Simulation Study On The Mechanisms Of Soil Erosion And Nutrient Losses On The Eroded Weathered Granite Slopes

The mountainous and hilly region of weathered granite in Southern China is one of the most ecologically sensitive and fragile areas.The weathered granite slopes in Zhejiang Province are widely distributed with high agricultural utilization.The loss of soil and water nutrients is serious under the combining conditions of heavy rainfalls and steep slopes.Accelerated erosion are prone to occur with the interference of human activities,affecting the sustainable development of ecological environment and social economics.It is of great significance to carry out researches on the mechanisms of soil erosion and nutrient losses in these areas.To reveal the dynamic characteristics and mechanisms of soil erosion,nitrogen(N)and phosphorus(P)losses on the weathered granite slopes,three typical eroded weathered granite slopes exposed with tillage layer(E1),laterite layer(E2)and sandy layer(E3)were selected as the research objects,for which the order of erosion degree was E1(27)E2(27)E3.Simulated rainfalls combining different rainfall intensities(30,60,90,120 and 150mm/h;RI)and slope steepness(8°,15°and 25°;SS)were conducted by employing the methods of field survey sampling,undisturbed soil removal and artificial rainfall simulation.The dynamics of soil erosion and the process of N and P losses were studied,and the hydrodynamic parameters of surface flow and interflow were calculated.The particle size distribution of eroded sediment and the loss forms as well as the paths of total nitrogen(TN)and total phosphorus(TP)were analyzed to systematically explore the dynamic characteristics of soil erosion and the influence of different factors on the nutrient loss amount and paths.The main results are as follows:(1)The hydrodynamic characteristics of rainfall runoff were closely related to the affecting factors including RI,SS and erosion degree.Runoff rate of surface flow(Rr-_S)showed a trend of increasing firstly and then stabilizing with the prolongation of runoff time,while the runoff rate of interflow(Rr-_I)changed as a single-peak curve with the trend of“increasing-stable-decreasing”.The form of runoff generation for surface flow gradually changed from infiltration excess runoff to saturation excess runoff with the prolongation of runoff time.Rr-_Sand the total discharge of surface flow(Q_S)positively correlated to RI but negatively correlated to SS.Rr-_Iand the total discharge of interflow(Q_I)were all positively correlated to SS.The measured velocity of surface flow(V_0-S)was linearly positively correlated to RI(p<0.05),and the measured velocity of interflow(V_0-I)was much smaller than V_0-S.Exponential functions could be used to characterize the relationships between V_0-Iand RI(p<0.05).Rr-_I,Q_Iand V_0-Ifor E2 were much smaller than that for E3.The average depth of surface flow(h)was negatively related to erosion degree and SS,but positively related to RI.Both Reynolds number(Re)and stream power(w)increased with increasing RI and SS with an order of E1>E2>E3,and Froude number(Fr)also increased with increasing RI.The trend of runoff shear stress(?)changing with RI was almost the same to that of h but it increased with increasing SS.Q_Swas highly linearly positively correlated to the average runoff velocity of surface flow,Re and w(p<0.01).Surface flow was laminar flow(Re<500)and slow flow(Fr<1),the velocity and pattern of which was greatly affected by erosion degree.The proportion order of interflow was E3>E2>E1,especially the rainfall runoff was dominated by interflow for E3 that accounted for more than 75.68%,indicating that interflow was more easily to produce on the slopes with greater erosion degree.(2)The dynamic indexes and grain size characteristics of eroded sediment were significantly different on the three eroded slopes with different erosion degree.The yielding rate(Sr)and loss concentration(Sc)of eroded sediment decreased with the prolongation of runoff time with slight fluctuations.The total loss amount of eroded sediment(S_L)increased with increasing RI and SS with an order of E1>E2>E3 while the order for Sc reversed.S_Lon different eroded slopes presented good linear correlations with V_S,?and w.Both the mean weight diameter(MWD)and enrichment ratio(Er)of fine sand-sized particles for eroded sediment positively correlated to erosion degree and extremely significantly positively correlated to RI(p<0.01).MWD positively correlated to all different hydraulic parameters of surface flow,among which the correlations to h,Fr and SS were the weakest.The correlation results of the three eroded slopes were significantly different.The actual effect of surface roughness on sediment transportation depended on the balance of different erosion processes.Large-sized particles(0.02-2 mm)of eroded sediment migrated through bed-rolling and jumping while small-sized particles(<0.02 mm)were transported in agglomerates via suspension.The composition of eroded sediment was richer in fine particles.The total erosion load was relatively large on the slopes producing interflow.Therefore,the effects of erosion degree of the original soils should be taken into consideration when conducting erosion prediction and modelling.Protecting the upper soils such as the topsoil from the tillage and laterite layers is the key to slowing down the deeply further processes of soil erosion on the eroded weathered granite slopes.(3)The process characteristics and paths of TN and TP losses were obviously different.TN loss concentration(C_TN)and TP loss concentration(C_TP)in surface flow sharply decreased firstly and then stabilized with the prolongation of runoff time.The total TN loss load(L_TN)and TP loss load(L_TP)in surface flow increased with increasing RI.L_TNin surface flow presented exponential relationships with RI(R²>0.79,p<0.05),while the relationships between L_TPin surface flow and RI on the8°,15°and 25°slopes could be characterized respectively by binomial functions(R²>0.98,p<0.05),power functions(R²>0.98,p<0.01)and logarithmic functions(R²>0.87,p<0.05).C_TNin interflow for E3 was larger than that for E2,but the order for C_TPin interflow reversed.The relationships between L_TNin interflow and RI could be characterized respectively by binomial functions(R²>0.62)and logarithmic functions(R²>0.94,p<0.01)for E2 and E3.As for L_TPin interflow for E2 and E3,the relationships could be characterized respectively by binomial functions(R²>0.80)and power functions(R²>0.92,p<0.01).C_TNin interflow was higher than that in surface flow while the order for C_TPreversed.Runoff TN was mainly lost in dissolved form and the most important part was NO₃^--N with NH₄⁺-N was an important supplementary form.The total amount of NO₃^--N and NH₄⁺-N accounted for more than 67.78%of the total TN loss.The morphological characteristics of TP loss in rainfall runoff were significantly different on the three eroded slopes.TP in surface flow was mainly lost in the form of DP on E1 and E2 slopes,and it was dominated by DP under light rainfalls but by PP under medium and heavy rainfalls on the E3 slope.TP lost via interflow was dominated by DP on E2 and E3 slopes while PP was an important supplementary form.C_TNand L_TNin eroded sediment presented an order of E1>E2>E3 while the order for C_TPand L_TPchanged alternately under different rainfall intensities.The relationships between L_TN,L_TPin eroded sediment and RI could be characterized respectively by power functions(R²>0.86,p<0.05)and exponential functions(R²>0.91,p<0.05).In general,interflow was the most important TN loss path(>50%)for E2 and E3,while the dominant loss path for E1 was surface flow.L_TPin eroded sediment was higher than that in rainfall runoff,indicating that eroded sediment was the predominant TP loss path(>49.26%),which was significantly different from TN loss characteristics due to the difference in N and P adsorption-desorption mechanisms.TN was mainly lost in the dissolved form with rainfall runoff,while TP was mainly transported through eroded sediment by being adsorbed on the soil surface in the form of particle phosphorus.(4)The results of model analysis showed that different influencing factors had a significant promotion effect on N and P migrations via different loss paths.N/P(mass ratio)in surface flow decreased with the prolongation of runoff time and the order was E1>E2>E3.N/P values in interflow were much greater than that in surface flow and it increased with the prolongation of runoff time with an order of 8°>15°>25°on the three eroded slopes.N/P in interflow for E2 was greater than that for E3.The N/P order among different loss paths was interflow>surface flow>eroded sediment.According to the values of N/P,N worked on nutrient limitation in surface flow and eroded sediment while P worked on nutrient limitation in interflow.The fitting effects of the statistical models between L_TN,L_TPand RI,SS were relatively good(p(27)0.01).Both L_TNand L_TPwere positively correlated to RI and SS,indicating that increasing RI and SS promoted nutrient losses.Therefore,engineering and plant protective measures should be taken to alleviate the increasing rate of soil erosion and nutrient losses on the eroded weathered granite slopes.