Salinity, Inundation, and Redox Gradients Shape Reactive Iron and Organic Carbon Spatial Patterns in Tidal Wetland Soils

Abstract

Tidal wetland soils are important blue carbon reservoirs; however, how hydrological and redox gradients regulate reactive iron (Fe) pools and their relationships with soil organic carbon remain poorly resolved. We examined soils at five sites along the brackish‐to‐freshwater Elbe estuary, spanning salinity, inundation, and vertical redox gradients, to assess how operationally defined reactive Fe pools covary with soil organic carbon (SOC). Since recurrent reducing conditions complicate conventional interpretations of dithionite‐citrate (Fe _d ) and acid ammonium oxalate (Fe _o ) extractions, we complemented these single‐step methods with a sequential Fe extraction scheme and reference‐mineral tests. The latter showed overlapping extraction selectivity, confirming that the sequential fractions are operational rather than uniquely mineral‐specific. Large ascorbate‐ and HCl‐extractable fractions were consistent with substantial contributions from short‐range ordered Fe(III) (oxyhydr)oxides and Fe(II)‐bearing phases, particularly in frequently inundated and reduced soils. Reactive Fe and SOC varied among sites without a consistent directional response along the estuarine salinity gradient, whereas fine‐grained and vegetated soils generally contained more SOC and reactive Fe. Fe _d and Fe _o tended to be higher in less frequently inundated low‐ and high‐marsh soils and remained positively associated with SOC after accounting for fine fraction, depth, site, and inundation zone. In contrast, no individual sequential Fe fraction showed a robust independent relationship with SOC. These results demonstrate that hydrological and redox gradients reshape operationally defined reactive Fe pools that covary with SOC, underscoring the role of Fe transformations in the context of sea‐level rise and tidal wetland blue carbon dynamics.