Effect of root exudates on soil mineralogy: possible implications on nutrient mobilization.

Plants release in the rhizosphere flavonoids and organic acids which exert multifunctional roles. In a recent study, we observed that the flavonoid rutin, alone or combined with genistein or organic acids (citrate, malate, oxalate), usually present in plant exudates, mobilized Fe from a calcareous soil with a great efficiency. However, the effects of these exudates on soil mineralogy, especially on Fe-bearing minerals and clays, remained unexplored. This work aims at evaluating the changes in the mineralogical composition of a calcareous soil after 24h-treatment with an aqueous solution of rutin (35 μM), alone or combined with other flavonoids (10 μM) or organic acids (1 mM). After centrifugation, the solid fraction was dried, homogenized with corundum (internal standard) and analysed by X-ray powder diffraction (XRPD), using the Rietveld method for mineralogical quantification. XRPD analysis of the natural soil revealed the presence of calcite (57.0%), illite (17.9%), smectite (5%), quartz (4.2%), heulandite (2.8%), and amorphous phases (13%). No Fe-oxides and oxyhydroxides were detected. In all the treatments with rutin, the sum of illite and smectite considerably increased, compared to the natural soil, especially when rutin was combined with organic acids or genistein. Conversely, the amorphous residue decreased. It can be assumed that the increase of illite+smectite was caused by the transformation of the amorphous phases into clay minerals. Rutin and its combinations with organics possibly mobilized Fe from soil along with other cations which could then find suitable conditions to recrystallize as new forming clay minerals. Na, K, and Ca could enter in the interlayer regions of phyllosilicates, while Al and Si could induce the formation of new sheets. Although the variation in soil mineralogy could not be easily related to Fe mobilization, it was evident that some treatments producing the higher Fe solubilization showed also the higher increase of the illite+smectite fraction.