Plants release in the rhizosphere a large variety of organic compounds, among which organic acids (OAs) and flavonoids. These molecules can interfere with nutrient cycles, protect plants against pests and diseases, and regulate plant-microorganism interactions and allelopathy. In a recent study, we tested the capacity of three flavonoids (rutin, quercetin and genistein) and three OAs (citrate, malate and oxalate) usually present in plant exudates, to mobilize major and trace elements from different types of soil. In particular we observed that rutin, both alone and combined with OAs or with genistein, mobilized Fe from a calcareous agricultural soil with a very high efficiency. Although we explained the chemical mechanisms involved in Fe mobilization from soil, the effects induced by these plant exudates on the soil mineralogy, especially on Fe-bearing minerals and clays, remained unexplored. Mineralogical investigations on mineral modifications by root exudates have always been a challenging task and only few reports are available in the literature. Therefore, the present study aimed at evaluating the changes in the mineralogical composition of a calcareous soil after 24 h of treatment in closed polyethylene tubes with an aqueous solution of rutin (35 µM), tested alone or in combination with other flavonoids (10 µM) or with OAs (1 mM). All solutions contained NaN3 (10 mM) as bacteriostatic agent. After centrifugation, the solid fraction was dried at 60°C, homogenized with 20% (w/w) of corundum (used as internal standard) and analysed by X-ray powder diffraction (XRPD). Quantitative analysis was performed by using the Rietveld method. XRPD analysis revealed the presence, in the natural calcareous soil, of calcite (57.0%), illite (17.9%), smectite (5%), quartz (4.2%), heulandite (2.8%) and amorphous phases (13%). No Fe-minerals were detectable by XRPD. In all treatments with rutin, the sum of illite and smectite considerably increased, compared to the natural soil, especially when rutin was tested in combination with OAs or genistein. In particular, illite increased by a minimum of 3% (rutin alone) to a maximum of 30% (rutin+oxalate), as well as smectite also increased, with the exception of treatment rutin+oxalate, by 25% (rutin and rutin+citrate) up to 33% (rutin+genistein). On the contrary, the amorphous residue decreased by a minimum of 12% (rutin) to a maximum of 41% (rutin+oxalate), while quartz diminished by about 10% in all treatments. On the basis of these results, it can be assumed that the increase of illite and smectite in the treatments with rutin was caused by the transformation of the amorphous phases into clay minerals. Most likely, rutin and especially its combinations with OAs or genistein, mobilized Fe from soil along with many other cations, such as Na, K, Ca, Al and Si, which could then find the suitable conditions to recrystallize as new forming clay minerals. In particular, Na, K, and Ca could enter in the interlayer regions of phyllosilicates, while Al and Si could induce the formation of their sheets. Even if the variation of the mineral composition of the soil could not be easily related to the extraordinary Fe mobilization efficiency of rutin, it was evident that the treatments producing the higher Fe solubilization showed in general the higher increase of the illite+smectite fraction. Results of this study seem to be confirmed by preliminary rhizo-test experiments that we are carrying out to evaluate the effects of exudation from Cucumis sativus plants on the mineralogy of the same soil. Also in this case, root exudation induced a slight increase of the illite+smectite fraction, especially when plants, before the transplanting on soil, were grown under Fe-deficiency conditions. In conclusion, exudates released by plants to increase nutrient availability can modify the mineralogical composition of soil, especially with regard to the clay fraction.
Effect of root exudates on soil mineralogy: possible implications on nutrient mobilization
L Medici;
2015
Abstract
Plants release in the rhizosphere a large variety of organic compounds, among which organic acids (OAs) and flavonoids. These molecules can interfere with nutrient cycles, protect plants against pests and diseases, and regulate plant-microorganism interactions and allelopathy. In a recent study, we tested the capacity of three flavonoids (rutin, quercetin and genistein) and three OAs (citrate, malate and oxalate) usually present in plant exudates, to mobilize major and trace elements from different types of soil. In particular we observed that rutin, both alone and combined with OAs or with genistein, mobilized Fe from a calcareous agricultural soil with a very high efficiency. Although we explained the chemical mechanisms involved in Fe mobilization from soil, the effects induced by these plant exudates on the soil mineralogy, especially on Fe-bearing minerals and clays, remained unexplored. Mineralogical investigations on mineral modifications by root exudates have always been a challenging task and only few reports are available in the literature. Therefore, the present study aimed at evaluating the changes in the mineralogical composition of a calcareous soil after 24 h of treatment in closed polyethylene tubes with an aqueous solution of rutin (35 µM), tested alone or in combination with other flavonoids (10 µM) or with OAs (1 mM). All solutions contained NaN3 (10 mM) as bacteriostatic agent. After centrifugation, the solid fraction was dried at 60°C, homogenized with 20% (w/w) of corundum (used as internal standard) and analysed by X-ray powder diffraction (XRPD). Quantitative analysis was performed by using the Rietveld method. XRPD analysis revealed the presence, in the natural calcareous soil, of calcite (57.0%), illite (17.9%), smectite (5%), quartz (4.2%), heulandite (2.8%) and amorphous phases (13%). No Fe-minerals were detectable by XRPD. In all treatments with rutin, the sum of illite and smectite considerably increased, compared to the natural soil, especially when rutin was tested in combination with OAs or genistein. In particular, illite increased by a minimum of 3% (rutin alone) to a maximum of 30% (rutin+oxalate), as well as smectite also increased, with the exception of treatment rutin+oxalate, by 25% (rutin and rutin+citrate) up to 33% (rutin+genistein). On the contrary, the amorphous residue decreased by a minimum of 12% (rutin) to a maximum of 41% (rutin+oxalate), while quartz diminished by about 10% in all treatments. On the basis of these results, it can be assumed that the increase of illite and smectite in the treatments with rutin was caused by the transformation of the amorphous phases into clay minerals. Most likely, rutin and especially its combinations with OAs or genistein, mobilized Fe from soil along with many other cations, such as Na, K, Ca, Al and Si, which could then find the suitable conditions to recrystallize as new forming clay minerals. In particular, Na, K, and Ca could enter in the interlayer regions of phyllosilicates, while Al and Si could induce the formation of their sheets. Even if the variation of the mineral composition of the soil could not be easily related to the extraordinary Fe mobilization efficiency of rutin, it was evident that the treatments producing the higher Fe solubilization showed in general the higher increase of the illite+smectite fraction. Results of this study seem to be confirmed by preliminary rhizo-test experiments that we are carrying out to evaluate the effects of exudation from Cucumis sativus plants on the mineralogy of the same soil. Also in this case, root exudation induced a slight increase of the illite+smectite fraction, especially when plants, before the transplanting on soil, were grown under Fe-deficiency conditions. In conclusion, exudates released by plants to increase nutrient availability can modify the mineralogical composition of soil, especially with regard to the clay fraction.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
