An interesting item to discuss, IMO anaerobic areas in the substrate also allow that Fe2+ exists, plants needs to spend less energy to assimilate it than Fe3+ , also due to this some elements like nitrate and sulphate can be "digested". Toxic elements like CH4 and SH2 will be oxidated easily when they reach more oxygenated areas of substrate changes into sulphates and CO2, both esentials for plants.
Rgds
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