The effect of freezing-thawing cycle on the combined forms and availability of the adsorbed Cd~(2+) in soil

The effect of the freezing and thawing frequency and soil water content on the combined forms and availability of absorbed Cd of brown soil collected from the Northeast China was investigated.20 mg · L-1 Cd was added to the soil and the soil was incubated at-30 ℃ and 20 ℃ for 24 h,respectively.Soil samples of different water content(10%,40%,70%,100% and 120% of the field capacity) were subsequently refrozen and thawed for 0,1,3,6,9 cycles.The different combined forms of Cd2+ were determined with sequential extraction method.The result showed that the order of the ratio of different combined forms of Cd to the adsorbed Cd2+ was exchangeable form residual form bound to carbonates form bound to Fe-Mn oxides form bound to organic matter form.The ratios of different combined forms of Cd to the adsorbed Cd2+ varied along with the change of freezing-thawing cycle frequency and soil water content.The effect of the freezing and thawing times on exchangeable Cd,bound to carbonates Cd,bound to Fe-Mn oxides Cd and residual Cd was significant,while the effect on bound to organic matter Cd was insignificant.The effect of soil water content on exchangeable Cd,bound to carbonates Cd,bound to Fe-Mn oxides Cd was significant,while the effect on bound to organic matter Cd and residual Cd was insignificant.The absorbed Cd in the sampled soil possessed very strong availability,and the availability index(K) and mobility index(M) were 65.9638% and 62.9851%,respectively.The K and M increased firstly,then decreased and finally reached a plateau with the increasing freezing and thawing frequency,which indicated that the soil had buffer capacity in some extent.