Purpose
Contamination of heavy metals in soil and its subsequent accumulation along the food chain is a potential risk to human health. Cu speciation in soil–plant system, particularly on the availability to plant roots, has obtained great attention. X-ray absorption near-edge structure spectroscopy (XANES) provides information about the bonding of Cu soil components at the molecular scale. In paddy soils, changes of redox conditions led to microbially mediated sulfur transformation, thus affecting heavy metal behavior. The objective of this work was to investigate how sulfur transformation in a paddy soil affected Cu biogeochemical processes.
Materials and methods
The Cu and sulfur species and their relationship in rice–soil system were investigated under flooded condition. The speciation of sulfur and copper in rice rhizosphere and bulk soil was investigated using integrated approaches including sequential extraction and XANES.
Results and discussion
Cu speciation exhibited some differences in rhizosphere and bulk soil of rice. In flooded paddy soil, most Cu in the rhizosphere existed as Cu (II), whereas part of Cu transformed to Cu (I) in the bulk soil. Sulfur XANES showed the presence of multiple both oxidized and reduced forms of sulfur in studied soil samples, with more oxidized sulfur in the rhizosphere than in the bulk soil. Copper and sulfur speciation changed depending on redox conditions. Changes in redox potential and microbial action shifted the sulfur oxidation and reduction reaction and affected the Cu speciation. Combined action of organisms maintained Cu homeostasis through cation binding to bioactive molecules. With higher Eh in rice rhizosphere, transformation of sulfur and organic compounds together contributed to more soluble and exchangeable Cu. Cu bond to sulfur containing groups and biomineralization by microorganisms could be defenses against toxic copper.
Conclusions
Our findings implied that Cu existed mainly as Cu (II) in rice rhizophere and part of Cu transformed to Cu (I) in anoxic bulk soil. With higher Eh in rice rhizosphere, transformation of sulfur and organic compounds together contributed to more soluble and exchangeable Cu. Combined action of organisms maintained Cu homeostasis through cation binding to bioactive molecules. Our results indicated the important role of sulfur in the transformation of Cu. Due to the complicated processes in soil, future work dedicating to the role of microbes is needed.