The mobilisation of phosphorus, organic carbon and ammonium in the initial stage of fen rewetting (a case study from NE Germany)

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 mu M, 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.