Sequential Fractionation as an Operational Tool to Study P Forms and P Release Potential in Soils and Sediments.
Antonio Delgado and Concepcion Saavedra. Univ of Seville, Dpt Ciencias Agroforestales, EUITA Ctra Utrera Km 1, Sevilla, 41013, Spain
An approximation to the knowledge of P forms in soils and sediments is important in agronomic and environmental terms. Sequential chemical extractions have been often used to study the nature of P forms. These methods are based on the selective extraction of operationally defined P fractions by using single extractants in a sequential manner. This can be used as a tool for establishing different “operational” P pools in soils or sediments with a differential ability to release phosphate to the solution. The main objectives of this work were: (i) to study P forms in 17 representative soils from Mediterranean areas and in sediments eroded from these soils by means of two sequential fractionation methods (Ruiz et al., 1997 and Diaz-Espejo et al., 1999), and (ii) identify the relationships of specific P fractions to P release potential in soil and sediments as determined using P sinks (anion exchange resins and paper strip impregnated with Fe oxide). The use of these fractionation schemes allow to discriminate in this group of soil: the more labile P forms (essentially adsorbed P), non-lithogenic Ca phosphates, most of low soluble pedogenic Ca phosphates, lithogenic Ca phosphates, P occluded in poorly crystalline Fe oxides, and P occluded in crystalline Fe oxides. The availability index for plants (Olsen P) was correlated with the P fractions including the most labile P forms: NaOH extractable P + Citrate-bicarbonate extractable P (NaOH-P + CB-P, essentially adsorbed and soluble precipitated Ca phosphates, r = 0.75, P < 0.001) and Ca-EDTA extractable P (Ca-EDTA-P, mainly adsorbed P, r = 0.79, P < 0.001). Amounts of P extracted by anion exchange resins at 1 h and 1 d were significantly correlated with NaOH-P + CB-P (r = 0.73 and r = 0.74, respectively, P < 0.001). Phosphorus fractions significantly affected the desorption kinetics of P release from soil: the ratio of P extracted at 1 h to the maximum extractable using resins was significantly correlated with the Ca-EDTA-P to total P ratio in soil (r = 0.66; P < 0.01). Eroded sediments from studied soils had a higher P concentration and a higher proportion of Fe-related P than original soils. The algal-available P in runoff from these soils (estimated using the Fe oxide impregnated strip - FeO strip) was equivalent to the sum of dissolved reactive P in runoff and fractions including the more labile P fractions in sediment (NaOH-P + CB-P) (Y = X, R2 = 0.85, P < 0.001).On average, 68 % of the P extracted with the FeO Strip was particulate P. An estimate of the long-term P release potential must take in account redox sensitive P forms, in particular P related to poorly crystalline Fe oxides, and mineralization of organic P in sediments. If redox sensitive P and NaOH extractable organic P is considered as potentially releaseble P, the long-term algal-available P might increase by 42 % of the amount estimated using the FeO Strip. References: 1)Ruiz, J.M., A. Delgado, and J. Torrent. 1997. Iron-related phosphorus in overfertilized European soils. 2) J. Environ. Qual. 26:1548-1554. Díaz-Espejo, A., L. Serrano, and J. Toja. 1999. Changes in sediment phosphate composition of seasonal ponds during filling. Hydrobiologia 392:21-28.