[Objective]To elucidate the effects of freeze-thaw cycles and initial water content on soil structural stability,rare earth oxides(REOs)were used as tracers to separate soil aggregates formation and breakdown processes.[Method]REOs-labelled soil was reformed and investigated herein.Two initial water contents(50％ field water holding capacity(T50)vs.100％ field water holding capacity(T100))and five freeze-thaw cycles(0,3,6,12 and 20 cycles)were involved in the simulation experiments.Soil aggregates distribution,mean weight diameter(MWD),and the aggregate transformation processes were measured accordingly.[Result]The results showed that freeze-thaw cycles significantly reduced MWD,＞0.25 mm aggregates and ＜0.053 mm aggregates proportions,but increased the contents of 0.25～0.053 mm aggregates under the same initial water content.After 6 freeze-thaw cycles,MWD was significantly higher under T50 compared with that under T100(P＜0.05),but there were no significant differences between the contents of 5～2 mm and ＜0.25 mm aggregates.Except for 5～2 mm aggregates,more intensive transformation between neighboring size aggregates was observed during the whole simulation experiments.Under the same freeze-thaw cycles,the transformation proportions from 5～2 mm to 0.25～0.053 mm aggregate were significantly higher under T100 compared with T50 treatment(P＜0.05).The freeze-thaw cycles promoted the breakdown of ＞0.25 mm aggregates and aggregation of ＜0.053 mm aggregales,thas leading to the accumulation of 0.25～0.053 mm aggregales both under T50 and T100 treatments.Also,MWD was significantly positively correlated with the relative formation of soil aggregates and negatively related with the relative breakdown of soil aggregates(P＜0.05).Soil aggregates turnover time remarkably increased with the freeze-thaw cycles(P＜0.05).Aggregate turnover time of ＞0.25 mm aggregates was higher than that of ＜0.25 mm aggregates.Comparatively,the aggregate turnover time was significantly higher under T100 than that under T50 with the same freeze-thaw cycle(P＜0.05).[Conclusion]The freeze-thaw cycles and soil initial water content significantly affect the soil structural stability by laying affects on aggregate formation and breakdown processes.The results will provide a theoretical basis for further exploration of the structural changes in Mollisols under freeze-thaw cycles.
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