Iron‐Related Phosphorus in Overfertilized European Soils

Abstract

AbstractIron‐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 (X̄ = 5.9 and 1.7 × 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.