Biogeochemical cycling of iron oxides in the rhizosphere of plants grown on ferruginous duricrust (canga)

Abstract

Goethite-cemented duricrusts, also known as canga, commonly occur as a capping rock protecting underlying iron ore deposits. The processes that govern canga formation are still unclear but include recurrent partial dissolution and recrystallisation of goethite through biogeochemical cycling of iron, hypothesised to be catalysed by plants and bacteria. In the present study, the effect of plant exudates on mobilisation of iron in canga was examined using model plants grown on crushed canga in RHIZOtest devices, which separate roots from substrate by a semi-permeable membrane. Moderate plant-induced acidification of the canga was detected, however the primary driver of mineral dissolution was the synergistic effect of reductive and ligand-promoted dissolution, identified by an increase in organic acids concentration and the presence of low concentrations of free ferrous iron. Whilst organic acids exudation lasted, iron cations were stabilised in solution; once the organic acids were degraded by microorganisms, the free cations precipitated as iron oxy-hydroxides. Mineralogical analysis and high-resolution microscopy confirmed our hypothesis that plants that grow in this iron-rich substrate contribute to iron dissolution indirectly (e.g., during phosphate solubilisation), and that the resulting surplus iron not taken up by the plants is redeposited, promoting the cementation of the residual minerals. Understanding the contribution of plants to the iron cycling in canga is crucial when formulating post-mining rehabilitation strategies for iron ore sites.