Hydrological complexity and nitrate removal versus phosphorus release in a shallow-drained riparian peatland

• Nutrient balances in a drained riparian peatland were quantified over three years. • A geological window enhanced groundwater inflow and influenced nutrient dynamics. • Groundwater-driven nitrate loads showed 74–75% removal under drained conditions. • Phosphorus loss risk after rewetting was comparable to current losses. • Legacy phosphorus release under drainage may offset rewetting-related risks. Peatland drainage has transformed natural carbon sinks into sources of greenhouse gases and nutrient pollution, yet the nutrient fluxes from drained systems remain poorly quantified. This study presents a comprehensive three-year nutrient balance assessment of a ditch-drained riparian peatland in Vejrumbro, Denmark, prior to rewetting. The site comprises two hydrologically distinct subareas – a western groundwater-dominated zone influenced by a sand aquifer and an eastern precipitation-fed zone. Water and nutrient fluxes were quantified using high-resolution flow measurements, automated and manual water sampling, and extensive soil and groundwater characterization. The western area exhibited high nitrate loads (703 kg N ha −1 yr −1 ), of which 74–75% was removed, primarily via denitrification under persistently anoxic conditions. In contrast, both areas consistently released ammonium, organic nitrogen, and total phosphorus (TP), with TP exports ranging from 1.2 to 2.4 kg P ha −1 yr −1 . Soil analyses revealed elevated degrees of P saturation in surface peat layers, indicating an elevated risk of P mobilization. The estimated P release risk upon rewetting (1.7 kg P ha −1 yr −1 ) was comparable to current losses under drained conditions, suggesting that rewetting may not exacerbate P export. These findings underscore the importance of subsurface hydrological connectivity and legacy nutrient pools in shaping nutrient dynamics and provide an empirical basis for evaluating the trade-offs of peatland rewetting in temperate lowland systems.