Effects Of Freeze-thawing Cycles On Brown Soil Aggre-Gates And Cadmium Transformations Within Particle Size Fractions

The transformation of heavy metal cadmium directly affects its bioavailability and tox-icity in soil,and it can also partly determine the true soil pollution.Seasonal freeze-thawing cycles is a common climatic phenomenon in northern China,and it can directly influence the soil structure and particle size.Thereby,it can indirectly affect the transformation of cadmi-um in soil.In this thesis,the brown soil-a typical soil in Shenyang area was used in this study.Freeze-thawing cycles treatments were conducted under different freezing temperatures and different soil moisture contents.Then,the particle sizes of aggregates and transformation of cadmium were determined to reveal the effect of freeze-thawing cycles on the aggregate sta-bility and the transformation of cadmium in brown soil,the main conclusions are as follows:(1)After the freeze-thawing cycles of brown soil,the amount of pH dependent charge was significantly decreased(P<0.05),but when the freezing temperature was lower than-20 ℃,the pH dependent charge amount was not obvious.The freeze-thawing cycles in-creased organic matter content in brown soil(P<0.05),but the freezing temperature had not significant effect on the organic matter content of brown soil.When the water content of brown soil was more than 40%of field water capacity,the effect of water content on organic matter content was not significant.(2)After different freezing temperature treatment of brown soil,>2 mm aggregates had a significant increase in the proportion of particles(P<0.05);1-2 mm,0.25-0.5 mm,and<0.25 mm aggregates had a significant decrease in the proportion of particles(P<0.05);0.5-1 mm aggregates did not change significantly.When the freezing temperature was-20 ℃>2 mm aggregates proportion increased with the increasing of soil water content,0.5-1 mm and 0.25-0.5 mm aggregates proportion decreased with the increasing soil water content,1-2 mm aggregates proportion was greatest when soil water content was 40%of field water capacity,and<0.25 mm aggregates proportion was greatest when soil water content was 30%of field water capacity.(3)After the freeze-thawing cycles of brown soil,the results of scanning electron mi-croscopy showed that there was a phenomenon of agglomeration in the soil,which increased the agglomeration ability between soil particles.When the water content of brown soil reached 60%of field water capacity,the stability of brown soil aggregates reached the maxi-mum.However,different freezing temperature had no significant effect on the stability of aggregates.(4)Compared with unfrozen soil,the content of cadmium in the aggregates of>2 mm,1-2 mm and 0.5-1 mm were significantly increased after freeze-thawing cycles under differ-ent freezing temperatures(P<0.05).The content of cadmium in the aggregates of 0.25-0.5 mm and<0.25 mm were significantly decreased after freeze-thawing cycles under different freezing temperatures(P<0.05).Freeze-thawing cycles promoted the enrichment of cadmium in soil aggregates and reduced the environmental risk of cadmium.(5)Compared with unfrozen soil,the contents of exchangeable Cd and organic bound Cd in brown soil were decreased after freeze-thawing treatment,and the content of Fe and Mn oxide bound form of Cd was increased.With the decrease of freezing temperature,the content of exchangeable Cd increased first and then decreased,and the content of Fe and Mn oxide bound form of Cd first decreased and then increased,and the residual Cd content de-creased.There was no significant change in the residual Cd after the freezing temperature lower than-20 ℃.(6)The effect of freeze-thawing cycles on the transformation of cadmium in brown soil aggregates was significant(P<0.05).Compared with unfrozen soil,the exchangeable Cd content and organic bound Cd decreased with the different freezing temperatures in the dif-ferent aggregates,and the content of Fe and Mn oxide bound form Cd and the residual Cd increased.