IRON-RELATED PHOSPHORUS IN OVERFERTILIZED EUROPEAN SOILS

Iron-related P forms directly influence the amount of phosphate releas ed from soils and sediments, particularly when subjected to reduction, In this work, we examined such P forms in 12 overfertilized, widely d ifferent European soils Bg using various single and sequential extract ion techniques, including the use of a mild and a strong reductant (as corbate 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 seque ntial extraction procedure of Olsen and Sommers. More than 50% of redu ctant-soluble P was released by the specific effect of ascorbate in mo st of the soils studied, The results of the sequential extractions sug gest that, on average, about one-half of the reductant-soluble P,vas o ccluded in Fe oxides. Poorly crystalline and crystalline Fe oxides gen erally contained similar amounts of occluded P in absolute terms, Howe ver, P/Fe mole ratios were higher in poorly crystalline than in crysta lline Fe oxides ((X) over bar = 5.9 and 1.7 x 10(-2), respectively), W e suggest that fertilizer P may have heel occluded in Fe oxides, parti cularly in the poorly crystalline forms. The contribution of the paren t material, pedogenesis, and fertilization to occluded P build-up is d ifficult to assess. Though obtained in vitro, our results suggest that reduction of overfertilized soil materials in aquatic environments ca n 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.