The role of pores in micro-zone distribution of Cd in a tropical paddy soil: Results from X-ray computed tomography combined energy dispersive spectroscopy analysis

Abstract

Accurate description of Cd micro-zone distribution and accumulation is the prerequisite for revealing Cd transfer and transformation processes. However, to date, the role of soil pores in the Cd micro-zone distribution characteristics in undisturbed soil is still unclear. In this study, the obvious heterogeneous distribution of Cd in and around the soil pores at the cross-sectional surface of the tropical undisturbed topsoil was visualized by the combination of X-ray micro-computed tomography and scanning electron microscope-energy dispersive spectroscopy. For both the air space and water-holding pores, the micro-zone distribution characteristics of Cd around the pores were dominated by pore sizes. For macropores and mesopores, Cd preferred to distribute in the micro-zone within 167.5–335 µm from pores. But for micropores, the highest content percentage of Cd was exhibited in the micro-zone within 67–167.5 µm from pores. The random forest model revealed that the occurrence of Fe (13.83%) and P (13.59%) contributed most to Cd micro-zone distribution around air space pores. While for water-holding pores, Fe occurrence (18.30%) contributed more significantly than P (11.92%) to Cd micro-zone distribution. Our study provided new insights into Cd retention mechanism, which is help for accessing Cd migration and transformation.