Phosphorus dynamics in acidic and neutral fen soils − does vivianite limit phosphorus release?

• Anoxic conditions during wet periods have triggered full Fe(III)oxide reduction. • Full re-oxidation of Fe(II) during dry periods. • Low phosphorus release in anoxic alkaline peat soil compared with acidic peat soil. • Vivianite controls phosphorus release in alkaline peat soils. • Geochemical factors are crucial for phosphorus risk assessments in peatland systems. Drainage and cultivation of organic lowland peat soils have profoundly altered their hydrological and biogeochemical functions, increasing greenhouse gas emissions and phosphorus (P) mobilization due to enhanced mineralization. Rewetting is increasingly promoted to mitigate these impacts. However, intensive agricultural use has led to P accumulation, primarily bound to Fe(III)(oxyhydr)oxide minerals, raising the risk of P release under anoxic conditions and challenging the restoration of these systems as nutrient sinks. Predicting P release remains difficult due to complex interactions, including P re-sorption to metal oxides with remaining P binding places and precipitation as Fe(II) or Ca phosphates such as vivianite and hydroxyapatite. This study investigates Fe(III)(oxyhydr)oxide reduction extent and associated P mobilization in two contrasting Danish lowland peatlands over 24 months: the acidic Vejrumbro and neutral-alkaline Løvenborg. Oxalate-extractable Fe, Al, and P (Fe ox , Al ox , P ox ) differed substantially between sites. Løvenborg exhibited higher Fe ox (198–241 mmol kg −1 ) and Al ox (41–49 mmol kg −1 ) than Vejrumbro (Fe ox : 27–122 mmol kg −1 ; Al ox : 4.5–40 mmol kg −1 ), resulting in greater P sorption capacity (PSC: 239–287 vs. 44–155 mmol kg −1 ). The degree of P saturation (DPS) remained low at both sites (<10%). Under winter or wet field conditions, Fe(III)(oxyhydr)oxide reduction extent reached up to 100%, indicating complete dissolution of the Fe ox pool. Despite extensive reduction, soluble P (P Sol ) remained low in Løvenborg (≤0.3 mg L −1 ), whereas Vejrumbro exhibited high P Sol concentrations (up to 4.7 mg L −1 ). Powder X-ray diffraction and Mössbauer spectroscopy confirmed vivianite formation in Løvenborg, demonstrating P immobilization via precipitation. Moreover, in Løvenborg, the saturation index calculated using the geochemical model Minteq showed values between 2.5 and 4.2 with respect to vivianite supersaturation, while for Ca-P precipitates such as hydroxyapatite, Løvenborg soils show near equilibrium (0.09) to slight undersaturation (−1.8). In contrast, P retention in Vejrumbro was mainly through adsorption onto residual Fe and Al oxide minerals. These results indicate that P release risk models must go beyond re-adsorption, and include precipitation pathways, which depend on metal-oxide content, cation availability (e.g., Ca 2+ , Fe 2+ ), pH, alkalinity, and the extent of Fe(III) reduction. Incorporating key geochemical indicators—such as pH, sorption capacity, calcium content, Fe(III) reduction extent, and alkalinity may lead to better classification of peat soils and guide rewetting strategies safeguarding minimal P release.