Redox processes affect the reactivity of iron oxides and this can affect the P availability in soils. To evaluate how this processes can affect phosphorus availability, this study was carried out with six Uruguayan soils, which differed in their parent material, and subjected to alternation of the flooded-drained conditions. Soil samples (0-15 cm) of the A horizon, were flooded during 0, 5, 15 and 45 days, drained and sampled after 21 days of air-drying. Fe extracted by ammonium oxalate at pH 6 (Fe6) increased with flooding, and after re-oxidation, decreased to levels greater than the initial levels. The organic carbon content of the soils was correlated with the poorly crystalline forms of Fe before and after flooding. Phosphorus availability (Bray-1) increased during the flooding period, and, except for Algorta soil, decreased during the drying period even lower than at the initial measurements. The alternation of the flooded-drained conditions increased the iron content in high reactivity forms resulting in increasing phosphorus adsorption, as indicated by low levels of P in the soil solution. A relationship between the iron content in high reactivity forms (extracted by ammonium oxalate at pH 6) and the adsorption of P resulting from a decreased P solution was found. These results suggest that after flooded-drained conditions, P availability to plants decreases due to the absorption of P with recently precipitated high reactivity forms of iron oxides.
phosphorus availability; phosphorus adsorption; iron oxides; flooding-drained soils; oxidation-reduction conditions