Effects Of Freeze-thaw Cycles And Initial Soil Water Content On Soil Shear Stress In Chinese Mollisol Region

Seasonal freeze-thaw cycles with different initial soil water contents have great impacts on soil shear strength.During the past decades,there were many studies conducted on the effects of the freeze-thaw cycle on soil anti-erosivity such as soil shear stress in China and in the World.However,the effects of a freeze-thaw cycle on hillslope water erosion mechanism are still weak,especially,there are less information related to the effects of freeze-thaw cycles on soil shear stress in the croplands in Chinese Mollisol region.Thus,this study collects tested soils from Binxian(BX)and Keshan(KS)counties in the Chinese Mollisol region to measure soil shear stress by using a direct shear test.The experimental treatments include five levels of freezethaw cycles(0,1,3,5,and 7),three levels of initial soil water contents(16.5%,24.8%,and 33.0%),and remolded and undisturbed soil samples from KS and BX sites.The main objective of this study is to investigate the influences of freeze-thaw cycles and initial soil water contents on soil shear stress and to compare differences of soil shear stress between remolded and undisturbed soils.The results can reveal the mechanisms of freeze-thaw effects on hillslope water erosion and provide a scientific basis for protecting Chinese Mollisol resources.The main results are as follows:(1)Soil shear stress,soil cohesion,and soil internal friction angle for both BX and KS tested soils declines with an increase of freeze-thaw cycle and initial soil water content.For BX tested soil,soil shear stress decreased by 8.6% to 9.2% and 13.1 to 18.0% when freeze-thaw shifts from 0 to 7 freeze-thaw cycles under 16.5%,24.8% and 33.0% of initial soil water contents,respectively.For KS soil,it decreased 15.1%,18.0% and 13.1%,respectively.In addition,soil shear stress decreased linearly with the increase of initial soil water content.Comparison between 16.5% and 33.0% of initial soil water content of BX soil shear stress showed that declined by 8.4% to 11.3 and for KS tested soil declined by 10.7% to 16.9%.Soil shear stress for the BX tested soils was lower than that for KS tested soils and the differences in soil shear stress between BX and KS soils decrease with an increase of initial soil water content.Soil cohesion decreased with an increase of freeze-thaw cycles for both tested soils,then soil cohesion stabilizes gradually after 5 freeze-thaw cycles.BX soil cohesion decreases by 70.9%,71.1%,and 65.9%,respectively,under five freeze-thaw cycles with three initial soil water content of 16.5%,24.8%,and 33.0%,and KS soil cohesion decreases by 56.6%,61.7%,and 71.9%,respectively.Compared with BX soil with freeze-thaw treatment,soil cohesion for KS soil is 0.8 to 2.2 times higher when freeze-thaw cycles shifts from 1 to 7 times under three initial soil water contents.Soil internal friction angle showed decreasing or increasing trends with increasing of freeze-thaw cycles and initial soil water content for both tested soils.(2)Soil shear stress for undisturbed soils at both BX and KS sites is lower than that for remolded soil.Under 24.8% and 33.0% initial soil water content,when freeze-thaw cycles increases from 0 to 5,soil shear stress for BX undisturbed soil decreased by 23.8% to 26.11% and 17.6%-21.3% respectively,compared with remolded soil;and for KS undisturbed soil,soil shear stress decreased by 0.2% to 11.0% and 4.2% to 9.7%,respectively.The main reason for that soil shear stress under remolded soil is larger than that under undisturbed soil is due to remold soil samples for remolded soil.(3)The initial soil water content and the freeze-thaw cycle are essential factors affecting soil shear stress.When the initial soil water content and freeze-thaw cycles are increased,soil shear stress decreased for both remolded and undisturbed soils.These results proved the effects of the freeze-thaw cycle and initial soil water content on soil shear stress in the Chinese Mollisol region,thus it is important to quantify mechanisms of freeze-thaw cycle influences on hillslope water erosion in the future work.