A comprehensive porewater survey of European peatlands reveals sustained elevated phosphorus levels after 10–20 years of rewetting

• Rewetted degraded fens increase nutrient and dissolved organic concentrations. • Seasonal shifts in dissolved phosphorus depend on helophyte colonization. • Phosphorus concentrations decline was 0–80 % over 5–10 years post-rewetting. • Phosphorus levels seem to drop faster in iron-poorer fens. Rewetting drained peatlands can lead to high nutrient mobilization, increased methane emissions, and a slow re-establishment of peat-forming vegetation. To guide effective restoration and management, understanding the temporal and spatial variability in porewater chemistry is essential. This study surveyed 64 natural and rewetted peatlands across Germany, Poland, Estonia, Sweden, Georgia, and Scotland from 1997 to 2017. A total of 812 anoxic porewater samples were collected using dialysis samplers (0–0.6 m depth). The rewetted fens exhibited a wide range of dissolved substances, spanning orders of magnitude for soluble reactive phosphorus (SRP: 0.1–18.9 mg L −1 ), ammonium (NH 4 + -N: 0.1–117.3 mg L −1 ), and dissolved organic carbon (DOC: 13–313 mg L −1 ). However, the mean concentrations were significantly higher than those observed in natural fens (p < 0.05). Depth-integrated mobilization rates for nutrients in rewetted fens were, on average, 23 times higher for SRP (1.8 mg P m −2 d -1 ) and 4.6 times higher for NH 4 + -N (3.6 mg N m −2 d -1 ) compared to their natural counterparts (0.1 mg P m −2 d -1 and 0.8 mg N m −2 d -1 ). Seasonal variation was also evident in rewetted fens densely colonized by helophytes, with SRP concentrations being lower in the growing season. Notably, SRP concentrations remained elevated 10–20 years after rewetting; however, a 50–80 % decrease was observed at sites characterized by comparatively low iron content in the peat (< 20 mg g −1 dry mass). Further investigations should explore how nutrient dynamics evolve over extended rewetting periods in different contexts, including climate change.