| Soil crusts formation is a common phenomenon in the cultivated soils of the Loess Plateau of China.It occurs when a thin layer with low porosity is formed at the soil surface after rainfall or irrigation. Soil crusts increase the bulk density and soil strength of the soil at the surface compared to non-crusted soils, and their formation may also increase runoff rates as a result of soil infiltration reduction. Soils with crusts have lower infiltration rates and generate higher volumes of runoff due to lower hydraulic conductivity. Special tillage treatments are designed to prevent loss of soil and water. However, the combined effects of soil crusts and special tillage treatments on soil erosion in the Loess Plateau of China have been seldom examined. In this study, we included traditional tillage treatments(contour tillage(CT), artificial digging(AD) and straight slope(CK)) in the Loess Plateau of China as the background, used the indoor artificial rainfall simulation and runoff scouring simulation to evaluate the formation and characteristics of different types of soil crusts and their effect on soil erosion. We also explored the effects on soil erosion from tillage treatment and the combined effects from soil crusts formation and tillage treatment. The main results were as follows:1. Tillage treatments created little changes of soil surface as mounds and depressions. The formation of soil crusts types was significantly affected by different tillage treatments. The structural crusts formed on the ridge where attacked by the raindrops directly while the depositional crusts formed in the depression between the ridges as sediment deposited. The structural crusts had lower thickness and bulk density, higher porosity and shear strength than the depositional crusts. The depositional crusts increased runoff rate by 75.80%, reduced the sediment rate and infiltration rate by 38.15% and 33.74% respectively while compared with uncrusted soil surfaces The structural crusts increased runoff rate by 26.87%, reduced the sediment rate and infiltration rate by 55.08% and 28.92 % respectively while compared with uncrusted soil surfaces.2. The results from the runoff scouring experiments showed that: soil crusts exerted significant effects on hydrodynamic characteristics. The Reynolds number under soil crusts surface was less than 500, indicating that the overland flow was laminar flow. The flow velocity was greater on crusted soil surface than uncrusted soil surface, while the flow depth, shear stress, stream power and soil erosion was much lower on crusted soil surface than uncrusted soil surface. The correlation between soil erosion and hydrodynamic characteristics of overland flow was significant(Regression coefficient R>0.90). Soil erosion had a positive linear correlation with Reynolds number, the logarithm of shear stress and stream power, while had a negative liner correlation with resistance coefficient. These results supposed that existence of soil crusts was helpful to reduce soil erosion from the runoff perspective.3. The results from the rainfall simulation experiments showed that: the initiation time of runoff was earlier under crusted soil surface than under uncrusted soil surface across the ranges of slopes and rainfall intensity tested in this study. The changing pattern of runoff intensity curve, sediment intensity curve and infiltration intensity cure was close to each other. The runoff discharge rate was significantly greater while sediment discharge rate and infiltration rate were lower under crusted soil surface than under uncrusted soil surface. Cumulative runoff yield increased linearly under crusted and uncrusted soil surface, and the growth rate of cumulative runoff yield was lower under uncrusted soil surface than under crusted soil surface. Cumulative sediment yield and infiltration increased as piecewise linear function separated by the stable time. The growth rates of cumulative sediment and infiltration yield were faster before stable time, and the growth rates were lower under crusted soil surface than under uncrusted soil surface.4. This study showed that the initiation time of runoff followed Contour tillage(CT) > Artificial digging(AD) > Straight slope(CK) across the ranges of slopes and rainfall intensities tested in this study. The amount of runoff yield followed CT < AD < CK, which was the same as the pattern of sediment yield. The infiltration rate was opposite to the runoff and sediment yield which followed CT > AD > CK. Cumulative runoff yield increased linearly, and the growth rate of cumulative runoff yield followed CT < AD < CK. Cumulative sediment yield and infiltration increased as piecewise linear function separated by the stable time. The growth rates of cumulative sediment and infiltration yield were faster before stable time. The growth rate of sediment yield was contrary to infiltration yield which followed CT < AD < CK.5. The results from correlation analysis showed that runoff, sediment and infiltration yield had significantly correlation relationship with rainfall intentisy, slope, tillage treatments and soil crusts(p < 0.01). Runoff and infiltration yield had maximum correlation with rainfall intensity while sediment yield had maximum correlation with slope factors. The results from multi-factor variance analysis showed that the single effect of rainfall intensity, slope, soil crusts and tillage treatments was significant on runoff, sediment and infiltration yield(p < 0.01). The double factors effects of rainfall intensity and slope, rainfall intensity and soil crusts were not significant on runoff, sediment and infiltration yield. The three factors effects of rainfall intensity, slope and soil crusts were not significant on runoff, sediment and infiltration yield. The four factors effects were still not significant on runoff, sediment and infiltration yield. The results from stepwise regression analysis showed that the linear equation among rainfall factor, slope factor and surface roughness factor reached the significant level(p < 0.01). The results of path analysis showed that the effect of rainfall and slope factors on runoff erosion is relatively serious while there was no correlation between tillage treatments and slope factors. The correlation coefficient was greater under crusted soil surface than under uncrusted soil surface. The main factor on infiltration under crusted soil surface was rainfall intensity and was tillage treatments under uncrusted soil surface.6. The runoff and sediment rates increased with slope following 5°<10°<15°<20°. The infiltration rate was contrary to runoff and sediment rates and followed 5°>10°>15°>20°. The variation tendency of runoff, sediment and infiltration rates under different rainfall intensity was the same as under different slopes. The runoff, sediment and infiltration rates increased with the increase of rainfall intensity. The effects of slope and rainfall intensity were obvious with differenttillage treatments. Runoff, sediment and infiltration rate changed significantly under contour tillage and artificial digging treatments which was not significantly under straight slope with the slope increased.Runoff, sediment and infiltration rate changed significantly under all tillage treatments with the rainfall intensity increased. The effects of depositional crusts and structural crusts decreased with the increment of slope and rainfall intensity. |
PDF Full Text Request