The Influence And Mechanism Of The Distribution Patterns Of Biological Soil Crusts On Sediment Yield Slope In The Hilly Loess Plateau Region,China

Soil erosion is the most important ecological problem in Loess Hilly areas.Biological soil crusts（referred to as biocrusts）,as a widespread ground cover in reclaimed land in Loess Hilly areas,have an impact on soil erosion that cannot be ignored.The current research on the effect of biocrusts on runoff and sediment yeild on slopes mainly focuses on the coverage and types of biocrusts,and rarely involves the distribution pattern of biocrusts.Although a small number of studies have focused on the distribution characteristics of biocrusts,relevant reports still focus on qualitative descriptions and lack quantitative characterization,which hinders people’s understanding of the soil and water conservation functions of biocrusts.For this reason,this paper takes the biocrusts naturally developed on the slopes and artificially filled soil trenches in the Loess Hilly area as the research object.Through field investigations,the principles and methods of landscape ecology are used to study the quantitative characterization of the distribution pattern of biocrusts.On this basis,through indoor simulated rainfall experiments,the influence of the distribution pattern of biocrusts on the sediment yeild on slopes was studied,and the mechanism of the influence of the distribution pattern of biocrusts on sediment yeild on the slopes was revealed.The following main conclusions are reached:（1）The five indexes of Percentage of landscape（PLAND）,Patch density（PD）,Landscape shape index（LSI）,Patch cohesion index（COHESION）and Splitting index（SPLIT）can be more comprehensive and low-redundan to describe the distribution characteristics of biocrusts.The suitable particle size range of the biolcrusts on the slope surface of the abandoned farmland is 5-10 mm in a 1.5m×1.5m square.On this basis,the 13 landscape indices are divided into five categories.The first category includes Proportion of like adjacencies（PLAND）,Aggregation index（AI）,clumpiness（CLUMPY）,and COHESION,which mainly reflect the connectivity of patches;the second category includes Edge density（ED）,PLAND,Largest patch index（LPI）,Effective mesh size（MESH）,mainly reflect the coverage and length of patches;the third type is the Interspersion and juxtaposition index（IJI）,which reflects the distribution of patches;The four types have PD and LSI,which reflect the number and shape of patches;the fifth type has Landscape division index（DIVISION）and SPLIT,which reflect the fragmentation of patches.Combining the results of factor analysis and cluster analysis and the ecological significance of the index,the PLAND,PD,LSI,COHESION and SPLIT are screened out.The five indexes can fully describe the distribution characteristics of biolcrusts under the condition of low redundancy.（2）When the coverage of the biocrusts changes,the distribution pattern of the biocrusts significantly affects the slope runoff and hydrodynamic characteristics of slope flow.The SPLIT of biocrusts is the main factor affecting the slope surface(Y=0.915-0.002SPLIT,R²=0.82^**),the PLAND,PD,COHESION,SPLIT of biocrusts and the roughness of the slope are the main factors that affect the hydrodynamic characteristics of slope flow.Compared with bare soil,biocrust slopes with different coverage in random patterns all reduce slope runoff,and the average value is 75.3%of that of bare soil.The influence of biocrusts on slope runoff increases with the increase of coverage.Compared with bare soil,biocrust slopes with different coverage have reduced runoff velocity by 36.6%,Reynolds number by 30.2%,Froude number by 39.7%,runoff power by 26.0%,and drag coefficient increased by 2.40 times.The runoff depth partly increases and partly decreases.Compared with bare soil,biocrusts with 21-38%coverage reduce runoff depth by 43.5%and 14.5%,respectively,while 54-83%coverage increases runoff depth by 33.3%,70.0%,and 71.0%,respectively.The SPLIT of biocrusts is the main factor affecting slope runoff(Y=0.915-0.002SPLIT,R²=0.82^**).The main influencing factors of the hydrodynamic characteristics of slope flow are different.The main factor is the PLAND,PD,COHESION,SPLIT of biocrust patches in the landscape and slope roughness.（3）When the coverage of biocrusts is constant（60%）,the distribution pattern of biocrusts is a key factor affecting slope runoff and hydrodynamic characteristics of slope flow.Among them,the main influencing factor of runoff is the SPLIT of biocrusts(Y=1.116+0.017SPLIT,R²=0.96^**).The main influencing factors of hydrodynamic characteristics of slope flow are the COHESION and SPLIT of biocrusts.The greater the degree of biocrusts fragmentation,the greater the slope runoff velocity,the greater the Reynolds number,the greater the Froude number,the smaller the resistance,and the greater the runoff power.When the coverage is constant（60%）,biocrust slopes with zonation,chessboard and random patterns all reduce the runoff of the slope compared with bare soil,and the degree of decrease is related to the distribution pattern of biocrusts.Compared with bare soil,the zonation,chessrboard and random patterns are reduced by 1.8%,21.1%,and 29.5%,respectively.The runoff rate of the random pattern slope is 71.7%and 89.6%of the zonation pattern and the checkerboard pattern,respectively.Compared with bare soil,the average runoff velocity of the slope covered by biocrusts under different distribution patterns was reduced by 54.6%,the runoff depth was increased by 86.2%,the Froude number was reduced by 67.0%,and the Darcy-weisbach resistance coefficient was increased by 10.68times..Under the premise of a constant degree of biocrust coverage,the SPLIT of biocrusts is the main factor affecting runoff(Y=1.116+0.017SPLIT,R²=0.96^**).The COHESION and SPLIT of biocrusts are the main factors affecting hydrodynamic characteristics of slope flow.The smaller the COHESION of biocrusts,the greater the degree of SPLIT,and the greater the degree of fracture.（4）Biocrust coverage（the PLAND of biocrusts）significantly affects the sediment yield on the slope.Under the experiment conditions,biocrusts with different coverages reduced the sediment yield by 88.8%compared with bare soil.The decrease rate showed an increasing trend with the increase of biocrust coverage.The PLAND of biocrusts influenced the slope sediment yield by affecting the slope Froude number.The larger the PLAND of biocrusts,the greater the biocrust coverage,the smaller the slope Froude number,and the lower the slope erosion.Compared with bare soil,biocrusts of different coverage significantly reduced the sediment yield on slopes,and the average value was 11.2%of that of bare soil,and the degree of decrease increased with the increase of PLAND（coverage）(Y=37.289-0.393PLAND,R²=0.80^**).When the coverage of the biocrusts changes,the PLAND of the biocrust is the main factor affecting the sediment production on the slope.At the same time,the Froude number is the main hydraulic parameter that affects the rate of soil erosion.The PLAND of biocrust affects the slope sediment yield by affecting the Froude number(Y=41.03+7.861X_Fr,R²=0.65^**).（5）Biocrust coverage is constant（60%）,the distribution pattern of biocrusts is affected by slope sediment yeild.The SPLIT of biocrust affects slope erosion dynamics and then affects slope sediment yield by influencing Reynolds number.The SPLIT of biocrust was greater,the Reynolds number was lower and the amount of sediment yeild on the slope was gteater.With a constant coverage（60%）,biocrust slopes with zonation,chessboard and random patterns reduced the sediment yield of slopes by 37.7%,74.8%,and 92.0%compared with bare soil,respectively.The degree of sediment yield reduction is related to the distribution pattern of biocrusts.The sediment yield on the slope of the random pattern is 12.9%of the zonation pattern and 31.8%of the chessboard pattern.When the cover degree of biocrust is constant（60%）,the SPLIT of biocrusts is the main pattern factor that affects the slope surface sediment yield,and the SPLIT of biocrusts affects the slope surface Reynolds number and then the sediment yield(Y=-90.537+2.791X_Re,R²=0.86^**).（6）The fragmentation degree（PD）of biocrusts with different distribution patterns under the same coverage（60%）is the main influencing factor of sediment yield.Fragmentation of biocrusts affects the sediment yield by affecting the slope resistance coefficient.The fragmentation of the biocrust was greater,the slope resistance was lower and the more severe the erosion.Compared with bare soil,biocrusts with different fragmentation degrees all increase slope runoff and reduce slope sediment yield,and their average values are 1.19 times and55.3%of bare soil,respectively.There is no significant difference in slope runoff rate between different fragmentation degrees,and there is a significant difference in soil erosion rate.Slope soil erosion rate is logarithmic function relationship with biocrust fragmentation(Y=6.9870ln（x）+18.9854,R²=0.84^**).With the increase of the fragmentation of biocrusts,the sediment yield on the slope first increased and then stabilized.Under the premise of a certain degree of biocrusts coverage,there is a significant correlation between the degree of biocrust fragmentation and hydrodynamic characteristics of slope flow except runoff power.Among them,the drag coefficient is the main erosion dynamic factor that affects the slope sediment yield,indicating that the fragmentation of the biocrusts mainly affects the slope sediment yield by affecting the slope resistance(Y=42.731-0.991X_f,R²=0.45^**).In summary,when the cover degree of biorusts differs greatly,the runoff on the slope is mainly affected by the degree of fragmentation,and the sediment yield is mainly affected by the degree of coverage.When the degree of biocrust coverage is constant,slope runoff,erosion dynamics and sediment yeild are mainly related to its fragmentation.With the increase of biocrust fragmentation,slope flow force increases,runoff resistance coefficient decreases,and sediment yield increases.