Plants are known to exude from their roots a variety of organic molecules capable of modifying the rhizospheric soil in order to increase nutrients' availability. Among these, citrate possesses a high affinity for micronutrients such as Fe, an element that may not be sufficiently available to plants despite its high total content in soil, especially in calcareous soils. Citrate has not only high affinity for Fe but also for Al, another major constituent of soil minerals. It has been demonstrated that citrate can mobilize Al from different soil minerals and promote its transport through the soil solution. Aluminum then stays in solution until it is trapped by hydroxide, phosphate or silicate anions or when the concentration of the complexing agent is reduced below the necessary stoichiometric amount. All these conditions can promote the formation of Al precipitates from the soil solution. Calcareous soils are usually alkaline; therefore complexed Al could easily undergo precipitation reactions forming Al oxy/hydroxides. This phenomenon can be particularly severe in the rhizosphere where the activity of microorganisms can quickly and dramatically reduce the amount of organic molecules (e.g., citrate) complexing Al. It is also known from the literature that Cu, an important plant nutrient, has a high tendency to be co-precipitated with Al oxides, hence strongly reducing its concentration in solution. In this research we provide for the first time experimental evidence that citrate can reduce Cu availability in calcareous soils by co-precipitation with amorphous Al oxy/hydroxides. Such a conclusion is supported by SEM-EDX (Scanning Electron Microscopy coupled to Energy Dispersive X-ray spectroscopy) observations from a real calcareous agricultural soil and laboratory experiments simulating the co-precipitation of Al and Cu from citrate complexes in calcareous soils. More specifically, by employing a SEM-EDX Automated Particle Analysis (APA) method specifically implemented for soil analysis, a large increase in the number of microscopic Al oxide particles was observed in a calcareous soil treated with 1mM citrate. Surprisingly approx. 40% of them contained Cu as the only other major constituent. Furthermore, the environmental conditions by which these phenomena may occur are also examined and mathematically described. In conclusion citrate exudation, which most plants adopt as a strategy to mobilize nutrients from the soil, under certain environmental conditions, like those encountered in calcareous soils, can reduce the availability of important elements for plant nutrition such as Cu.
Citrate Exudation can Reduce Copper Availability in Calcareous Soils
L Medici;A Lettino;S Fiore;
2013
Abstract
Plants are known to exude from their roots a variety of organic molecules capable of modifying the rhizospheric soil in order to increase nutrients' availability. Among these, citrate possesses a high affinity for micronutrients such as Fe, an element that may not be sufficiently available to plants despite its high total content in soil, especially in calcareous soils. Citrate has not only high affinity for Fe but also for Al, another major constituent of soil minerals. It has been demonstrated that citrate can mobilize Al from different soil minerals and promote its transport through the soil solution. Aluminum then stays in solution until it is trapped by hydroxide, phosphate or silicate anions or when the concentration of the complexing agent is reduced below the necessary stoichiometric amount. All these conditions can promote the formation of Al precipitates from the soil solution. Calcareous soils are usually alkaline; therefore complexed Al could easily undergo precipitation reactions forming Al oxy/hydroxides. This phenomenon can be particularly severe in the rhizosphere where the activity of microorganisms can quickly and dramatically reduce the amount of organic molecules (e.g., citrate) complexing Al. It is also known from the literature that Cu, an important plant nutrient, has a high tendency to be co-precipitated with Al oxides, hence strongly reducing its concentration in solution. In this research we provide for the first time experimental evidence that citrate can reduce Cu availability in calcareous soils by co-precipitation with amorphous Al oxy/hydroxides. Such a conclusion is supported by SEM-EDX (Scanning Electron Microscopy coupled to Energy Dispersive X-ray spectroscopy) observations from a real calcareous agricultural soil and laboratory experiments simulating the co-precipitation of Al and Cu from citrate complexes in calcareous soils. More specifically, by employing a SEM-EDX Automated Particle Analysis (APA) method specifically implemented for soil analysis, a large increase in the number of microscopic Al oxide particles was observed in a calcareous soil treated with 1mM citrate. Surprisingly approx. 40% of them contained Cu as the only other major constituent. Furthermore, the environmental conditions by which these phenomena may occur are also examined and mathematically described. In conclusion citrate exudation, which most plants adopt as a strategy to mobilize nutrients from the soil, under certain environmental conditions, like those encountered in calcareous soils, can reduce the availability of important elements for plant nutrition such as Cu.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
