Silicon solubilisation from soil minerals and soil by root exudate compounds

Organic root exudate compounds (ORECs) can enhance mineral dissolution, releasing silicon (Si), but OREC-induced processes of Si solubilisation from different soil Si pools are poorly understood. This study explored the impact of two carboxylates (citrate, oxalate) and a phenolate (catechol) on Si solubilisation from soils and soil minerals. We extracted six different soils, seven primary minerals, and four clay minerals with the three ORECs and NH₄NO₃ as a background electrolyte (control) at a realistic rhizosphere concentration of 5 mmol L⁻¹. The extraction solutions were not buffered to mimic the conditions of real rhizospheres. Silicon extractability from the experimental soils was positively related to clay content, whereas other soil chemical and mineral characteristics showed no obvious relation. The ORECs extracted up to 11 times more Si than the control from primary minerals, clay minerals and soils during 2-hour extraction periods. Citrate consistently increased Si solubility in soils and soil minerals. Catechol mainly increased Si solubilisation from soil minerals, while oxalate had the greatest effect in soils. Overall, the Si solubilisation efficiency of ORECs varied and followed the order citrate > catechol > oxalate for primary minerals, catechol > citrate > oxalate for clay minerals, and oxalate > citrate > catechol for soils. We conclude that ORECs can strongly enhance Si solubilisation from soils and soil minerals at concentrations as low as observed in plant rhizospheres. These OREC-induced mineral weathering processes contribute to Si dynamics in the rhizosphere and facilitate soil formation.