The effect of soil flooding on the transformation of Fe oxides and the adsorption/desorption behavior of phosphate

As repeatedly reported, soil flooding improves the availability of P to rice. This is in contrast with an increased P sorption in paddy soils. The effects of soil flooding on the transformation of Fe oxides and the adsorption/desorption of P of two paddy soils of Zhejiang Province in Southeast-China were studied in anaerobic incubation experiments (submerging with water in N-2 atmosphere). Soil flooding significantly increased oxalate-extractable Fe (Fe-ox), mainly at the expense of dithionite-soluble Fe (Fe-DCB), as well as oxalate-extractable P (Pox), but decreased the ratio of P-ox/Fe-ox. Flooding largely increased both, P adsorption and the maximum P adsorption capacity. The majority of newly sorbed P in the soils was P-ox, but also more newly retained P was found to be not extractable by oxalate. Flooding also changed the characteristics of P desorption in the soils. Due to a decrease of the saturation index of the P sorption capacity, P adsorbed by flooded soils was much less desorbable than that from non-flooded soils. There are obviously significant differences in the nature of both, the Fe-ox and P-ox fractions under non-flooded and flooded conditions. The degree of the changes in Fe-ox, P-ox, P adsorption and P desorption by flooding depended on the contents of amorphous and total Fe oxides in non-flooded soils. Our results confirm that the adsorption and desorption behavior of P in paddy soils is largely controlled by the transformation of the Fe oxides. The reasons of the often-reported improved P availability to rice induced by flooding, in spite of the unfavorable effect on P desorbability, are discussed.