Phosphorus and iron mobilization in flooded soils from brazil

The dynamics of phosphorus in flooded soils has much agricultural and environmental significance. One specific feature of such dynamics is that it is influenced largely by changes in Fe forms. In this work we investigated, in the laboratory, the effects of continuous flooding and 25-day redox cycles on the P and Fe dynamics in subsoils of seven Oxisols (Orthoxs, Humoxs, and Udoxs) and surface horizons of 19 lowland soils (Fluvents and Aquepts) from the State of Minas Gerais, Brazil. The amounts of Fe and P extracted by acetate/EDTA (Feae, Pae), citrate at pH 6 (Fec, Pc), and citrate/ascorbate at pH 6 (Feca, Pca), which provided a measurement of Fe forms with decreasing solubility (from soluble Fe to the more stable ferrihydrites), were determined before and after the continuous flooding and redox cycles treatments. In most Oxisols, the Fe forms were barely affected by continuous flooding or redox cycles. In the lowland soils, acetate/EDTA-Fe and citrate-Fe, i.e., the most labile Fe forms, were increased by such treatments, particularly in the soils containing more organic matter. In contrast, citrate/ascorbate-Fe either did not change or it decreased, suggesting that (i) no significant amounts of crystalline Fe (hydr)oxides were reduced and transformed to more soluble forms and (ii) the acetate/EDTA-Fe and citrate-Fe increases were largely the result of the transformation of some of the less soluble citrate/ascorbate-Fe forms. Continuous flooding and redox cycles also induced changes in acetate/EDTA-P, citrate-P, and citrate/ascorbate-P that were greater in the lowland soils than in the Oxisols, but there was no consistent increase or decrease in any of these extractable P forms. However, the citrate/ascorbate-P, on average, increased, suggesting that some organic P was mineralized. As a result of the aforementioned changes in Fe and P forms, the P/Fe mole ratio, on average, decreased in the more soluble (acetate/EDTA-and citrate-extractable) and increased in the less soluble (citrate/ascorbate-extractable) Fe forms. In the continuous flooding treatment, the Fe and P concentrations in solution generally increased until days 20 to 50 and then decreased; the P concentration occasionally showed a second maximum after 50 to 170 days of continuous flooding, probably caused by organic matter mineralization. After 209 days of continuous flooding, the P concentration in solution was of the same order of magnitude as that at the beginning of flooding. The average Fe concentration in solution during the flooding period was related to acetate/EDTA-Fe and citrate/ascorbate-Fe. The variable best suited to predict average P concentration in solution was citrate/ascorbate-P. The variance accounted for by this variable alone (33%) increased significantly (to 66%) when clay and citrate/bicarbonate/dithionite-extractable Fe were included as variables in the regression. This suggests that P concentration in solution is controlled by the amount of P released on reduction of the citrate/ascorbate-extractable Fe forms and by the P-adsorbing properties of the crystalline soil minerals (kaolinite, gibbsite, goethite, and hematite). It has been suggested that the acetate/EDTA and citrate/ascorbate methods will be useful for evaluating the Fe and P availability in waterlogged soils and sediments, if there is analysis of organic matter and phosphate sorption capacity.