Aluminium-organic acid interactions in acid soils: II. Influence of solid phase sorption on organic acid-Al complexation and Al rhizotoxicity

Malic acid has been implicated as an inducible root mechanism for the detoxification of rhizotoxic Al. The chemical reactions of malate were therefore investigated in four acid soils ranging in pH from 4.30 to 5.00. All the soils behaved similarly, with malate predominantly complexing Al in solution in favor of other metal cations (Fe, Mn, Cu, Zn, Ca and Mg). Comparison of experimental extraction results with theoretical predictions made using Geochem-PC showed a similar agreement, indicating that the behavior of Al and malate in soil was similar to their behavior in an ideal solution. Competitive anion sorption studies with synthetic Fe and Al(OH)_3(S) gels showed similar sorption sites for phosphate, citrate and malate, however, malate was weakly sorped in comparison to phosphate and citrate. Sorption was little affected by solution pH. Malate sorption did not appear to be largely influenced by metal complexation in soils; however, sorption onto Fe and A1(OH)₃ gels was markedly reduced in the presence of complexing metal cations. The implications of these results in understanding the role of malate exudation from the root in Al detoxification in the rhizosphere are discussed.