Microbial activity as a major factor in the mobilization of iron in the humid tropics

The relation between the biodegradation and mineralization of soil organic matter and the bacterial solubilization and mobilization of iron in soils of the humid tropics is not clear. We therefore studied how microbial activity mobilized iron in a typical swamp soil of the humid tropics using two complementary approaches. By analysing soil water we found that the swamps (20% of the catchment) are the major sites of mineral weathering. Bacteria caused the solubilization of ferrous iron, produced ammonium, released organic soluble compounds, and caused the disappearance of nitrate. The waterlogged soil of the swamp (Gleysol) degraded and mineralized organic matter, producing CO2 and NH4+, and the amount of that activity depended on the amount, availability and quality of the organic matter. The nature and biomass of autochthonous bacteria were also important. The solubilization of iron was detected in the upper horizons (L1, 0-30 cm; L2, 30-70 cm; L3, 70-100 cm), where both iron and organic matter are available. Highly significant correlations were found between mineralization of organic matter and iron reduction. Quantitatively, we found that 100 mug of organic C being mineralized could reduce and dissolve 38, 97 and 115 mug Fe2+ in the L1, L2 and L3 horizons, respectively, during 30 days. After 30 days, there was a marked change in the relation, suggesting that lack of iron limited reduction. The coupling of iron reduction to the carbon cycle (soil organic matter biodegradation) is significant.