Abstract
It is known that low molecular weight organic molecules with complexing capability (eg carboxylic acids, aromatic compounds, aminoacids, phenolics, etc.), released into the rhizosphere, can interact with soil minerals, causing structure destabilization and desorption processes from mineral surfaces. As a consequence, the solubility of many elements is enhanced thus increasing their availability for plant uptake. It has been demonstrated that one of the most effective organic ligands released by plants is citrate, which can be particularly efficient in increasing mineral nutrients mobility (especially Fe) in alkaline soils. Experimental evidence showed that in a calcareous soil (pH 8.2, CaCO3 61.8% w/w, Corg 0.86% w/w) citrate (1 mM) was not as effective as for Al, Fe and Mn in solubilizing Cu from soil. In addition, Automated Particle Analyses (APA) using Scanning Electron Microscopy coupled to Energy Dispersive X-ray spectroscopy (SEM-EDX) and Cluster Analysis evidenced the formation of a large number of aluminum oxide (AlOx) particles, about 50% of which contained a significant amount of Cu. The aim of this research is to demonstrate that under peculiar soil conditions (ie alkaline pH, high microbial activity) typically found in the rhizosphere of plants grown in calcareous soils, citrate efficacy in increasing Cu mobility is strongly reduced by coprecipitation phenomena within Al (hydr) oxides.
To do this we simulated the process under experimental conditions similar to those of the real soil (100% calcite, pH 8.2,[Al]= 17 mg/l,[Cu]= 7 mg/l), gradually reducing the concentration of citrate and thus mimicking the action of soil microorganisms feeding on citrate (as also observed experimentally).