| Plant root system plays important role in soil erosion control, To investigate characteristics of root systems of herb vegetation for erosion control in the Three Gorges Reservoir region and their influences on slop reinforcement and soil erosion resisitance, We used bare land as a control,4typical herb plants of erosion control, Vetiveria zizanioides(Lin.) Nash, Paspalum notatum Flugge, Cynodon dactylon(L) Pers. and Medicago sativa L., were selected and investigated in the experiments. Circular knives were used to take samples from the test area for analysis. We measured soil initial infiltration, average infiltration, stable infiltration and accumulative water quantity as soil infiltration capability, Double-rings method was applied to analyze the soil infiltration. Direct shear apparatus was applied to analyze soil shear strength. Root analysis system (WinRHIZOPro.2004c) was used to characterize the root systems. The results indicated that:(1) Vetiveria zizanioides(Lin.) Nash had the largest root length density and root surface area density. Root length density, root surface area density and root weight density of all typical herb plants were decreased with the increasing of soil depth, but the difference on root length density, root surface area density and root weight density among different herb plants was not significant with the increasing of soil depth. The root length density, root surface area density and root weight density were0.92cm/cm3,0.29cm2/cm3and2.10mg/cm3for Cynodon dactylon(L) Pers.,1.01cm/cm3,0.77cm2/cm3and34.72mg/cm3for Medicago sativa L.,8.12cm/cm3,3.12cm2/cm3and27.70mg/cm3for Vetiveria zizanioides(Lin.) Nash and1.66cm/cm3,0.71cm2/cm3and9.26mg/cm3for Paspalum notatum Flugge, respectively.(2) As soil depth increased, soil porosity, non-capillary porosity, water capacity and saturated water holding capacity decreased, but soil bulk density increased; the improvement of soil moisture was mainly realized by roots of diameters in the range of0.5mm<D≤1mm and1mm<D≤2mm (L stands for root diameter)(3) The soil infiltration rates were significantly amongst the different herb plants and at different soil layers under the same herb plants. The soil infiltration rates of soil under herb plants were significantly higher that of bare soil and declined with the increase in soil depth. The soil infiltration rates from high to low were in the order of Vetiveria zizanioides(Lin.) Nash, Medicago sativa L., Paspalum notatum Flugge, Cynodon dactylon(L) Pers. The initial, average and steady infiltration rates were2.42,1.65and1.38mm/min for Cynodon dactylon(L) Pers.,3.87,2.42and2.11mm/min for Medicago sativa L.,4.84,3.37and3.00mm/min for Vetiveria zizanioides(Lin.) Nash and2.93,1.79and1.39mm/min for Paspalum notatum Flugge and1.31,0.86and0.71mm/min for bare land respectively., The soil initial, steady and average infiltration rates as well as cumulative infiltration were increased with increases in root length density and root surface area density, which were evidently correlated with root diameter from0.50to5.00mm. Root length density and root surface density had significant impacts on Kostiakov’s model parameters, K and a. The K value, representing initial infiltration rate increased with increase in root length density and root surfer area density and the a value, representing the decrease rate of infiltration decreased with them. The results will be useful for hillslope vegetation protection and soil erosion control.(4) Soil shear strength were significantly different in4herb plants, Even in the same herb plant it had different patterns with the difference of soil layers. The internal friction angle f and cohesion c declined with the increasing of soil depth. We found that the internal friction angle f and cohesion c of all herb plants were higher than that of bare land. The sequence from high to low were: Vetiveria zizanioides(Lin.) Nash> Paspalum notatum Flugg> Medicago sativa L.> Cynodon dactylon(L) Pers. The internal friction angle f, cohesion c and shear displacement were14.79°,15.96k Pa and3.43mm for Cynodon dactylon(L) Pers.,17.44°,20.93k Pa and4.89mm for Medicago sativa L.,24.04°,31.36k Pa and2.88mm for Vetiveria zizanioides(Lin.) Nash and18.71°,30.34k Pa and3.63mm for Paspalum notatum Flugge and9.45°,14.18k Pa and5.51mm for bare land, respectively. The internal friction angle f was logarithmic growth and cohesion c was linear growth with the increase of root length density and root surface area density, and soil shear strength had obvious correlation relationship with root systems that the diameter was less than5mm. It is concluded that root length density and root surface area density are the best indexes to characterize soil shear strength, especially root length density and root surface area density of roots with D≤5mm.(5) We found that the ANS of all herb plants were higher than that of bare land. The sequence from high to low were:Vetiveria zizanioides(Lin.) Nash (3.17min/g)> Paspalum notatum Flugg (2.52min/g)>Cynodon dactylon(L) Pers.(1.78min/g)>Medicago sativa L.(1.41min/g); The ANS had a significant positive linear correlation with organic matter content; With the increase of the initial, f and steady infiltration rates ANS showed the power function growth. ANS also had the logarithmic function growth with the increase of c. The ANS and root length density and root surface area density were the unary square multinomial change. And the ANS had obvious correlation relationship with root systems that the diameter was less than5mm. The stepwise regression analysis between ANS and the different parameters showed that ANS had good linear relation with cohesion c and root surface area density that the diameter was less than5mm (multiple correlation coefficient R2=0.980). |
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