Heterogeneous oxygenation of wetland soils with increasing inundation: Redox potential, water depth, and preferential flow paths

Saturated wetland soils are typically depleted in oxygen due to rapid consumption by biological processes. Fine‐grained sediments limit the vertical flux of oxygenated surface water. In forested wetlands potential exists for advective transport along preferential flow pathways created by decomposing roots or tree remains, resulting in isolated zones of oxygenated pore water. This potential is enhanced in wetlands that are perched above the regional water table, where increases in surface‐water depth create significant downward hydraulic gradients. Along the Mississippi River floodplain, many wetlands are now perched as a result of groundwater mining. This study was conducted at Sky Lake, MS, through a growing season, with high temporal‐resolution monitoring of redox potential as a proxy for the delivery of oxygenated surface water at 30 and 60 cm depths at six locations. Results showed reducing conditions at all locations under moderate inundation, but redox potential rose significantly at a subset of locations when surface‐water exceeded one meter in depth. Rising redox potential is argued to represent localised advective transport of oxygen in excess of the rate of biological consumption. Wetland plants growing in perennially saturated soils may benefit from periodic increases in water depth and subsequent increase in the flux of oxygen along preferential flow paths. In heterogeneous sediments of forested wetlands, pockets of oxygenation can develop and persist even under long‐term inundation due to advective vertical transport of surface water along preferential flow pathways. At Sky Lake, MS (USA), in the alluvial floodplain of the Mississippi River, increases in surface water depth produced increases in wetland‐soil redox potential at isolated locations. Higher redox potentials persisted during periods of elevated surface‐water depth (high vertical hydraulic gradient).