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Long-term phosphate (P) desorption from soil is described using two discrete P "pools" in the soil: one available and one strongly fixed pool. The P release kinetics for each pool are described with a firstorder rate equation. A new desorption method is used with hydrous iron oxide inside dialysis tubing acting as a P sink. The widely used iron-impregnated filter paper desorption method overestimates initial P desorption by a factor of up to 4 and underestimates the quantitative progression of desorption as a function of time. P desorption continued with substantial rates for periods longer than 1600 h. A wide range in P desorbability was observed: 15-70% of oxalateextractable P (Pox) desorbed after 1600 h. P desorbability decreased with increasing Feox + Alox content of the sample. The relative size of the quickly desorbing pool increased with increasing initial degree of P saturation oto = P0X/[Fe0X + Alox] of the soils. This fact is of direct importance for the estimation of P losses from phosphate-rich soils. This study furthermore provides evidence that all oxalate-extractable P potentially is desorbable: no irreversibly fixed Pox exists.

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The sorption of phosphate on amorphous aluminium hydroxides was investigated using 27Al and P solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, following the effect of different exposures to soluble phosphate. The spectra obtained were compared with the spectrum of amorphous aluminium phosphate. Aluminium i n the unreacted hydroxide had a 100% octahedral co-ordination. When dried at 200°C and exposed to soluble phosphate, very little (maximum 0.1 %) amorphous aluminium hydroxide transformed to a tetrahedral co-ordination (A1 bound by oxygen bridges to four P atoms), even after 120 d. The tetrahedral co-ordination exists in aluminium phosphate gel, although most of its A1 atoms exhibit an octahedral co-ordination. For the aluminium hydroxide dried at 200"C, no formation of aluminium phosphate in which aluminium is in octahedral co-ordination could be detected, not even when the aluminium hydroxide was exposed to a phosphate solution for 120 d. We concluded that the formation of aluminium phosphate is restricted to the surface of the hydroxide. Most of the phosphate which is bound to the aluminium oxide however may not have formed a 'bulk solid' aluminium phosphate, but is adsorbed on the internal and external surface of the oxide. The same amorphous aluminium hydroxide, dried at 70°C instead of 2OO"C, is converted much more rapidly to aluminium phosphate when exposed to soluble phosphate. We propose a P-induced weathering mechanism to describe P sorption on amorphous aluminium hydroxides at high P concentrations. In addition to NMR, phosphate adsorption experiments conducted on aluminium hydroxides dried at different temperatures produced evidence that the porosity of the aluminium hydroxide aggregated particles can also be a factor controlling the rate of phosphate uptake from solution, if the aggregate is stable (is not resuspended) in solution. 71

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