The influence of local variations in soil hydro-physical properties on ammonium release during flooding events in a coastal peatland

Coastal peatlands are vulnerable to environmental changes, including salinity fluctuations caused by storm surge-induced seawater intrusion. This study investigates ammonium (NH₄⁺) release patterns during simulated sea flooding event in two locations of a coastal peatland in Northeast Germany (Hütelmoor): a near-natural location and a historically drained and rewetted location. Undisturbed soil samples (N=18) were collected from two depths (0–10 cm and 30–40 cm) at each location. A leaching experiment was conducted using three salinity treatments (N=3): groundwater (control, <1 ppt), Baltic Sea water (10 ppt), and mean seawater salinity (35ppt). Soil hydro-physical properties were determined following leaching experiment.

Results showed that NH₄⁺ release varied with salinity, soil depth, and land management. In the topsoil (0–10 cm), both locations exhibited high NH₄⁺ release at <1 ppt initially; however, higher salinity treatments (10 ppt and 35 ppt) continued to release elevated NH₄⁺ over time. In the subsoil (30–40 cm), rewetted samples under 10 ppt salinity released the most NH₄⁺, highlighting them as hotspots for nutrient mobilization during Baltic Sea flooding events.

Soil hydro-physical properties varied significantly across locations and depths, with a notable negative correlation between NH₄⁺ release and both saturated hydraulic conductivity (Ks) and macroporosity. This correlation was primarily driven by subsoil samples. While differences in hydro-physical properties were evident between near-natural and rewetted topsoils, they did not significantly influence NH₄⁺ release, suggesting that other factors, like soil organic matter (SOM), may play a more critical role in topsoil NH₄⁺ dynamics. In the subsoil, near-natural peat, characterized by higher Ks and macroporosity, retained less NH₄⁺ and released smaller amounts. Conversely, the rewetted subsoil, with lower Ks and macroporosity, accumulated and released more NH₄⁺, identifying it as a hotspot for nutrient mobilization.

Overall, by examining how local variations in soil hydro-physical properties across different locations within a single site influence NH₄⁺ release, this research identifies key hotspots for nutrient mobilization in a rewetted peatland. The findings highlight the necessity of accounting for both spatial and vertical soil property variations in coastal peatland restoration and management, especially regarding the prediction of environmental risks associated with nutrient release. Future research should examine how biogeochemical processes and microbial activity interact with soil hydro-physical properties to influence nutrient dynamics, especially under changing climate scenarios.

*Note: The first and second authors contributed equally to this work and share first co-authorship.