Selecting Characteristics Of Soil Aggregates During Rainfall Erosion In Black Soil Sloping Field

To reveal the characteristics of soil aggregate selecting during rainfall erosion,the surface soil of black soil sloping field（0～10 cm）was taken as the research object.The influence of rainfall intensity and slope gradient on the characteristics of soil aggregate selecting in black soil sloping field was clarified through the simulation tests of two rainfall intensities(78,127mm·h^-1)and three slope gradients（2°,4°,6°）by using the method of simulation rainfall.Firstly,the process of soil loss and the total amount of soil loss under different rainfall intensities and slope gradients were analyzed to clarify the characteristics of soil loss during rainfall erosion.Secondly,by exploring the fraction size distribution and loss process of aggregates under different rainfall intensities and slope gradients,the structural parameters of aggregates（mean weight diameter MWD and fractal dimension D）were calculated,the effects of rainfall intensity and slope gradient on the loss characteristics of aggregates can be proved.Finally,the fraction size distribution of mechanically stable aggregates and water-stable aggregates in surface soil after rainfall erosion was measured,and the failure rate of aggregates(PAD_0.25)and structural parameters of water-stable aggregates（MWD,D）were calculated,the distribution characteristics of surface aggregates after rainfall erosion can be clarified.The main conclusions are as follows:（1）The characteristics of soil loss during rainfall erosion were clarified.Overall,the soil loss under simulated rainfall conditions reached its maximum at the beginning of rainfall runoff and decreased rapidly with rainfall duration and then stabilized.When the slope gradually increased from 2°to 6°,the total soil loss significantly increased by 0.70～1.42 times.When the rainfall intensity increased from 78 mm·h^-1to 127 mm·h^-1,the total soil loss did not change significantly at the slope of 2°,but increased by 1.94 and 2.41 times at the slope of 4°and 6°,respectively.Rainfall intensity is the main factor affecting soil loss,with the contribution rate of 44.37%～54.52%.（2）The effects of rainfall intensity and slope gradient on aggregate loss characteristics were proved.Under simulated rainfall conditions,the aggregate with fraction size<0.053 mm is the main loss fraction size,and the loss proportion is 68.20%～92.08%.With the increase of rainfall intensity and slope gradient,the loss proportion of aggregates with fraction sizes of 5～1,1～0.25 and 0.25～0.053 mm decreased,while the loss proportion of aggregates with fraction sizes<0.053 mm increased.When the slope gradually increased from 2°to 6°,the MWD of the lost aggregates decreased by 47.22%～79.72%,and the D value increased by 0.01～0.03times with the increase of slope only when the rainfall intensity was 78 mm·h^-1.Under the same slope gradient condition,there was no significant difference in MWD and D values between the two groups of rainfall intensities.Rainfall intensity was the main factor affecting the total loss of aggregates with diameters of 1～0.25,0.25～0.053 and<0.053 mm,and the contribution rates was 27.42%,28.09%and 47.09%,respectively.Slope gradient is the main factor affecting the total loss of 5～1 mm aggregates,with a contribution rate of 45.59%.This indicates that the loss of small size aggregates is more sensitive to rainfall intensity changes,while the loss of large size aggregates is more sensitive to slope gradient changes.（3）Under simulated rainfall conditions,the influence of rainfall erosion on the distribution characteristics of surface aggregates was clarified.Rainfall erosion reduces the content of organic and water-stable aggregates>0.25 mm in surface Soil.When the rainfall intensity increased from 78 mm·h^-1to 127 mm·h^-1,the content of>0.25 mm aggregates in the mechanically stable aggregates of the eroded surface soil decreased,and the content of>0.25mm aggregates in the water-stable aggregates of the eroded surface soil increased;the PAD_0.25of surface aggregates decreased by 12.21%～30.75%,MWD significantly increased by0.38～0.75 times,D value significantly decreased by 3.67%～5.81%.When the slope gradually increased from 2°to 6°,the content of>0.25 mm aggregates in the mechanically stable aggregates and water-stable aggregates of the eroded surface soil increased;the PAD_0.25and D values of surface aggregates did not change significantly after erosion,MWD increased by 0.35times at the rainfall intensity of 78 mm·h^-1,and had no significant change at the rainfall intensity of 78 mm·h^-1.