A Simulation Study On The Effect Of Freeze-Thaw Alternation On The Soil Erodibility Of Biological Crusts In The Loess Plateau

Biological crust is the widely distributed ground cover.When it comes to winter and spring,Biological crust is affected by the alternation of freeze-thaw,the physical and chemical properties,hydrological characteristics and soil erodibility of the soil covered by the biological crust（BCS）change significantly and are significantly different from the bare soil,which affect the soil erodibility assessment and soil erosion control in the area.Due to the existence of pioneer species such as algal and moss,the physical and chemical properties of the soil have been significantly improved,and the interaction between the alternation of freeze-thaw and the crust has caused changes in soil erodibility.This study takes the windy sand soil covered by biological crusts in typical seasonal freeze-thaw areas as the research object.Through field sampling,indoor cultivation,and indoor simulation experiments,the effect of the alternation of freeze-thaw on the physical and chemical properties of the biological crust-covered soil is systematically studied.The EPIC model is used to calculate the soil erodibility K value and build an empirical model to estimate the soil erodibility K value under freeze-thaw conditions.The soil separation rate is obtained through the soil erosion separation test.The WEPP model is used to estimate the soil water erodibility value and aims to explore the influence mechanism of freeze-thaw alternations on the soil erodibility of BSC,so as to provide a theoretical basis for soil erosion control during the unfreezing period time in the water erosion area of the Northern Loess Plateau,and the main conclusions obtained are as follows:（1）With the increase of the initial soil moisture content,the bulk density of the algal crust layer under freeze-thaw conditions increases;the alternation of freezing and thawing makes the saturated hydraulic conductivity,soil cohesive force,hardness,organic matter,nitrogen content,and biomass of the algal crust layer etal.significantly reduced,and the particle composition of the soil changes significantly.The depth of the soil and the initial water content affect the accumulated temperature of the soil,thereby affecting the unfreezing rate and the intensity of the freeze-thaw action.The number of freeze-thaw cycles（p<0.05）and the initial soil moisture content（p<0.01）significantly affect the bulk density of crust layer and the underlying soil,with the increase of soil initial water content under freeze-thaw conditions,the bulk density of algal crusts increased by an average of 4.58%,and the changes in clay particles of algal or moss covered soil increased by 36.0%and 26.4%separately on average compared to bare soil.The change of particle composition of moss crusts under freeze-thaw conditions is fewer than that of the algal crust layer;when the water content is 8%,the change of the particles of the crust covered soil is the most obvious.The alternation of freezing and thawing further reduced the saturated hydraulic conductivity of BCS,when the soil have frozen and thawed for 10 times,the saturated hydraulic conductivity of bare soil is 10.13 times that of algal crust covered soil.Soil aggregates of moss crust covered soil are more sensitive to freeze-thaw alternation than algal crust soil aggregates and are significantly affected by moss coverage（p<0.05）;as the coverage of algal and moss increases,the degree of variation of soil structure index GSSI under freeze-thaw conditions is lower,the5～10cm soil is tend to form a dense layer that cannot thaw after repeated freeze-thaw.Under freeze-thaw conditions,the algal crust coverage,soil initial water content,soil structure and the physical and chemical properties of the algal crust have a strong correlation,while the strong correlation factors of the physic and chemstry properties of the moss-covered soil include moss coverage,the number of freeze-thaw cycles,Organic matter and bulk density.The content of chlorophyll and organic matter in the crust layer is highly sensitive to freezing and thawing,and the temperature,moisture content and water-stable aggregates of the underlying soil are highly sensitive to freezing and thawing.（2）The erodibility factor K value of biological crust covered soil is significantly affected by the number of freeze-thaw alternation times,initial soil moisture content,crust type and crust coverage.Under freeze-thaw conditions,the main influencing factors of soil erodibility of biological crusts are soil particle composition（texture）,freeze-thaw alternation times,types of biological crusts,soil organic matter,biomass,total nitrogen content,and soil structure.Affected by freezing and thawing,the erodibility K value of algal crust soil increased by2.60%on average compared with that before freezing and thawing,and the erodibility K value of soil covered by moss crust increased by 4.69%compared with that before.Under freeze-thaw conditions,the erodibility factor K value of algal crust soil increased by 0.72%compared with moss covered soil,which is significantly different（p<0.05）.With the increase of algal crust coverage,the soil erodibility factor K value of algal crust coverage decreased by 1.40%;while with the increase of coverage of moss crust,the soil erodibility K value increased by 0.84%under freeze-thaw conditions,The average K value of the erodibility factor of algal crust soil[0.1671 t·ha·h/（ha·MJ·mm）]is 1.01 times of moss crust soil[0.1658 t·ha·h/（ha·MJ·mm）].As the initial soil water content increases,when the soil erodibility is similar,the soil with high water content underwent more freeze-thaw cycles.（3）Before and after freeze-thaw,the relationship between the erodibility of the algal crust covered soil was:K_冻融=1.047K_未冻融。When the algal crust covered soil was frozen-thawed more than 15 times,the K value gradually approaches the stable value at 1.730[t·ha·h/（ha·MJ·mm）],the development of crust weakens the erosion and corrode effect of runoff on the soil,the soil erodibility factor K value of BSC has significant relationship between silt,organic matter,Total nitrogen,biomass,aggregates,cohesive force and structure index.Freez-thaw and the enrichment of biological crusts both affect the migration of soil clay and silt between different soil layers,thereby significantly affecting the K value of soil erodibility.（4）Freeze-thaw effect significantly increase the soil separation rate of crust-covered soil.The soil separation rate is mainly affected by slope,followed by runoff.Freeze-thaw alternation times（FT）,structural index（GSSI）,crust coverage（C）,clay content（CLA）,cohesive force（T）,bulk density（ρ_B）,biomass（B）,hardness（H）)are the key factor which affecting the water erodibility of BCS during freeze-thaw.Under freeze-thaw conditions,the separation rate of algal crust soil is 1.46 times that of moss covered soil.When the erosion slope is greater than 15°,the soil separation rate increases significantly（p<0.01）.Under a certain slope,with the increase of runoff,the soil separation rate increases accordingly.After the freeze-thaw cycle,the water erodibility Kτof the soil covered with algal crust is 0.361s/m,the Kτof moss crust covered soil is 0.215s/m,and the Kτof soil with algal crust is 1.68 times of that of moss crust covered soil,the covering effect of moss crust layer effectively weakens the separation ability of water flow to the soil.Under freeze-thaw conditions,the fitting relationship between the water erodibility parameter Kτand the erodibility factor of algal crust soil is:K_τ=0.11e^0.017FTT^-0.648GSSI^-0.059C^-0.053CLA^-0.079（R²=0.464,NSE=0.346）;Moss crust covered soil:Kτ=0.02e^0.002FTρ_B^0.1B^-0.723H^-0.019GSSI^0.002C^-0.09（R²=0.889,NSE=0.860）.