| Five simulative experiments were carried out to study the effect of soil flooding on the transformation of iron oxides and the adsorption/desorption behavior of phosphate in a red earth (Ultisol) and a deglyed dark paddy soil (Entisol), which were widely distributed in Zhejiang Province of China. Three kinds of synthetic iron oxides with different crystallinity (i.e. goethite, hematite, and ferrihydrite) were used to study the relationship between their characteristic of adsorption/desorption and phosphorus saturation. Different degree of phosphorus saturation of ferrihydrite was used to study the relationship between degree of phosphorus saturation and rice adsorbed phosphorus. The effect of the special oxidation condition in the rice rhizosphere on P dynamics, compared to the situation in the anaerobic bulk soil, and on P availability was studied. The effects of the exudation of organic acids and protons as well as their interactions on the fixation and release of ammonium in the rhizosphere was studied. The results obtained from these experiments are summarized as follows.1. The effect of soil flooding on the transformation of Fe oxides and theadsorption/desorption behavior of phosphate1) Soil flooding significantly increased oxalate-extractable Fe (Feox), mainly at the expense of dithionite-soluble Fe (FeDCB), as well as oxalate-extractable P(Pox), but decreased the ratio of Pox/Feox.2) Flooding largely increased both, P adsorption and the maximum P adsorption capacity. The majority of newly sorbed P in the soils was Pox, 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.3) Due to a decrease of the saturation index of the P sorption capacity, P adsorbed by flooded soils was much less desorbable than from non-flooded soils. There are obviously significant differences in the nature of both, the Feox and Pox fractions under non-flooded and flooded conditions. The degree of the changes in Feox, Pox, P adsorption and P desorption by flooding depended on the contents of amorphousand 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.2. Phosphorus adsorption properties of three synthetic iron oxides inrelation to phosphorus adsorption saturation1) P adsorption properties of three synthetic iron oxides could be described by Langumir equation with a correlation coefficient larger than 0.9 at 1% significant level. It was found by comparing Qm (maximum quantity of adsorption), K (adsorption constant) and MBC (maximum buffing capacity) of three synthetic iron oxides that ferrihydrate (amorphous) was much larger than crystalline iron oxides (goethite and hematite) in both intensity and capacity of P adsorption.2) P adsorbed by ferrihydrate was much more difficult to be desorbed than those by goethite and hematite. The effect of P desorption enhanced by the large active surfaces of ferrihydrate with very poor crystallinity was not observed in the experiment.3) It was suggested by our results that P adsorption saturation may be available for use as an integrative index for estimating the intensity and capacity of P adsorption-desorption in soils or iron oxides.3. Phosphorus Adsorption Saturation of Ferrihydrate as an Index ofPhosphorus Availability to Paddy Rice (Oryza sativa L.)The results obtained from experiments showed that the artificially synthesized ferrihydrate had a very large adsorption potential to P, with 45045 μg P g-1 of maximum adsorption capacity (Qm). P adsorption isotherm of ferrihydrate could be fitted very well with Langmuir equation. Desorption of P adsorbed by ferrihydrate was mainly controlled by P adsorption saturation of ferrihydrate. P adsorbed by ferrihydrate with saturation less than 30% was hardly desorbed. P desorption slowly increased with the increase of P saturation from 30% to 60%, but sharply increased with P saturation larger than 60%. Th...
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