Effect of organic matter on fractional transformation of Zn in black soils under freeze-thaw cycle

Freeze-thaw cycle (FTC) has a strong influence on fractional transformation of the heavy metal Zn in soils with different contents of organic matter. For 8 FTC cycles, every one of which is 24 h, in a black soil with 150 mg Zn per kg soil, increase in organic matter leads to a decrease in exchangeable form of Zn and Fe-Mn oxide form of Zn, and an increase in organic form of Zn and residual form of Zn. While, for no FTC, exchangeable form of Zn and Fe-Mn oxide form of Zn decrease gradually, and organic form of Zn increases gradually, residual form of Zn has no distinct change, with the increase of organic matter content. As organic matter content increases in the soil, FTC further improves decreases in bio-available form of Zn. Fitting organic matter and contents of different forms of Zn with or without FTC, Zn in all forms exhibits a quadratic polynomial function correlation with organic matter content with correlation coefficients of 0.664~0.995.