Rewetting a pump-drained fen converts a nutrient hotspot into a sink and reduces organic carbon export

Abstract

Peatland drainage for agricultural purposes has transformed wetlands into major nutrient sources, but rewetting offers potential for nutrient retention and climate mitigation. We quantified hydrological and nutrient fluxes in a 14.5 ha former pump-drained fen (Strande Enge, Denmark) across 1 year before and 2 years after rewetting. Restoration involved removing pumping infrastructure and reconnecting upstream catchments with the aim to restore natural flow paths. Prior to rewetting, the site exhibited high hydraulic loading (5904 mm yr⁻¹) dominated by groundwater inputs and acted as a strong nutrient source, with net losses of total nitrogen (TN: 123 kg ha⁻¹ yr⁻¹), total phosphorus (TP: 23 kg ha⁻¹ yr⁻¹), and total organic carbon (TOC: 1336 kg ha⁻¹ yr⁻¹). Following rewetting, hydraulic loading increased more than fourfold (24,258–27,160 mm yr⁻¹), and the system shifted to a flow-through wetland dominated by surface water inputs. Despite substantially higher nutrient loads after rewetting, the fen became a large sink for TN (209–242 kg ha⁻¹ yr⁻¹; 21% retention) and retained 44–72% of soluble reactive phosphorus. However, although their losses declined, particulate phosphorus and organic matter were still exported, particularly during high-flow events. At catchment scale, rewetting reduced the yearly TN export to the downstream lake by 27–38%, while the annual TP export varied strongly after rewetting (12% reduction in year 1 and 74% increase in year 2). These findings demonstrate that rewetting of pumped fens can rapidly transform nutrient source hotspots into effective nitrogen sinks, but additional measures may be needed to control particulate losses of nutrients and organic carbon, if these prove to persist over time. Overall, our results show that rewetting can reduce nutrient losses, including TOC losses, thereby contributing to mitigation of aquatic pollution and helping preserve peatland carbon stocks.