Factors Related to Acid-Induced Iron and Aluminum Release from Sandy Coastal Plain Soils.

Sandy Soils of SE USA are mostly acidic in nature and thus iron (Fe) and aluminum (Al) are the prevailing metals in these soils. Accumulation and redistribution of these metals have much pedological significance. Results of recent studies suggest that Fe may be an inhibitor of Al mobilization via podzolization. The present study addressed this possibility by testing the hypotheses that (i) Al release into soil solution inversely relates to crystalline Fe content because surfaces of these oxides mitigate organo-Al complexing potential, and (ii) Fe release, as by biochemical reduction, will be accompanied by Al release due to loss of Fe oxide surfaces that can otherwise compete with Al oxides for ligand reactions. Eighteen soils with a range of soil drainage and Fe concentrations were collected. Release of Fe and Al by I mmol oxalic acid and HCl and by natural soil dissolved organic matter (DOM) was monitored and compared with standard extractions by acid ammonium oxalate and sodium citrate-dithionite-bicarbonate Oxalic acid was found to be most effective in extracting metals at lower metal-acid interaction time, but DOM also extracted significant quantities of Al and Fe. Metal release by 1 mmol HCl was minimal compared to the organic acids, indicating that organo-complexation played an essential role in metal release. Release of both metals by DOM was significantly higher after 7 days, when Al concentration in soil solution correlated with but greatly exceeded that of Fe. Results are generally consistent with the idea that the presence of Fe oxides, but not necessarily their concentration, inhibits organo-Al complexation, possibly via surface retention of organic ligands until redox-related dissolution. Results are important in explaining response of Al to water table rise or soil reducing condition and thus increased organo-metal complexation in flatwood landscapes.