Influence Of Freeze Thaw Cycles On Structure Of Top Black Soil And Brown Soil Investigated By CT Scanning

Black and brown soil are main dryland farming in northeastern China,to carry out its mechanism of freeze-thaw erosion will help to prevent the freezing and thawing of soil,and to develop reasonable soil and water conservation measures play a key role.In this paper,black and brown soil were used as the research objects.The factors of initial soil moisture content and freezing and thawing cycles were selected by the method of indoor simulated freezing and thawing cycle and CT scanning.Through the quantitative analysis of CT number and macropore characteristics,the effect of freeze-thaw on the content of water-stable aggregates was evaluated to evaluate the erodibility of black and brown soil,conclusions followed:(1)In the aspect of CT number:The interaction between the initial water content?the initial water content and the freeze-thaw cycle both have significant effects on CT number of black and brown soil.The effect of freeze-thaw cycles on CT number of the black soil mainly accord in 0 to 5 cycles.The effect of freeze-thaw cycles on CT number of the brown soil mainly accord after 5 cycles.When the initial water content was close to the withered coefficient,CT number of the soil section did not change significantly,and the effect of freezing and thawing on the soil structure was not significant.When the initial soil water content increased to the field water holding capacity,the CT number of the soil section decreased significantly.When the initial water content continues to increase to near saturation and supersaturation,the damage variable is negative,and the freezing and thawing effect strengthens the connection between the soil particles.When the initial water content of black soil is 40%,the effect of freeze-thaw cycle on the structure damage at 5 cm is the largest.When the initial water content of brown soil is 30%,the effect of freeze-thaw cycle on the structure damage is 6cm.(2)In the aspect of soil macropore characteristics:the change of macropores in black soil during 0 to 5 freeze-thawing cycles is more intense.To brown soil,5 to 10 freeze-thawing cycles more intense.When the initial water content is close to the wilting coefficient,the number and area ofmacropores do not change significantly,and the porosity does not change obviously.When the initial water content is close to field water holding capacity,the number and area of macropores are significant.The number and area of macropores in the section decrease,and the porosity decreases.When water content is near saturation or supersaturation,the number and area of the macropores are decreased.The change of soil macropores is consistent with the trend of soil CT.(3)In the aspect of water stability aggregates?The effect of freezing and thawing on the water stability of black soil aggregates was more intense in 0 to 5 freeze-thaw cycles,and more moderate in 5 to 10 cycles.In the case of brown soil,after 5 cycles gradually reached a significant level.The water-stable aggregates in the black soil and brown soils with different initial moisture content were different.The freeze-thaw effect of the aggregates was broken down,and the water-stable aggregates in the small-particle group increased.The initial water content is close to the wilting coefficient of the soil,freeze-thawing can reduce the content of water-stable aggregates,but can not achieve significant differences;with the initial moisture content gradually increased to close to the field of water holding capacity,water stability aggregate content.The higher the water content,the stronger the damage to the water-stable aggregates.When the initial water content exceeds the near-saturation or supersaturation of the field,the total aggregates increased,and the freeze-thaw effect promotes the increase in aggregates.(4)In the aspect of relationship between aggregates,CT number and macropores:When changes of aggregates is small,it is difficult to affect the CT number.When the change of aggregates is increased to a certain extent,the CT number will be affected.There was a negative correlation between CT number and large porosity of the section,and reached a significant correlation level when the water content was close to field water holding capacity and 60%water content of black soil.When the initial water content was less than field water holding capacity,the large porosity and particle size were>5mm,5?2mm,2?1mm,the initial water content was higher than that of field water holding capacity.The porosity was positively correlated with>5mm and 5?2mm particle size aggregates.And the negative correlation between 2?1mm,1?0.5mm and 0.5?0.25mm particle size.It is inferred that the increase of CT number in near saturation and supersaturated water content is mainly due to the large particles breaking into small particles aggregates.Small particles of aggregates in the external pressure and mutual bonding under the action of the formation caused slightly larger size of the aggregates.