Variations in Iron-bound carbon in Scirpus mariqueter rhizosphere and bulk soils across different salinities and tidal elevations

Aims: The objective of this study was to investigate the response of the iron (Fe) (hydr-) oxides-bounded organic carbon (Fe-OC) process, characterized by the interaction of Fe (hydr-) oxides with soil organic carbon (SOC) in coastal wetlands, to different flooding and soil salinity conditions. Methods: The study concentrated on Scirpus mariqueter in the Yangtze River Estuary, analyzing variations in various forms of Fe (hydr-) oxides, Fe-OC, and Fe redox cycling bacteria in the rhizosphere and bulk soils under diverse flooding and soil salinity conditions. Results: The study found that during the growing season, the levels of crystalline (Fe_DH) and amorphous (Fe_HH) Fe (hydr-) oxides were elevated compared to the non-growing season. In the rhizosphere, around 80% of SOC was associated with Fe (hydr-) oxides, significantly higher than the 50% in bulk soil. Elevated areas in the tidal flat exhibited increased Fe_DH, Fe_HH, and total Fe content in rhizosphere soils. Soils with higher salinity had more Fe (hydr-) oxides compared to low salinity soils. Furthermore, salinity reduced the prevalence of Gallionella and Geobacter, which are conducive to the preservation of Fe_HH, leading to a greater Fe_HH-OC ratio in the total SOC. Conclusions: The bond between Fe (hydr-) oxides and SOC in soils becomes stronger during the growth season of S. mariqueter. The Fe-OC process’s reaction to environmental factors like salinity and flooding profoundly affects the dynamics of SOC in wetland ecosystems. These findings are essential for wetland management, emphasizing the need to preserve these ecosystems for their role in SOC sequestration.