90% of total precipitation) and relatively low groundwater recharge (<5% of total precipitation) prior to riparian rehabilitation in 2000. Increasing groundwater levels from 2000-2017 and the relatively linear nature of the catchment storagedischarge relationship, indicate a gradual increase in groundwater recharge (buts still <10% of total precipitation). Wetland rehabilitation, greatly enhanced by increasing beaver populations, resulted in longer water transit times in the stream network, less linear storage-discharge relationship and a loss of daily stream variability, increased DOC concentrations, isotopic evaporative enrichment downstream, and moderated stream temperatures. There was limited long-term water quality improvements from wetland rehabilitation on either nitrate or total phosphorus concentrations, with unchanged seasonal summer and winter peak concentrations for phosphorus and nitrate, respectively. This likely reflects the long-term legacy of fertilizer use on nutrient reservoirs in the catchment's soils, aquifers, and stream network. These long-term changes in hydrology and stream chemistry resulting from riparian rehabilitation and changes in agricultural management practices provide invaluable insights into catchment functioning and an evidence base for future planning in relation to long-term climatic changes.