Possible role of the polyuronic components in accumulation and mobilization of iron and phosphate at the soil–root interface

With the aim to investigate the role of the polyuronic components in the accumulation of iron and phosphate at the soil–root interface, the interactions of Ca-polygalacturonates (PGAs) with Fe(III) and P ions and of Fe(III)–Ca-polygalacturonates (Fe–PGAs) with P ions were studied at pH 4.7. The role of citric, malic and pyruvic acids in the mobilization of Fe(III) and P, in the presence and absence of Ca(II) 2.5 mM, was also investigated.The sorption kinetics evidenced that P diffuses freely through the calcium polysaccharidic matrix whereas Fe(III) accumulates as an hydroxypolymer. The sorption kinetics of P by the Fe–PGA indicated that the amount of P sorbed increases with increasing its initial concentration up to a constant value equal to 0.98 μmol/3.87 μmol mg−1 of Fe(III)-polymer trapped. The FT-IR spectra of the P–Fe–PGA systems, show bands attributable to P-O(H) stretching vibrations.The study of systems with a constant initial P amount and varying Fe(III) amounts allowed to hypothesize that phosphate settles down inside holes formed by the carboxylate groups of galacturonic units.Citric and malic acids showed to be active in the mobilization of both Fe and P whereas pyruvic acid appeared inactive.