| In order to explore the improvement effects of polyacrylamide(PAM)and grass roots on purple soil in badlands,this article set up 9 treatments,including CK(blank control),PAM1(liquid application of 30 g/m~3 polyacrylamide),PAM2(liquid application of 60 g/m~3 polyacrylamide),GY(Cynodon dactylon planting alone),SY(Trifolium repens planting alone),GY-PAM1(C.dactylon planting and liquid application of 30 g/m~3 polyacrylamide),GY-PAM2(C.dactylon planting and liquid application of 60 g/m~3 polyacrylamide),SY-PAM1(T.repens planting and liquid application of 30 g/m~3 polyacrylamide)and SY-PAM2(T.repens planting and liquid application of 60 g/m~3polyacrylamide).Circular knives were used to take soil samples/root-soil systems from flowerpots in May 2016,July 2016,August 2017 and October 2017.Circular knife method and drying method were applied to analyze soil bulk density,moisture content and porosity of different treatments.Degree of aggregation,dispersion coefficient,MWD,GMD and fractal dimension(D)were used as evaluation indexes,and micro-aggregates distribution of different treatments were measured using Kachinsky method.Internal friction angle(φ)and cohesion(c)were used as evaluation indexes,and shear strength of soil samples/root-soil systems was measured using a direct shear test experiment.Using a root analysis system[WinRHIZO(Pro.2004c)],five diameter classes(d≤0.2 mm,0.2<d≤0.5 mm,0.5<d≤0.7 mm,0.7<d≤0.9 mm,d>0.9 mm)were divided for root-soil systems’root length,root surface area and root volume for statistical analysis.The main conclusions were as follows:(1)In different test months,there were differences in root system overall parameters and diameter parameters of root-soil systems.Root system overall parameters of“C.dactylon+PAM”were the best,followed by GY,while that root system overall parameters of SY was the worst.The order of root system overall parameters with different treatments was GY-PAM1>GY-PAM2>GY>SY-PAM1>SY-PAM2>SY.Root system diameter parameters of d≤0.2 mm was the best,followed by 0.2<d≤0.5 mm.All root system diameter parameters showed a decreasing trend with the increase of diameter class.There were also differences in root system parameters of different treatments among the same diameter classes.For example,root length density(RLD),root surface area density(RSAD)and root volume density(RVD)with the diameter class of d≤0.2 mm ranged from“C.dactylon+PAM”>GY>“T.repens+PAM”>SY,other diameter class were basically the same.(2)In different test months,there were differences in soil bulk density,moisture content and porosity of different treatments.Soil bulk density of different treatments was lower than that of CK,and moisture content was higher than that of CK.With liquid application of 30 g/m~3 polyacrylamide,soil bulk density showed“grass+PAM”<grass planting/polyacrylamide application,and liquid application of 60 g/m~3 polyacrylamide was basically the same.Soil bulk density of low-concentration PAM was lower than that of high-concentration PAM,showing PAM1<PAM1,GY-PAM1<GY-PAM2,SY-PAM1<SY-PAM2.In addition to CK,porosity indexes(total porosity,capillary porosity and non-capillary porosity)of the same treatment increased with the extension of test time,and the effect of“grass+PAM”on porosity indexes was the best.The diameter class of d≤0.2 mm was a critical diameter class that affects soil bulk density,moisture content and porosity,followed by the diameter class of 0.2<d≤0.5 mm.(3)In different test months,there were differences in micro-aggregates distribution,structural stability characteristics and fractal characteristics of different treatments.Compared with CK,the proportion of large particle size micro-aggregates of each experiment treatment increased while the proportion of small particle size micro-aggregates decreased.The content of 1~0.05 mm particle size micro-aggregates ranged from“C.dactylon+PAM”>“T.repens+PAM”>polyacrylamide application>grass planting>CK,of which GY-PAM1 had the largest content of this particle size micro-aggregates.1~0.25 mm particle size was the dominant particle size in different treatments,and 0.25~0.05 mm particle size was the sub-dominant one,while the content of<0.001 mm particle size micro-aggregates was the smallest.In different test months,degree of aggregation,MWD and GMD of different treatments ranged from“C.dactylon+PAM”>“T.repens+PAM”>polyacrylamide application>grass planting>CK,while dispersion coefficient was the opposite.The soil erodibility K of different treatments(except for CK),which based on the content of micro-aggregates,were not significantly different,but all tended to decrease with the prolongation of test time.The fractal dimension of micro-aggregates in different treatments(except for CK)also decreased.The diameter class of d≤0.2 mm was a critical diameter class that affects the micro-aggregates structural stability of purple soil in barren hillside badlands,followed by the diameter class of 0.2<d≤0.5 mm.RLD was the major factor affecting the micro-aggregates structural stability,followed by RSAD.(4)In different test months,there were differences in shear strength,internal friction angle and cohesion of different treatments.With the extension of test time,the shear strength,internal friction angle and cohesion of root-soil systems increased with different degrees,and these indexes of“C.dactylon+PAM”were the best.Shear strength,internal friction angle and cohesion all have highly significant or significant correlation with RLD,RSAD and RVD,which in the diameter class of d≤0.2 mm and 0.2<d≤0.5 mm.d≤0.2 mm diameter was a critical diameter class to strengthen shear resistance,followed by 0.2<d≤0.5 mm,and the other diameter class of the strengthening effect was smaller.RLD was the main factor to enhance shear resistance of root-soil systems,while RSAD was the secondary factor and RVD had little effect on the shear strength.Grass roots strengthening shear resistance of root-soil systems was mainly achieved by affecting the cohesion,rather than the internal friction angle. |
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