Rhizosphere effects on cadmium fractionation in contaminated soils: a comparative study of ultisols and alfisols

Elucidating the impact of rhizosphere interactions on soil heavy metal accumulation and speciation is essential for accurately assessing heavy metal bioavailability and remediating contaminated soils. The rhizosphere effects of wheat on soil organic matter (SOM) content, pH levels, and Cd fractions in two Cd-contaminated soils, Ultisols (red soil) and Alfisols (yellow cinnamon soil), were studied through pot experiments using root bags. The results indicated that the rhizosphere of the red soil had a higher SOM content but a lower pH than the bulk soil. Conversely, the yellow cinnamon soil exhibited lower SOM and higher pH in the rhizosphere relative to the bulk soil. Meanwhile, Tessier sequential extraction identified Fe/Mn oxide-bound Cd (Fe/Mn-Cd; rhizosphere: 23.2%-50.7%, bulk: 35.9%-55.1%) and exchangeable Cd (Ex-Cd; rhizosphere: 31.6%-40.3%, bulk: 24.6%-33.7%) as the dominant fractions in the red soil. Similarly, the yellow cinnamon soil exhibited predominant fractions of Fe/Mn-Cd (rhizosphere: 22.4%-27.3%, bulk: 48.5%-55.9%), Ex-Cd (rhizosphere: 18.3%-49.6%, bulk: 17.5%-20.4%), and carbonate-bound Cd (Ca-Cd; rhizosphere: 8.9%-37.2%, bulk: 8.9%-20.5%). Furthermore, wheat roots tended to enhance the accumulation of Ca-Cd and residual Cd (Re-Cd) fractions, particularly under high Cd stress. Conversion sources varied by soil type: in the red soil, the primary sources were Ex-Cd and Fe/Mn-Cd, with rhizosphere processes dominated by SOM and bulk soil dynamics regulated by SOM and pH. In the yellow cinnamon soil, the sources included Ex-Cd, Fe/Mn-Cd, and OM-Cd, with rhizosphere transformations governed by SOM and bulk soil mechanisms controlled by pH. The findings are crucial for assessing Cd contamination in soil-plant environments and developing tailored phytostabilization approaches for Cd in specific soil types.