Paddy cultivation significantly alters the forms and contents of Fe oxides in an Oxisol and increases phosphate mobility

To evaluate the effect of paddy cultivation on phosphorus environmental risk in an Oxisol-derived paddy soil, we examined phosphate (P) affinity on bulk soils and their different size fractions with adsorption/desorption isotherms. The results showed that intermittent flooding in Oxisol-derived paddy soil, led to a significant decrease in free Fe oxides and an increase in amorphous Fe oxides in the surface (0-20 cm) and subsurface (20-40 cm) soils. A disproportional ratio of P adsorption capacities to the contents of free Fe oxides was observed in the studied Oxisol, when compared with that in the paddy soil, owing to more reactive sites exposed on amorphous Fe oxides than on crystalline Fe oxides. Furthermore, P adsorption/desorption isotherms revealed that non electrostatic adsorption was the main mechanism responsible for P binding to the Oxisol and its derived paddy soil, representing 83.94%-99.62% of the adsorption capacities. Examination of P adsorption by different size soil fractions further revealed that an equivalent mass of Fe oxides in the paddy soil particles retained more P than those in the Oxisol.