| The Three Gorges area is prone to soil erosion due to topography and meteorological factors.The preferential flow generated in the macropores can accelerate the surface hydrological cycle,improve soil infiltration performance,reduce surface runoff and reduce soil erosion.But human activities can easily affect the structure and distribution characteristics of soil macroporcs.The study aimed to investigate the effccts of human activities on the basic properties of soil,macropore distribution and preferential flow.We selected four different levels of human disturbances(vegetation destruction,tillage and trampling),and the intensity of human disturbance increased with elevation lowering.Lowest area covered with coniferous trees(LF)>middle mountain areas covered with tea garden(TG)>middle mountain areas covered with deciduous trees and mixed shrubs(MF)>subalpine areas covered with evergreen coniferous trees(HF).We used CT scanning technique to quantity the characteristic parameters of soil macropores,solute breakthrough curve experiment and model fitting to determine the preferential flow patterns and obtain different water flow characteristic parameters.Quantitative analysis of the relationship between basic physical and chemical properties of soil,soil macropores and water flow characteristics,and analysis of the effects of different levels of human disturbance on preferential flow.The dye tracing experiment was used to analyze the preferential flow morphological characteristics of each sample site,and the difference between the three methods.The main conclusions obtained are as follows:(1)Human activities also significantly changed the physical and chemical properties of the soil.The bulk density of LF(1.24 g/cm3)>TG(1.12 g/cm3)>HF(0.88 g/cm3)>MF(0.86 g/cm3),and the bulk density increases with the increase of depth.The soil organic matter content and root length density were both HF>MF>TG>LF,and the organic matter contents were 50.61,42.50,28.61 and 23.19 g/kg,respectively,which gradually decreased with the increase of depth.The root length density was 1.02,0.81,0.33 and 0.23 cm/cm3,respectively.The changes of soil clay content in the four samples were HF>MF>TG>LF,and the sand content was opposite.Soil properties have a very important influence on saturated hydraulic conductivity and water infiltration.The highest saturated hydraulic conductivity is HF(66.28 ctn/h),MF(61.60 cm/h),TG(18.45 cm/h)and the smallest LF(10.92 cm/h).(2)Human disturbance will rapidly change the structure of soil macropores,and the CT scanning technique is used to quantify the soil macropore characteristics.Macroporosity,surface area density,macropore number density,length density and node density are all sorted by HF>MF>TG>LF.Mean macroporosity is 0.115,0.111,0.037,and 0.015 m3/m3 mean surface area density is 279.43,205.69,93.98 and 44.82 m2/m3,mean macropore number density is 20.74,14.76,11.28 and 7.40×105 N/m3,mean length density is 34.47,29.59,5.23 and 2.19 km/m3,mean node densities is 33.73,26.65,6.16 and 3.36×106 N/m3,respectively.This shows that human disturbances affect the distribution and structure of macropores.(3)The time required for the solute breakthrough curve to penetrate the soil on average is LF>TG>MF>HF,the faster the penetration occurs,the higher the relative concentration of the outflow.Both the equilibrium convection dispersion model(CDE)and the non-equilibrium mobile-immobile model(MIM)can fit the breakthrough curve,However,the average coefficient of determination of MIM fitting Rm2 is between 0.94 and 0.98,which is higher than the equilibrium CDE model(R2 is between 0.90 and 0.94).There are widespread preferential flows in HF and MF,the water flow situation of TG and LF tends to be homogeneous,human disturbance can affect the form of soil water flow.(4)The correlation between soil basic physical and chemical properties,soil macropore characteristics and penetration curve fitting parameters proves that there is a significant(P<0.001)negative correlation between bulk density and macroporosity,surface area density,pore size,pore number density and average angle,significantly(P<0.001)positively correlated with the hydraulic radius,organic matter and root length density are opposite with bulk density.There was a significant negative(P<0.001)correlation between soil bulk density and average pore water velocity(V),solute dispersion coefficient(D),average pore water velocity of the mobile region(Vm)and solute dispersion coefficient of the mobile region(Dm),organic matter and root length density are opposite with bulk density.Macroporosity,macropore surface area density and macropore size have significant(P<0.001)positive correlations with the breakthrough fitting parameters V,D,Vm and Dm.Long-term human trampling,cultivation or deforestation will lead to an increase in soil bulk density,a decrease in vegetation roots and soil organic matter content.It is difficult to form a stable macropores structure,and the number of macropores is reduced,which weakens the preferential flow of soil.(5)The stained area ratio of the four sites is LF(38.61%)>MF(35.23%)>TG(33.14%)>HF(20.12%),The morphology of HF and MF with less human disturbances stained areas is relatively close,the stained area of TG is very evenly distributed and has a wide range,and the stained area of LF site at the junction of the 10-20 cm soil layer and the parent layer is very high,and the water flow flows preferentially along the rock gap.The results obtained by the dye tracing experiments are inconsistent with the soil macropore characteristics and the water flow characteristic parameters fitted by the CT scan,the main reason for the difference is that the resolution of the CT scanner,richness of soil preferential flow path,degree of development of macropores,water repellent phenomenon caused by high soil organic matter content,the distribution of the soil layer,and there is no repeated experiment.Therefore,the dye tracing experiment has certain limitations in judging the characteristics of the preferential flow,It is only applicable to the comparison of preferential flow characteristics between the same soil type,And intuitive observation of the migration characteristics of preferential flow in the soil,It is not possible to accurately quantify the structure of large pores in the soil and the characteristics of water infiltration.Studies have shown that human disturbance(vegetation damage,tillage and trampling)can significantly reduce macropores in forest soil,reduce preferential flow,increase surface runoff,and reduce soil fertility. |
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