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

Abstract

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 m2g−1) and whether the effect of maize mucigel (MU) is comparable to PGA. We measured the PO4desorption kinetics of goethites in batch experiments over 2 wk at pH 5. One part of the goethite was equilibrated with organic substances before PO4addition, another part after addition of PO4Before the desorption experiments, the porosity of our samples was analyzed by N2gas adsorption. In each treatment a rapid initial desorption was followed by a slow desorption reaction, which is assigned to the diffusion of PO4out of mineral pores. No consistent relation between the micro‐ and mesoporosity and the rate of the slow PO4desorption was observed. Compared with the C‐free control, only PGA and MU affected the fraction of PO4mobilized by the fast and slow desorption reaction: when PGA was sorbed to goethite before PO4, twice as much PO4was mobilized via the fast reaction than in the treatment where PO4was sorbed before PGA, suggesting a decreased accessibility of goethite pores to PO4Mucigel, 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 PO4solution concentrations by pore clogging and sorption competition is small.