Copper isotopic fractionation in soil-rice systems near the multimetallic mine: Implication for Cu uptake and transport mechanisms

This study analyzed Cu isotopes in rice plants (root, straw, husk, and grains), Fe plaque and rhizosphere soil collected at sites downstream of the Dabaoshan sulfide mine in South China. Cu isotope fractionation from soil to the available pool (Δ^65/63Cu_{available pool-soil} = 0.24‰ to 0.64‰) was positively correlated with soil pH. Rice plants were enriched in light Cu isotopes under low available Cu supply (Δ^65/63Cu_{whole plant-available pool} = 0.53‰ to 0.33‰), whereas the opposite result occurred under high available supply. Cu isotopes in straw were identical to or lighter than those in roots under high available Cu supply (Δ^65/63Cu_straws-roots = 0.13‰ to 0.06‰), but contrasting results were obtained for low available Cu supply. Results confirm that soil pH strongly influences soil Cu desorption from soil to the available pool, which in turn affects uptake and transport pathways in the available pool-Fe plaque/root-straw continuum. Fe plaque serves as a source and sink of Cu in rice plants and rice plants acquire Cu through high- and low-affinity pathways under low and high soil-available Cu, respectively. Cu immobilization in the cell wall helps reduce the upward transport of excess Cu. These findings highlight the potential of Cu isotope fractionation for determining suitable levels of available Cu to ensure rice grain quality.