The Process And Dynamic Mechanism Of Soil Organic Carbon Erosion On Slope

In this study,two independent experiments were carried out.The first experiment selected cinnamon soil as the research object which was a typical soil type in the Rocky Mountains of Beijing,and 10 continuous simulated rainfall events were carried out under every rainfall intensity(30 mm/h,60 mm/h,90 mm/h).The second experiment selected loess soil as the research object which was a typical soil type in the Loess Plateau,and 20 simulated rainfall events were carried out under different rainfall intensity(60 mm/h,90 mm/h),slope gradient(2.5°,5°,7.5°,10°,15°,20°)and slope length(5 m,10 m).The two independent experiments carried out were used to study the characteristics and process of SOC erosion and its hydrodynamic mechanism on slope of the cinnamon soil and the loess soil,respectively.The main conclusions were as follows:(1)The characteristics and process of SOC erosion on the cinnamon soil slope of Rocky Mountains of Beijing were systematically analyzed under different rainfall intensity and continuous rainfall,and the relationship between soil erosion and SOC loss of cinnamon soil was clarified.Under the rainfall intensity of 30 mm/h,60 mm/h and 90 mm/h,for the 10 consecutive rainfall on slope,the cumulative sediment yield was 0.85 kg/m2,7.11 kg/m2,17.20 kg/m,,respectively,and the cumulative SOC loss was 4.44 g/m2,31.41 g/m2 and 72.15 g/m2,respectively.The relationship between SOC loss intensity and soil loss intensity was further analyzed and it was found that there was a most significant and positive correlation between them(R2=0.965,P<0.01).With the increase of rainfall intensity and rainfall duration,the SOC content of sediment decreased.The SOC content of sediment under 90mm/h rainfall intensity was 0.77 times and 0.90 times of that under 30mm/h and 60mm/h,respectively.The SOC content of sediment in the early stage of rainfall event was higher,and with the increase of rainfall duration,it showed a decreasing trend of logarithmic function,but the sediment yield under 60mm/h and 90mm/h rainfall intensity increased first and then decreased.Further analysis of the relationship between sediment SOC content and soil erosion intensity revealed logarithmic function relationship with significant negative correlation between them(R2=0.209,P<0.01).(2)Through the in-depth analysis of the hydrodynamic characteristics of runoff on the cinnamon soil slope during the erosion process,the hydrodynamic mechanism of SOC erosion on the cinnamon soil slope was revealed.During the whole rainfall erosion process of the cinnamon soil slope,the hydraulic characteristics of runoff were constantly changing with the variation of the underlying surface shape.The mean flow velocity and Froude number decreased exponentially with the increase of rainfall duration under of 30 mm/h and 60 mm/h rainfall intensity,and they both decreased with the power function under 90mm/h rainfall intensity,and the Darcy resistance coefficient both increased with the logarithmic function under 30,60,90 mm/h.The average flow velocity,shear force and runoff power had significant negative correlation with the SOC content,respectively,and had significant positive correlation with the SOC loss,respectively.The runoff shear force was a superior hydrodynamic parameter forecasting the SOC content of sediment.The average flow velocity was a superior hydrodynamic parameter forecasting the amount of SOC loss.(3)The characteristic and process of SOC erosion on loess soil slope were analyzed systematically under different rainfall intensity,slope gradient and slope length,and the relationship between loess soil erosion and SOC loss was clarified.With the increase of rainfall intensity,the amount of sediment yield and SOC loss on the loess soil slope increased significantly.Under the same rainfall intensity,the sediment yield and SOC loss on the loess soil slope increased with the slope length,and increased first and then decreased with slope gradient,and there was a critical slope(15°).The SOC loss intensity of the loess slope was significantly positively correlated with the soil erosion intensity(R2=0.772,P<0.01).The SOC enrichment ratio was higher in the initial stage of rainfall event,and it gradually decreased,and finally stabilized with runoff duration.There was significant linear relationship between the SOC enrichment ratio and the fine enrichment ratio in the sediment(R2=0.703,P<0.01),indicating that the SOC-rich and fine particulate matter during erosion process was preferentially transported by runoff and it leaded to the accumulation of SOC in sediment.The relationship between sediment SOC content and soil erosion intensity was found that there was logarithmic function relationship with significant negative correlation between them(R2=0.578,P<0.01).(4)The hydrodynamic mechanism of SOC erosion on the loess soil slope of the Loess Plateau was explored,and the better hydrodynamic index of loess soil SOC loss was revealed.The SOC loss was significantly and positively correlated with the average flow velocity,runoff depth,shear force and runoff power,respectively.The average flow velocity was a superior hydrodynamic parameter forecasting the SOC loss of loess soil.The SOC content of sediment had significant and negative correlation with the unit runoff power,and the unit runoff power was a superior hydrodynamic parameter forecasting the SOC content in loess sediment.