Abstract

Dicotyledons cope with iron (Fe) shortage releasing low molecular weight organic compounds into the rhizosphere to mobilize Fe through reduction and complexation mechanisms. The effects induced by these root exudates on soil mineralogy and the connections between Fe mobilization and mineral weathering processes have not been completely clarified. Within a batch experiment, we tested two different classes of organics commonly exuded by Fe-deficient plants, i.e. three organic acids (citrate, malate, oxalate) and three flavonoids (rutin, quercetin, genistein), alone or in combination, for the ability to mobilize Fe from a calcareous soil and modify its mineralogy. In addition, the effect of root exudates on soil mineralogy was assessed in-vivo by cultivating Fe-deficient and Fe-sufficient cucumber plants (Cucumis sativus L.) in a RHIZOtest device. Mineralogical analyses were performed by X-ray powder diffraction. The batch experiment showed that citrate and, particularly, rutin (alone or combined with organic acids or genistein) promote Fe mobilization from the soil. The combinations of rutin and organic acids modified the soil mineralogy by dissolving amorphous fractions and promoting the formation of illite; these mineralogical alterations were significantly correlated with the amount of Fe mobilized from the soil. RHIZOtest experiments revealed a drastic dissolution of amorphous components in the rhizosphere soil of Fe-deficient plants, possibly caused by the intense release of phenolics, amino acids and organic acids, but without any formation of illite. Both batch and RHIZOtest experiments proved that exudates released by cucumber under Fe-deficiency concurred to the rapid modification (few days) of the mineralogy of a calcareous soil.

Full paper: http://www.sciencedirect.com/science/article/pii/S1002016015601047