Impacts of natural field freeze–thaw process on the release kinetics of cadmium in black soil: Soil aggregate turnover perspective

Seasonal freeze–thaw action has been shown to influence the behaviors of heavy metals in soil by changing soil aggregate formation, stabilization, and breakdown. Using rare earth oxides tracing method, this study conducted in-situ seasonal freeze–thaw experiments to examine the links between aggregate turnover behaviors and cadmium (Cd) release kinetics in black soil. Higher stability and longer turnover time of soil aggregates after spring freeze–thaw process than those just experienced autumn freeze–thaw process were found. Changes of the Cd distribution in different aggregates during the freeze–thaw process were mainly driven by soil aggregate transformations. Autumn freeze–thaw events increased the exchangeable Cd amounts in bulk soil and ≥ 0.25 mm aggregates, while no difference was found for the soils subjected to spring freeze–thaw events. The freeze–thaw process decreased the amounts of Cd released from soil, and the lowest amounts were found in the spring snow-removed treatment. Redundancy analysis indicated that the exchangeable, reducible, and residual Cd fractions accounted for most of the variations in Cd release. This study is the first to apply rare earth oxides as an approach to uncover the role of soil aggregate turnover dynamics in explaining the environmental behaviors of heavy metals in seasonal freeze–thaw regions.