Solubilization of ferrihydrite iron by plant phenolics: A model for rhizosphere processes

Abstract

Several naturally‐occurring phenols, including some which are exuded by iron deficient roots, promote the reduction and release of ferrihydrite iron. The initial rate of iron reduction brought about by each phenol was determined by spectrophotometric measurement of the Fe(ferrozine)² ₃ ⁺ complex which absorbs maximally at 562 nm. The rate of iron release was dependent upon the concentration of phenol and ferrihydrite, but not on the concentration of added ferrozine. The order of reactivity was: caffeic acid > chlorogenic acid > ferulic acid > p‐coumaric acid. A direct correlation was observed between the reducing ability for each phenol and its ability to release ferrihydrite iron. The optimum pH range was 5.0–6.0. Cu(II) and Ni(II) inhibited the reduction of ferrihydrite iron by the phenols. This study introduces a model for the investigation of chemical processes occurring at the soil‐root interface.