Combined effect of organic acids and flavonoids on the mobilization of major and trace elements from soil
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Combinations of different molecules usually co-exuded by plant roots in soil can significantly affect the mobilization of mineral elements from soil. The flavonoids rutin and quercetin appeared to be highly efficient in Fe and Mn mobilization from soil, being rutin about 25 times more effective than citrate in extracting Fe from an alkaline calcareous soil, and 15 times in mobilizing Mn from a slight acidic agricultural soil. Quercetin was particularly effective in mobilizing Mn from the acidic soil and 50 times more efficient than citrate. A significant synergistic effect was detected when either quercetin or rutin was combined with citrate, extracting, respectively, 1.7 and 1.5 times more Mn than what is expected by a simple combination of the two. Sorption processes on soil particles were more relevant for flavonoids than for organic acids for which microbial degradation often prevails, with the only exception of oxalate. Citrate was usually the most efficient organic acid in mobilizing major and trace elements from soil even if oxalate was a better extractant for Cu, Zn, and Ni from the alkaline soil. In the acid soil, a synergistic effect among organic acids was observed only for Mn while in alkaline soil it was observed for Si. The mechanism by which Fe is extracted by rutin in the alkaline soil is a reductive one, with one molecule of rutin being capable of mobilizing two atoms of Fe. Also for Mn in the acid soil, quercetin and rutin solubilize this element by a reductive process. However, when quercetin and rutin are combined with citrate, a complex-forming dissolution mechanism also occurs which increases the mobilization of Mn over the expected rates.