Reduced phosphorus loss in a pump-drained wetland after harvesting and rewetting

The rewetting of low-lying organic soils previously used for agriculture can mobilize legacy phosphorus (P), increasing the risk of eutrophication in downstream water bodies. One potential mitigation strategy is biomass harvesting, which may remove mobile P fractions from the soil. At the 44-ha, pump-drained former shallow lake Gammelgaard, biomass was harvested annually from 2020 to 2023, prior to the re-establishment of the lake in July 2023. Over this period, biomass harvesting removed a total of 701 kg P. In comparison, 254 kg of total P was exported via drainage pumping, corresponding to an average annual loss of 1.5 kg P ha^{- 1}. Following rewetting, the P mass balance revealed an inlet load of 206 kg P and an outlet loss of 238 kg P, resulting in a net P loss from the rewetted area of 0.7 kg P ha^{- 1}. Soil cores collected before and after 3 years of biomass harvesting period showed a reduction in the redox-sensitive P pool of 222 kg P ha^{- 1}. This reduction exceeded the sum of harvested P and outlet P fluxes, indicating that additional processes-such as internal redistribution or transformation into more stable binding forms-likely contributed to the observed changes. Although biomass harvesting appears to significantly reduce the mobile P pool in the soil, further research is needed to clarify the extent to which it effectively mitigates P losses from rewetted wetlands.