Processes And Mechanisms Of Soil Erosion And Nutrient Transportation In The Black Soil Region Of Northeast China

Black soil region is a major food production area in China, while serious soil erosion andnutrient loss directly threaten grain security strategy of our country. Therefore, deepunderstanding mechanisms of soil erosion are an urgent demand to protect black soilresources. In view of the insufficiency research of soil erosion processes and nutrient loss inthe black soil region of Northeast China, this study took the black soil as the research objectto study processes and mechanisms of soil erosion, and nutrient migration mechanisms basedon rainfall simulation experiments, the137Cs tracer technology, GIS technology and laboratoryanalysis. Mechanisms of soil erosion were studied under two kinds of erosion dynamics thatwere raindrop impact, and the combining action of seepage flow and surface flow. Then thestudy evaluated the effects of rainfall characteristic (rainfall intensity and storm patterns),slope gradient, and soil surface cover on soil erosion processes, and illustrated hydro-dynamicmechanism of soil erosion from the perspective of flow hydraulics and hydrodynamicmechanism of sediment yielding. Migration mechanisms of nitrogen and phosphorus wereexplored under different soil erosion conditions. Finally, this study clarified spatialdistribution of soil erosion at the watershed scale. Main conclusions of this study were asfollows:(1) The role of raindrop impact on hillslope soil erosion processes was investigated.Sediment yield induced by raindrop impact occupied72.29%-96.20%of the total sedimentyield. When raindrop impact was eliminated, runoff depth, flow velocity, Reynolds number,and Froude number decreased3.85%-39.34%,20.9%-67.5%,25.82%-70.29%,and18.75%-62.50%, respectively; whereas Darcy-Weisbach coefficient increased39.90%-430.91%. Flow velocity was the key hydraulic parameters of affecting raindroperosion according to the correlation analysis and path analysis. Further, raindrop impactresulted in the critical shear stress, the critical stream power, and the critical unit stream power increased above2times, compared with the condition without raindrop impact.(2) Effects of the interaction of seepage flow and surface flow on hillslope soilerosion processes were analyzed. Compared with the treatment without seepage flow,sediment yield from the treatment with seepage flow and surface flow increased50.84%-74.34%, these corresponding hydraulic parameters of flow velocity, Reynoldsnumber, and Froude number increased2.74%-50.11%,4.03%-77.22%, and2.08%-37.51%,respectively, while Darcy-Weisbach coefficient decreased4.52%-60.44%, and shear stress,stream power, and unit stream power increased1.42%-24.64%,4.04%-77.22%,respectively.(3) Processes and hydro-dynamic mechanism of soil erosion under different stormpatterns were revealed. Sediment yield from the storms of varying rainfall intensity wassignificantly greater than that from the storms of stable rainfall intensity. Among, the risingstorm pattern gave a consistently higher sediment yield relative to other storm patterns.Sediment yield from the rising, falling, and falling-rising storm patterns was1.83-5.17,1.62-3.79, and1.13-3.79times, respectively, greater than that from the even storm pattern.Also, significant difference in sediment yield was observed between different steps ofrainfall intensity occurrence in different storm patterns. Compared with even-intensitystorms, flow velocity, Reynolds number, and Froude number from varying-intensity stormsincreased significantly and hydraulic parameters from the rising storm pattern showed thelargest increasing amplitude. Shear stress, stream power, and unit stream power fromvarying-intensity storms increased by10.28%-101.46%,8.89%-173.33%, and6.75%-88.40%, compared with those from even-intensity storm.(4) The estimation equations of soil erosion were established under experimentalconditions. Based on the sensitivity analysis, path analysis, and factor analysis, it was foundthat unit stream power (ψ) and Froude number (Fr) were the main-control factors of soilerosion for the treatment with raindrop impact, and the relationship between sediment yieldper unit width (Dr) and these main-control factors was expressed by: Dr=4.787Fr3.547ψ-0.008;unit stream power and Reynolds number (Re) became the key factors of soil erosion for thetreatment only with runoff erosion, and the relationship between Dr and these key factors tookthe following form: Dr=0.002Re0.76ψ0.53. Verified by the information of actual measurement,the two equations showed higher precision.(5) Nutrient migration mechanisms under different soil surface water regimes wererevealed. NO3-N, NH4-N, and PO₄-P concentrations from the seepage treatment were228.7-294.0,38.4-42.9, and7.3-10.2times greater than those from the free drainage treatment, respectively. Both nutrient losses in runoff and sediment yield ranked in order: the seepagewith rainfall treatment>the saturation treatment>the free drainage treatment. NO3-N, NH4-Nand PO₄-P concentrations in runoff from the saturation and seepage with rainfall treatmentswere significantly greater than those from the free drainage treatment, with magnitudes of4.06-5.75and14.8-20.96times greater for NO₃-N,1.26-1.36and1.74-2.09times greater forNH₄-N, and1.29-1.90and1.57-2.20times greater for PO₄-P, respectively. Further, it wasfound that the effective erosion transport depth by runoff of NO3-N was less than3mm forthe free drainage treatment, while it was more than10mm for the saturation and seepage withrainfall treatments.(6) The spatial distribution of soil erosion and deposition in a small catchment of blacksoil region was revealed using¹³⁷Cs tracer technique and GIS technology. Soil erosionplayed a dominated role in the upstream of the catchment, and erosion and depositionappeared alternately in the middle reach of the catchment, while deposition took a leadingposition in the downstream of the catchment. Along the slope length, soil erosion wasslightly at the top of slope, and erosion became more serious at the middle of slope, whiledeposition occurred at the toe of slope. Slope erosion occupied95.7%of the catchmenterosion. The mean erosion modulus was571.5t km-2a-1for this catchment.