Physicochemical Properties Of Topsoil Of Loess During Freeze-thaw Period

Freeze-thaw cycles are the process of freezing and thawing of soil or rock in a certain deepth range due to one day,one year and years of climate change.Freeze-thaw cycles are widely distributed on the global surface land,and is also a natural phenomenon in middle,high latitudes and mountainous areas.Nowadays,global climate change is one of the most serious challenges facing mankind,and the change of climate directly affects the change of freeze-thaw characteristics,causing changes in particle granularity and finaly in soil physicochemical properties.Other phase transitions and coupling effects in freeze-thaw soil also lead to changes in the response of soil physicochemical properties,thus affecting the chemical reaction of nutrients and the environmental changes in the soil.However,studies on impact of freeze-thaw cycles on soil physicochemical properties of seasonal frozen areas like hilly-gully loess plateau lag behind slightly.In view of this,this paper takes topsoil of wheatland,grassland and bare land in the fifth sub-region of hilly-gully loess plateau as the research object.The physicochemical properties of three land use types and there difference during freeze-thaw period are studied by field sampling and laboratory experiments.And the correlation and variation characteristics of soil physicochemical properties during freezethaw period are analyzed.The main conclusions are given as follows:1)Several indexes of soil physicochemical properties are significantly correlated in different degree.> 0.25 mm WSA(Water Stable Aggregates)and total mass of WSA are correlated with organic matter(P < 0.01).Field capacity of three land use types both has a linear correlation with bulk density and porosity(P < 0.05),the main factors affecting field capacity are bulk density and porosity,and the bulk density has a greater impact on the field capacity.Soil saturated hydraulic conductivity is significantly correlated with bulk density,porosity,field capacity and organic matter(P<0.05).2)The change characteristics of soil physicochemical properties during freeze-thaw period are classified into three types by cluster analysis.Clay,MWD(Mean Weight Diameter),GMD(Geometric Mean Diameter),fractal dimension,bulk density,saturated hydraulic conductivity and organic matter as a category;> 0.25 mm WSA,total mass of WSA,sand and field capacity as a category;silt and porosity can be clustered into one type.The results of principal component analysis show that the contribution rate of water physical factors,soil water stable factors,soil pellet factors and soil particle factors is 23.000%,22.866%,21.034% and 19.352% respectively,which reflects four main factors causing influence on soil environment during freeze-thaw period.3)Variation characteristics of 13 types of soil physicochemical properties can be divided into 3 types after comprehensive correlation analysis,clustering analysis and principal component analysis results.Bulk density,porosity,field capacity and saturated hydraulic conductivity are classified into one category;> 0.25 mm WSA,total mass of WSA,MWD,GMD,fractal dimension and organic matter are the second category;sand,silt and clay are the third category.