| A scientific instrument for quantitatively assessing soil erosion intensity,comprehending the effects of changing land uses on soil erosion,and carrying out planning for soil and water conservation is the soil erosion model.The soil erodibility factor（K factor）,which is strongly connected to the physicochemical characteristics of the soil,is a measure of soil parameters used in soil erosion prediction models.Grain to Green Project（GTGP）on the Loess Plateau,through the increase of soil organic carbon content and refinement of soil particles,influences the K value.The variation in K value on restored lands may be one of the reasons for reducing erosion,but the differences in K values between cultivated and uncultivated lands are not yet clear.Another important parameter that affects the accuracy of soil loss estimation in soil erosion prediction models is the cover and management factor（C factor）,which has been a focal point and challenge for many scholars’research.The magnitude of the C value in non-agricultural land is determined by factors such as land use type and coverage.Regarding grasslands,most studies have focused on the influence of green vegetation and litter on the C value,while there is limited reporting on the development and coverage of biocrusts and their impact on the C value.As one of the widely distributed ground covers in arid and semi-arid regions,biocrusts directly affect water and soil loss on slopes through their covering effect.However,their role is generalized in soil erosion prediction models,which may affect the accuracy of soil loss estimation.In order to enhance the accuracy of soil erosion model estimation,it is important to clarify the application and inclusion of biological soil crust（biocrusts）in the model.This study focused on the Loess Plateau as the research area and conducted field surveys and laboratory analysis experiments to investigate the changes in soil erodibility following Grain to Green Project（GTGP）.The objective was to assess the continued applicability of the calculation equation for the erodibility factor（K）.Furthermore,the indoor and field simulated rainfall experiments were carried out to quantify the impact of biocrusts（development and coverage）on soil erosion.A calculation equation for the biocrusts factor（development and coverage）was developed,and the accuracy of this factor was verified through field runoff plot experiments.Lastly,by comparing the calculated values of the original model with the measured values,it was determined whether the soil erosion model incorporated the biological crust factor.If not,the biocrusts factor was adjusted and integrated into the original model,and the modified model was validated.（1）After GTGP in the Loess Plateau,the significant increased in soil organic carbon content in the 0-20cm soil layer did not led to a significant decrease in the erodibility factor（K）.Compared to cultivated land,the soil organic carbon content in restored land on the Loess Plateau increased significantly by 1.22-2.83 g·kg-1,while soil particle composition and erodibility remained unchanged.Different types of restored land on the Loess Plateau（including forested land,shrubland,and grassland）showed a significant increase in soil organic carbon content,with an increment of 1.22-2.83 g·kg-1.Specifically,forested land,shrubland,and grassland exhibited average increases of 71%,49%,and 46%,respectively,while soil particle composition and soil erodibility（K value）did not show significant changes.（2）The chlorophyll a content of cyanobacteria crusts can quantitatively describe the early development succession of biocrusts.During the early stage of biocrust development,cyanobacteria crusts dominate,while the development of moss crusts was relatively low（moss height less than 2mm,diameter approximately 0.2mm）.The number of moss plants during the early development stage had no significant effect on chlorophyll a content.Therefore,the chlorophyll a content of cyanobacteria crusts can be directly measured to quantitatively describe the early developmental succession process of biocrusts.（3）The runoff rate and soil loss rate on the slope significantly decreased as the biocrusts developed.And the changes in runoff rate,soil loss rate exhibited a threshold phenomenon with the development of the biocrusts.Under a rainfall intensity of 90 mm·h-1,when the biomass of the biocrusts was less than 9 ug·g-1,the runoff rate did not show a significant difference compared to bare soil.When the biomass exceeded 9 ug·g-1,the runoff rate on the slope decreased significantly by 25%.When the biomass was less than 1ug·g-1,the soil loss rate on the slope did not show a significant difference compared to bare soil.When the biomass was between 1-9 ug·g-1,the soil loss rate on the slope significantly decreased with a sediment reduction efficiency of 66.3%.When the biomass exceeded 9ug·g-1,the slope was able to fully resist the erosion caused by erosive rainfall.Furthermore,through a two-year field monitoring experiment of natural rainfall,the influence of the biocrusts on runoff only showed a decreasing trend,but it still had a significant effect on soil losse and the threshold for sediment control remained at 9 ug·g-1.Hydraulic parameters also exhibited a threshold phenomenon with the development of biocrusts.The changes in hydraulic parameters with the development of biolcrusts were as follows:runoff velocity and runoff power increased first and then decreased,while runoff depth,runoff shear force,and resistance coefficient decreased first and then increased.（4）The development of the biocrusts primarily reduced soil loss indirectly by increasing the thickness of the biocrusts.Additionally,although the development of the biocrusts also significantly affected the runoff power and Darcy-weisbach resistance coefficient,but,it did not significantly reduce soil loss through these factors.Under a rainfall intensity of 90 mm h-1,the order of influence on soil loss was as follows:biocrusts thickness（-0.773）>biocrusts biomass（-0.589）>soil cohesion（-0.198）>runoff resistance（0.124）>runoff power（0.114）.（5）The biocrust（development and cover）factor was constructed based on the reference of C factor.The BSC development factor equation is primarily applicable to slopes that are initially returning to fallow in cultivated land.Within the range of 1-9 ug·g-1biomass of BSC,the expression BBD=0.466e-0.211（Cha）is utilized for estimating slope erosion.The estimated and measured values demonstrate an even distribution on both sides of the 1:1line.The model exhibits Nash coefficients of 0.82 and 0.66 for the years 2021 and 2022,respectively,with corresponding RMSE values of 0.10 and 0.08.Similarly,the biological crust coverage factor equation is mainly applicable to forests and grasslands in the middle and later stages of returning to fallow.When the coverage of BSC less than 45%,the expression BBC=0.8586e-0.05（C）is used for estimating slope erosion.The calculated and measured values also show an even distribution on both sides of the 1:1 line.The model exhibits Nash coefficients of 0.61 and 0.74 for the years 2020 and 2022,respectively,with corresponding RMSE values of 0.04 and 0.08.（6）Regardless of whether it was simulated rainfall or natural rainfall,there was a significant discrepancy between the calculated values of the original model and the corresponding measured values.The validation results of the original model for simulated rainfall showed a Nash coefficient of-26.01 and an RMSE value of 3.21.For natural rainfall,the validation results were a Nash coefficient of-25.84 and an RMSE value of 2.43.After incorporating the coverage factor of the biological crust into the erosion model and adjusting it,the calculated values of the new model became much closer to the measured values,resulting in a significant improvement in the model’s fitting accuracy.The validation results of the new model for simulated rainfall showed a Nash coefficient of 0.56 and an RMSE value of 0.41.For natural rainfall,the validation results were a Nash coefficient of0.48 and an RMSE value of 0.34.This indicates that the new model can accurately predict soil erosion on grassland slopes at the scale of the Loess Plateau.The biocrusts factor was one of the crucial components in the soil erosion model since it had the ability to save soil and water,while the duration of the loess plateau must be taken into account when choosing biocrust factor.The biolcrusts development factor was suitable for the early stage of slope and with biorusts biomass lower than 9ug-g-1.The biocrusts cover and factor was appropriate for forested grassland in the middle and late stages of slope and with biocrust cover lower than 45%.The Grain to Green Project’s advantages for soil and water conservation of Plateau Plateau may be effectively assessed using biocrusts factorization and modification of the soil erosion model. |
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