The release of phosphorus from heavily fertilized soils to dilute electrolytes: effect of soil properties

Soils heavily fertilized with P can contribute to the enrichment of P in surface runoff and can accelerate the eutrophication of streams and lakes. To describe this enrichment, information is needed on the capacity and rate of P release from soils. Thus, we carried out laboratory studies in which the release of P to a dilute electrolyte (0.002 M CaCl2) was characterized in 12 widely different, heavily fertilized European soils. We studied: a) long-term P release under 'static' conditions tin soil suspensions allowed to settle and left undisturbed for long periods); and b) short-term P release under 'turbulent' conditions (rapidly stirred suspensions). The relationships between the amount of P released under static conditions and the solution/soil ratio (W) and time differed widely among soils. These differences were the probable result of differences in the content of the relatively soluble metal phosphates (capable of buffering P solution in the 10(-6)-10(-5)M concentration range) and in the P adsorption capacity of the soils. The ratio between P released at 340 d for W = 10 000 and P exchanged isotopically at 1 d was generally > 2 in soils rich in soluble phosphates and with a high degree of surface saturation with P, while it was < 1 in soils with low surface saturation with P (acid soils with high P adsorption capacity). In soils rich in organic matter, the amounts of dissolved organic P and molybdate-reactive P released were comparable. Under turbulent conditions, the amount of P released after 2 h of soil-electrolyte contact ranged between 7 and 27 % of the P released at 340 d under static conditions. This stresses the importance that surface runoff and initial stream transport have in loading surface waters with P. The release of P under turbulent conditions was slower in acid sandy soils, where poorly crystalline Fe and Al oxides were the most important P adsorbents. ((C) Inra/Elsevier, Paris.)