Kinetics of Phosphorus Release from Spodosols: Effects of Oxalate and Formate
- T. R. Fox ,
- N. B. Comerford and
- W. W. McFee
Organic anions have been implicated in the release of P to soil solution and may thus increase the availability of P to plants. Oxalate and formate have been identified as the most prevalent low-molecular-weight organic anions in the soil solutions in Spodosols under forest vegetation in northern Florida. The purpose of this study was to investigate the effect that these two organic anions have on the kinetics of inorganic- and organic-P release from the A, Bh, and Bt horizons of a representative forested Spodosol. Soil samples were equilibrated for various periods of time with solutions of either oxalate, formate, or water. The extracts were analyzed for pH, inorganic P, organic P, Al, and organic acids. Results showed that oxalate had little effect on P release in the A horizon. This was due to the lack of Al oxides to hold P. Conversely, oxalate greatly increased solution P in the Bh and Bt horizons. In the subsoil horizons, the release of P was rapid, followed the disappearance of oxalate from solution, and was coincident with an increase in pH. These facts suggested that P was released via a ligand-exchange reaction. A longer term reaction was consistent with the reprecipitation of the P as an amorphous Al phosphate when oxalate was either totally sorbed or degraded. The presence of formate did not increase P release.
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