Soil Science Society of America Journal, Sept-Oct, 2006, Vol.70(5), p.1731(10)
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 (P[O.sub.4]) into and out of pores of a synthetic goethite (147 [m.sup.2] [g.sup.-1]) and whether the effect of maize mucigel (MU) is comparable to PGA. We measured the P[O.sub.4] desorption kinetics of goethites in batch experiments over 2 wk at pH 5. One part of the goethite was equilibrated with organic substances before P[O.sub.4] addition, another part after addition of P[O.sub.4]. Before the desorption experiments, the porosity of our samples was analyzed by [N.sub.2] gas adsorption. In each treatment a rapid initial desorption was followed by a slow desorption reaction, which is assigned to the diffusion of P[O.sub.4] out of mineral pores. No consistent relation between the micro- and mesoporosity and the rate of the slow P[O.sub.4] desorption was observed. Compared with the C-free control, only PGA and MU affected the fraction of P[O.sub.4] mobilized by the fast and slow desorption reaction: when PGA was sorbed to goethite before P[O.sub.4], twice as much P[O.sub.4] was mobilized via the fast reaction than in the treatment where P[O.sub.4] was sorbed before PGA, suggesting a decreased accessibility of goethite pores to P[O.sub.4]. 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 P[O.sub.4] solution concentrations by pore clogging and sorption competition is small.
Phosphates -- Research ; Sorption -- Research