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Iron-Related Phosphorus in Overfertilized European Soils

Departamento de Ciencias y Recursos Agrícolas y Forestales, Universidad de Córdoba, Apdo. 3048, 14080 Córdoba, Spain.

^* Corresponding author (crltocaj{at}uco.es).

ABSTRACT

Iron-related P forms directly influence the amount of phosphate released from soils and sediments, particularly when subjected to reduction. In this work, we examined such P forms in 12 overfertilized, widely different European soils by using various single and sequential extraction techniques, including the use of a mild and a strong reductant (ascorbate and dithionite, respectively). Reductant-soluble P (defined as the amount of P released by the specific action of a strong reductant) ranged from 2 to 63% of the total amount of P extracted by the sequential extraction procedure of Olsen and Sommers. More than 50% of reductant-soluble P was released by the specific effect of ascorbate in most of the soils studied. The results of the sequential extractions suggest that, on average, about one-half of the reductant-soluble P was occluded in Fe oxides. Poorly crystalline and crystalline Fe oxides generally contained similar amounts of occluded P in absolute terms. However, P/Fe mole ratios were higher in poorly crystalline than in crystalline Fe oxides ( = 5.9 and 1.7 x 10^–2, respectively). We suggest that fertilizer P may have been occluded in Fe oxides, particularly in the poorly crystalline forms. The contribution of the parent material, pedogenesis, and fertilization to occluded P build-up is difficult to assess. Though obtained in vitro, our results suggest that reduction of overfertilized soil materials in aquatic environments can release significant amounts of P; these in turn increase saturation of reduction-resistant P-adsorbing surfaces and thus help increase the P equilibrium concentration in solution.

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