Saturday, 15 July 2006

P Desorption in Andisols with Anion Exchange Resin Membranes Sequential Extraction.

Miguel A. Negrin1, Jose Manuel Hernández-Moreno1, Francois Bartoli2, and Montserrat Espino1. (1) Univ de La Laguna, Dpto. Edafología y Geología, Facultad de Biología, Avda. Astrofísico Fco.Sánchez s/n, La Laguna, 38204, Spain, (2) Laboratoire Sols et Environnement ENSAIA/INRA, BP 172, Vandoeuvre-les-Nancy, 54505, France

Volcanic soils (Andisols) are characterized by large amounts of poorly-ordered minerals (allophanes, ferrihydrite), mostly positively charged and therefore by large amounts of mineral (sulphates and phosphates) anions or organic polyanions. Al-humus colloids also belong to the reactive colloids of Andisols. An important fraction of phosphorus in Andisols should be linked to in Organic Carbon (OC)-Al-Fe associations. Previous studies by the authors in Andisols showed that P sorbed by Anion Exchange Membranes (AEM) was always associated with OC, Al, Si, and Fe, and in some cases, AEM promoted some soil dispersion. To test the hypothesis that P dynamics in Andisols is linked to the aggregation processes involving noncrystalline and OC components, an AEM sequential extraction experiment was designed. Studied soils were forest and cultivated Andisols from the Canary Islands, Spain. Soil aggregate suspensions were prepared from bulk air-dried sieved soils (< 2 mm) whereas organo-mineral clay suspensions were prepared from different soil clay fractions (obtained using Na resin as dispersant agent. The AEM, immersed in a 10 kDa dialysis membranes, were shaken in the soil aggregate or clay suspensions. Dialysis bags and AEM were removed and replaced after 2, 4, 8, 16, 32, 64 and 128h. The membranes were eluted with NaHCO3 and P, Al, Si, Fe, and OC analysed. The results showed a fast initial P desorption step decreasing up to 40-50h, followed by a constant rate desorption. The same trend was observed for OC, Al, Si, and Fe for the clay fractions but not for the bulk soils in which the final desorption rate increased. This was attributed to aggregate disruption when the energy induced by end-over shaking was sufficient. This disaggregation rendered more accessible the organo-mineral clays which concentrated both poorly-ordered minerals, organic matter and counteracting anions such as phosphates. The amounts of P recovered at the end of the experiment represented two to five times the available Olsen P contents. The similar shape of the curves of cumulative sorbed elements suggests that anionic complex forms of P, Al, Si and OC could participate as sorptives. This is consistent with the finding by the authors of negative z-potential in the readily water dispersible clay fractions of the studied soils. The results were interpreted in terms of different kinetic expressions. The results bring useful information for the interpretation of P availability in Andisols for which usual calibrations relating extractable P to expected yields and fertilizer requirements are not adequate.

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