Water and Redox Conditions in Wetland Soils—Their Influence on Pedogenic Oxides and Morphology

Abstract

For wetland soil studies, it would be desirable to estimate soil redox status and its effect on element translocations based on diagnostic redoximorphic features. However, their identification and interpretation is often complicated and require a regional calibration. This study was conducted (i) to determine the relationship between water table and redox potential (EH), (ii) to investigate their influence on mass balances of pedogenic Mn- and Fe-oxides and soil morphology, and (iii) to evaluate the use of soil color to identify redox status in hydromorphic soils. Three representative geomorphic units were chosen from low relief positions in the cool-humid Allgäu (Germany): an alluvial plain and two pond margins. Within each unit two paired study sites were established along a wetness gradient. Thus in six wetland soils (Aquepts) groundwater level (GWL) and EH were measured over 1 yr. Monitoring results were presented as (i) percentage of time of water saturation, (ii) percentage of time the EH is <170 mV (onset of Fe-oxide reduction), and (iii) related to static soil properties (mass balance of pedogenic oxides). Both, GWL and EH were linearly correlated (r2 = 0.88) and indicated increased Fe mobilization with increasing duration of saturation. Thus, higher Fe losses occurred with increasing duration of EH < 170 mV, whereas no correlation existed between Mn and the duration of reducing conditions. Calculation of Mn and Fe mass balances indicated losses of both elements within the pedons when reducing conditions were located near the surface (<10 cm). However, no losses were detected when the reductive conditions occurred at depths >50 cm. The element redistribution induced by soil redox conditions was reflected by the soil color index of subsoil horizons (C2h), within a sensitive range between 6 and 12. It was shown, that this index is an adequate tool to delineate wetland soils according to the duration of water saturation and Fe-reducing conditions. It could be concluded that C2h may be used as a proxy to estimate the intensity of water/redox conditions.