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Soil Chemistry

Phosphate Desorption from Goethite in the Presence of Galacturonate, Polygalacturonate, and Maize Mucigel (Zea mays L.)

^a Institute of Ecology, Berlin Univ. of Technology, Salzufer 12, D-10587 Berlin, Germany
^b Institute for Plant Nutrition, Univ. of Hohenheim, Fuwirthstr. 20, D-70593 Stuttgart, Germany

^* Corresponding author (christian.mikutta{at}tu-berlin.de)

Uronates are important constituents of maize mucilage and polyuronates are used as a simplified model of the soil–root interface. We tested whether galacturonate (GA) and polygalacturonate (PGA) impair the diffusion of phosphate (PO₄) into and out of pores of a synthetic goethite (147 m² g^–1) and whether the effect of maize mucigel (MU) is comparable to PGA. We measured the PO₄ desorption kinetics of goethites in batch experiments over 2 wk at pH 5. One part of the goethite was equilibrated with organic substances before PO₄ addition, another part after addition of PO₄. Before the desorption experiments, the porosity of our samples was analyzed by N₂ gas adsorption. In each treatment a rapid initial desorption was followed by a slow desorption reaction, which is assigned to the diffusion of PO₄ out of mineral pores. No consistent relation between the micro- and mesoporosity and the rate of the slow PO₄ desorption was observed. Compared with the C-free control, only PGA and MU affected the fraction of PO₄ mobilized by the fast and slow desorption reaction: when PGA was sorbed to goethite before PO₄, twice as much PO₄ was mobilized via the fast reaction than in the treatment where PO₄ was sorbed before PGA, suggesting a decreased accessibility of goethite pores to PO₄. Mucigel, however, showed reversed effects, which is ascribed to its differing chemical composition. In conclusion, PGA seems inappropriate as a model substance for maize MU collected from non-axenic sand cultures. Under the experimental conditions chosen, the efficacy of all organic substances to increase PO₄ solution concentrations by pore clogging and sorption competition is small.

Abbreviations: DOM, dissolved organic matter • GA, galacturonate • MU, mucigel • OM, organic matter • PE, polyethylene • PO₄, phosphate • PGA, polygalacturonate