The influence of microtopography on soil carbon accumulation and nutrient release from a rewetted coastal peatland

[Display omitted] •Microtopography controls carbon accumulation and nutrient release.•Soil organic matter content and carbon:nitrogen ratio were negatively correlated with surface elevation.•High ammonium concentrations are released from organic rich low-lying soils when flushed with brackish water.•Local geomorphological depressions are hotspots of carbon turnover and nutrient release in coastal peatland. Coastal peatlands have been frequently blocked from the sea and artificially drained for agriculture. With an increasing awareness of ecosystem functions, several of these coastal peatlands have been rewetted through dike removal, allowing seawater flooding. In this study, we investigated a recently rewetted peatland on the Baltic Sea coast with the aim to characterize the prevailing soils/sediments with respect to organic matter accumulation and the potential release of nutrients upon seawater flooding. Eighty disturbed soil samples were collected from two depths at different elevations (–0.90 to 0.97 m compared to sea level) and analyzed for soil organic matter (SOM) content and carbon:nitrogen (C:N) ratio. Additionally, nine undisturbed soil cores were collected from three distinct elevation groups and used in leaching experiments with alternating freshwater and Baltic Sea water. The results demonstrated a moderate to strong spatial dependence of surface elevation, SOM content, and C:N ratio. SOM content and C:N ratio were strongly negatively correlated with elevation, indicating that organic matter mineralization was restricted in low-lying areas. The results also showed that the soils at low elevations release more dissolved organic carbon (DOC) and ammonium (NH4+) than soils at high elevations. For soils at low elevations, higher DOC concentrations were observed when flushing with freshwater, whereas higher NH4+ concentrations were found when flushing with brackish water. Recorded NH4+ concentrations in organic-rich peat reached 14.82 ± 9.25 mg L–1, exceeding Baltic seawater (e.g., 0.03 mg L–1) by two orders of magnitude. A potential sea level rise may increase the export of NH4+ from low-lying and rewetted peat soils to the sea, impacting adjacent marine ecosystems. Overall, in coastal peatlands, geochemical processes (e.g., C and N cycling and release) are closely linked to microtopography and related patterns of organic matter content of the soil and sediments.