Study On The Influencing Factors Affecting The Soil Detachment Of Root-Soil Composite And Its Effect Mechanism In Loess Hilly Region

The loess hilly region is one of the most serious areas suffering from the soil erosion on the Loess Plateau,which seriously threatens the sustainable development of local economy and society.With the recovery of vegetation in this area,soil erosion has been effectively controlled.Roots play an important role in the effect of vegetation on the erosion reduction.Soil detachment is the initial stage of soil erosion.Exploring the mechanism of roots on soil detachment is important for analyzing the vegetation effect on erosion reduction.Some work about the effect of roots on the soil detachment on the loess plateau has been carried out.But the root is not existed independently,but it is composited with the soil.And the reduction effect of root to soil detachment actually acted by the root-soil composite.Therefore,the quantitative expression of roots to soil detachment is easily affected by the soil properties.The study on the influencing factors of soil detachment capacity of root-soil composite and its mechanism,is of great significance for revealing the reduction mechanism of roots to erosion,promoting the development of soil erosion process model,and selecting suitable plant measures for soil and water conservation in this region.In this study,undisturbed root-soil composite samples,under different grassland succession years,plant mixed growth patterns and land use types,in the loess hilly region were taken as the research objects.The hydraulic flume experiments were conducted to estimate the soil detachment capacity.And the response mechanisms of soil detachment capacity to stream power,root traits and soil properties were explored.The critical zones of root and soil properties played its dominated effects on soil detachment capacity were quantified.And the effect mechanism of roots and soil factors affecting the soil detachment capacity of root-soil composite was analyzed.And the prediction model of soil detachment capacity was developed.The main research conclusions are as follows:（1）The differences of soil detachment capacity of different of root-soil composite were compared.With the grassland succession years increased,the root density and soil properties were all increased,leading to the soil detachment capacity decreased as a power function,and gradually tended to be stable at 11a.The root density of mixed plant sites was higher than that of single plant sites,resulting in the soil detachment capacity was about75.65%lower than that of the single plant sites.The soil detachment capacity of the pattern of the fibrous roots mixed with the tap roots was higher than that of the pattern of the fibrous roots mixed with the fibrous roots,but lower than that of the pattern of the tap roots mixed with the tap roots.Under different land uses,the root density of woodland and grassland was lower than that of cropland and orchard,resulting in the soil detachment capacity of the woodland was the lowest,followed by the grassland,cropland,last was the orchard.（2）The relationship between soil detachment capacity of root-soil composite and stream power was clarified.With the grassland succession years increased,the response of soil detachment capacity to the stream power gradually decreased.Compared with the single shrub site,the response of soil detachment capacity to the stream power was weakened under the shrub-grass mixed site.However,compared with the single grass site,there was no significant variation for the response of soil detachment capacity to the stream power under the shrub-grass mixed site and grass-grass site.With the land use transferred from farmland and orchard to grassland and woodland,the response of soil detachment capacity to the stream power gradually decreased.When the root length density was higher than 152.42km·m^-3,the dominated factors affecting soil detachment changed from hydrodynamic parameters to root-soil combinations factors.（3）The effects of root parameters on soil detachment capacity of root-soil composite were clarified.With the grassland succession years increased,the relationship between soil detachment capacity and root parameters gradually increased.The relationship between soil detachment capacity and root parameters in mixed plant sites was stronger than that in single plant sites.With the land use transferred from farmland and orchard to grassland and woodland,the response of the soil detachment capacity to the root parameters gradually increased.For cropland,orchard and grassland at the early succession stage,its quantitative expression of roots to soil detachment capacity were easily affected by hydrodynamic characteristics.For grassland at the late succession stage and woodland,the relationship between soil detachment capacity and root parameters was easily affected by soil properties.At this case,its relationship could be analyzed in three zones:when the root mass density was greater than 5.07 kg·m^-3,or the root length density was greater than 198.70km·m^-3,or the root cohesion was greater than 10.18 Kpa,the dominated factors affecting the soil detachment capacity was root traits;when root parameters were less than the above critical values,and the soil bulk density was higher than 1.29 g·cm^-3,or the clay content was higher than 14.43%,or organic matter was higher than 10.43 g·kg^-1,the dominated factors were soil properties;the dominated factors of other condition were root traits and stream power.Compared with single plant sites,the critical root parameters of mixed plant sites were higher,and the critical soil properties were no significant difference.The relationship of soil detachment capacity with root cohesion was better than that with root mass density and root length density.And the soil detachment capacity had the best correlation with the root cohesion with the 0-1mm diameter class.（4）The effects of soil properties on soil detachment capacity of root-soil composite were clarified.The relationship between soil detachment capacity and soil physicochemical properties and mechanical properties was better than that of soil hydrological properties.With the grassland succession years increased,the effect of soil properties on the detachment capacity increased.The correlation between soil detachment capacity and soil properties was stronger in the shrub-grass mixed site than in the single shrub site.Compared with the single grass site,there was no significant difference of this correlation under the shrub-grass mixed site and the grass-grass mixed site.With the land use transferred from farmland and orchard to grassland and woodland,the response of soil detachment capacity to soil properties gradually increased.（5）The variation mechanism of soil detachment capacity and rill erodibility parameters of root-soil composite were revealed.The direct effect of roots on soil detachment capacity was stronger than the indirect effect.Roots mainly indirect affect soil detachment by affected the physicochemical properties.When fibrous roots mixed with tap roots,the effect of fibrous roots on soil detachment was stronger than that of later.And the fibrous roots mainly indirect affect soil detachment by affected the mechanical properties,while tap roots mainly indirect affected by soil hydrological properties.The prediction model of soil detachment capacity under different land uses was constructed.For the sites with the dominated factors was root-soil recombination properties,the prediction accuracy of the model could be effectively improved by zoning fitting.The variation of rill erodibility parameters was also analyzed.With the grassland succession years increased,the rill erodibility parameters gradually decreased.The rill erodibility parameters under the pattern of fibrous roots mixed with tap roots were higher than that under the pattern of fibrous roots mixed with fibrous roots,lower than that under the pattern of tap roots mixed with tap roots.Under different land uses,the rill erodibility parameters of the orchard was the highest,followed by the cropland and grassland,last was the woodland.The effect of roots on rill erodibility parameters was consistent with the results of the soil detachment capacity.The root parameters were the most correlated with rill erodibility parameters was root cohesiveness（Cr）,and the soil parameters were soil organic matter（SOM）and aggregate mean weighted diameter（MWD）.Based on the above three parameters,the prediction model of rill erodibility parameters was constructed:K_r?（28）0.03e^{-0.6 6Cr}SOM^1.59MWD^0.02(NSE=0.97,R²=0.97,P.O._0.5-2=55.00%).Meanwhile,combined with the previous research results,and based on 3 parameters of root weight density（RMD）,soil cohesion（COH）and SOM,a prediction model suitable for a wider area of the Loess Plateau was constructed:K_r?（28）1.4316e^-0.99RMDCOH^-0.⁵⁹SOM^-0.34-0.0040.And this model also had a better calibration accuracy(NSE=0.73,R²=0.78,P.O._0.5-2=45.00%).