Modulation of Ammonium Release by Soil Hydro-Physical Properties Following Flooding Events in a Coastal Peatland

Abstract Coastal peatlands are increasingly threatened by seawater intrusion and salinity fluctuations under climate change. This study investigated nitrogen dynamics following a storm surge event in a coastal peatland (Hütelmoor) in Northeast Germany, combining surface water monitoring and controlled soil leaching experiments at two locations: a near-natural location (Heiligensee) and a rewetted, historically drained location (Moorhof). Surface water observations revealed pronounced spatial differences in post-flood nitrogen responses. At Heiligensee, ammonium (NH 4 + ) concentrations reached up to ~ 179 µmol L –1 in early 2019 and remained elevated for several weeks, whereas Moorhof showed lower concentrations with rapid declines following the flood. Soil leaching experiments demonstrated that NH 4 + release responded to salinity exposure but was also modulated by location- and depth- specific soil hydro-physical properties. Particularly, subsoils (30–40 cm) at Moorhof, characterized by low porosity and low saturated hydraulic conductivity (Ks), released the highest NH 4 + amounts (up to 3.86 mg per sample) under brackish and saline conditions, identifying them as hotspots for nutrient release. Principal component analysis (PCA) showed that the first two components explained 81.5% of the total variance in NH 4 + release patterns, separating soils along a structural gradient (PC1: soil organic matter content, bulk density, porosity) and a hydraulic–nutrient gradient (PC2: macroporosity, Ks, NH 4 + release). These findings emphasize the need to account for both surface water dynamics and subsurface soil heterogeneity in coastal peatland restoration. Identifying location- and depth-specific nitrogen risks is essential to improve nutrient management and ecosystem resilience under future salinization scenarios. Graphical Abstract