Root-induced changes in pH and dissolved organic matter binding capacity affect copper dynamic speciation in the rhizosphere

Abstract

Due to only few experimental evidences, the importance of root-induced alteration of metal dynamic speciation in the rhizosphere in the determination of metal bioavailability to plants is still a matter for debate. The present study thus investigated how root-induced changes in pH and dissolved organic matters (DOM) altered copper (Cu) dynamic speciation in the rhizosphere of durum wheat (Triticum turgidum durum L.). Plants were exposed to a Cu-contaminated soil previously alkalised by liming to cover soil pH values ranging from 4.8 to 7.5. A range of analytical techniques was deployed on soil exposed (i.e. in the rhizosphere) or not (i.e. in the bulk soil) to plant roots, including the measurement and the modelling (using the Humic Ion-Binding Model VI) of Cu2+ activity, the measurement of labile Cu concentration and Cu lability by Differential Pulse Anodic Stripping Voltammetry (DPASV) and Diffusive Gradients in Thin films (DGT). Due to root-induced alkalisation, pH reached about 7.3 in the rhizosphere whatever the initial bulk soil pH. Compared to the most acidic bulk soil (pH _ 4.8), Cu2+ activity decreased by three orders of magnitude in the rhizosphere while DPASV-Cu concentration decreased by 6-fold. DOM became the key driver of Cu dynamic speciation in the rhizosphere, where roots induced up to an order of magnitude increase in DOM concentration compared to bulk soils. This resulted in an increase in labile-Cu (both DPASV and DGT) concentrations, in spite of a decrease in Cu2+ activity. Model VI calculations supported a decrease in DOM binding capacity towards Cu in the rhizosphere. DPASV measurements unequivocally demonstrated that the increase in Cu lability in the rhizosphere solution can be attributed to a greater lability of organically-bound Cu. Collectively, our data introduce a consistent picture of root-induced changes of Cu dynamic speciation in the rhizosphere that were notably related to substantial alterations of DOM binding capacity. (Résumé d'auteur

The effect of excess copper on growth and physiology of important food crops: a review

“…Moreover, Cu bioavailability is more influenced by rhizosphere pH than the bulk soil pH in an acidic Cucontaminated soils (Chaignon et al 2009). Dissolved organic matter (DOM) and rhizosphere pH can alter Cu dynamic speciation (Michaud et al 2007;Chaignon et al 2009;Bravin et al 2012). The plant roots also play vital role in bioavailability of Cu, but it depends upon plant species and availability of Cu in soils (Hinsinger 1998;Chaignon et al 2002;Yruela 2009).…”

“…Copper was first identified as a plant nutrient in the 1930s (Sommer 1931;Arnon and Stout 1939). Copper solubility in soil is greatly dependent on soil pH and dissolved organic matter (DOM) content (Bravin et al 2012) and becomes readily available at a pH below 6 (Adriano 2001;Brun et al 2001). The requirement of Cu for healthy plant growth and development varies with plant species and cultivars (Mantovi et al 2003).…”

Influence of Soil Chemistry and Plant Physiology in the Phytoremediation of Cu, Mn, and Zn

“…As summarized in Figure 2, bioavailability of trace metals depends on environmental conditions such as oxidation state, moisture and temperature, soil properties such as soil organic matter (SOM) and soil pH, and biological activity promoted by microorganisms (Bravin et al, 2012;Gadd, 2010;Yang et al, 2012).…”

“…Most common forms of Cu in soil solutions are soluble organic chelates of this metal (Kasmaei and Fekri, 2012;van Schaik et al, 2010). The two major parameters that influence the variability of total Cu concentration as well as the available Cu concentration in soil are the CEC and SOM (Bravin et al, 2012;Vega et al, 2010;Wu et al, 2010). Cu mobility is especially reduced in the presence of large mineral colloids in the clay fraction, as the presence of Fe and Mn oxides enhances Cu fixation to the soil components (Komarek et al, 2008).…”

Potential for Phytoextraction of Cu by Sesamum indicum L. and Cyamopsis tetragonoloba L.: A Green Solution to Decontaminate Soil

“…Organic carbon (OC) content of the soil was 2.20%, while organic matter (OM) of the tested soil was found to be 3.79%. Among soil properties, mobility and bioavailability of Cu strongly depend on pH of soil and organic matter (OM) contents present in soil (Bravin et al 2012). Soil pH has direct affect on the solubility of heavy metals together in soil as well as soil solution.…”