Assessing the fixation and availability of sorbed phosphate in soil using an isotopic exchange method

Abstract

An isotopic exchange method was used to characterize quantitatively the fixation and plant availability of phosphate previously sorbed by soils. In general, the exchangeability of the sorbed phosphate was much higher than its desorbability for both soils and clay minerals. Isotopic exchangeability of the sorbed phosphate increased with sorption saturation during the initial stage (1 5-60% saturation), but the increase was less with increasing saturation from 6&90% for all soils tested. Therefore a sorption saturation of 60% was recommended as the upper limit of P fertilization in terms of economical efficiency. For clay minerals, with increasing sorption saturation, the isotopic exchangeability of the sorbed P increased significantly for kaolinite and sesquioxides, but decreased for montmorillonite. Most of the phosphate sorbed by montmorillonite and kaolinite was found to be isotopically exchangeable, but only a small amount of the P sorbed by goethite could be exchanged. The P sorbed by Al oxide exhibited isotopic exchangeability between that of kaolinite and Fe oxide. The isotopically exchangeable phosphate pool could readily account for the P uptake of plants and the available P determined by some commonly used chemical methods, such as Olsen-P and Bray-P.

Desorption and plant-availability of phosphate sorbed by some important minerals

“…It is interesting that P uptake of plants (see Table 4) was larger than the total extractable P lost during the rice growth for Fe and A1 oxide-P complex. In addition, compared to the previous results (He et al, 1991) the recovery of sorbed P by rice plants was larger than the desorbability in 0.02 M KC1 solution for the complexes except for montmorillonite-P at 50% saturation, and was even larger than the isotopic exchangeability for the goethite-P complex. Apparently, a part of the P sorbed by sesquioxides, though not extractable by those chemical reagents, still remains available to plants, whereas P sorbed by montmorillonite may be unavailable to plants although it is readily desorbed even by anion-exchange resin.…”

“…The samples of minerals have been described previously (He et al, 1991). Briefly, the montmorillonite and kaolinite are natural products.…”

“…At pH 7.0, kaolinite has a small net negative charge, goethite has a small net positive charge and amorphous A1 oxide is positively charged whereas montmorillonite is negatively charged according to their PZC values (kaolinite6.45, goethite7.1s, amorphous A1 oxide9.9o and montmorillonite2.1o) (He et al, 1989(He et al, , 1991. Therefore, apart from the type of mineral groups, surface charge may be also an important factor responsible for the difference in phosphate adsorption between these two types of mineral.…”

“…Particularly this is the case for the surface of goethite that may form a bidentate complex with phosphate (Hingston, et al, 1974;Martin and Smart, 1987;Parfitt, 1978). However, He et al (1991) showed that a large proportion of the specificallysorbed P on the surfaces of Fe, A1 oxide and kaolinite was isotopically exchangeable within three days although it was very difficult to desorb by a simple aqueous equilibration. The isotopic exchangeability of sorbed P was also found to increase with increasing sorption saturation.…”

Phosphorus in humped and hollowed soils of the Inchbonnie catchment, West Coast, New Zealand: I. Variation with age and distribution

“…The slope of the linear regression denotes the annual change in P concentration. much greater than olsen P due to the occupation of P sorption sites with added P. Furthermore, studies using radioactive 33 P have shown the changes in labile pools tend to be quicker than others (He et al 1991;Daroubetal.2000).…”