| Since the implementation of the national project of returning farmland to forest and grass in the Loess residual plateau and gully area,the vegetation coverage has been increasing,the physical and chemical properties of soil and soil structure have been improved,and the soil consolidation function of plant roots has been gradually strengthened after long-term comprehensive management for soil and water conservation,but there are still some problems of single vegetation type,simple structure and relatively low water and soil conservation benefits.Soil anti-scourability reflects the ability of soil to resist runoff erosion,which is an important aspect of soil erosion mechanism research in the Loess Plateau.In order to explore the ability of the main vegetation types in the Loess Plateau to improve the soil properties,enhance soil erosion resistance,and study the quantitative relationship of the impact factors on the soil anti-scourability,to provide a scientific basis for the stability of soil and water conservation benefits and the effective play of soil and water conservation functions of vegetation in the Loess Plateau,this paper takes the soil of main vegetation types,Pinus tabulaeformis forest,Robinia pseudoacacia forest,Armeniaca sibirica forest,Vitex negundo land and grassland in Caijiachuan watershed of Jixian County,Shanxi Province in the Loess Plateau Gully Area as research objects,the dynamic change of soil anti-scourability,the root morphology characteristics and the basic physical and chemical properties of surface soil under different vegetation and slope conditions were studied by using the outdoor undisturbed soil scouring method and the indoor root scanning analysis system and the determination of the physical and chemical properties of soil.The main factors affecting the surface soil anti-scourability model were constructed by using SEM structural equation model.The main research results are as follows:(1)In the process of undisturbed soil scouring experiment,the trend of runoff yield under different vegetation types and slope conditions was relatively stable with the prolongation of scouring time.The sediment concentration was high in the initial stage of runoff production.With the extension of scouring time,the sediment concentration decreased at first and then tended to be stable.The index of soil anti-scourability increased with the extended scouring time.The relationship between the index of soil anti-scourability and scouring time was linear or power function.(2)The differences of each index in the process of soil scouring experiment under different vegetation types and slope conditions are shown as follows: the runoff and sediment concentration of Robinia pseudoacacia forest and Pinus tabulaeformis forest land were lower,the index of soil anti-scourability was larger,showing stronger anti-scourability.The runoff and sediment concentration of wasteland,Vitex negundo land and Armeniaca sibirica forest land were larger,and the index of soil anti-scourability was smaller,the soil anti-scourability was relatively weak.The soil anti-scourability under different slope conditions was shown as 10°>15°>20°>25°>30°,which means that the soil anti-scourability under gentle slope was stronger than that on steep slope.(3)The total root length density,root surface area density,root volume density and root biomass density in surface soil under different vegetation types and slope conditions were generally larger in Robinia pseudoacacia and Pinus tabulaeformis forest land,smaller and similar in Armeniaca sibirica forest,Vitex negundo land and grassland.The total root parameters in gentle slope(10°?15°?20°)were larger than those in steep slope(25° ? 30°).There was a positive correlation between the total root parameters.The root length density of various diameters level decreased with the increase of root diameter.The change of root surface area density with root diameter showed different characteristics.The root volume density showed that the volume density of fine root was smaller than that of thick root.(4)The contrast experiment of remolded soil erosion under the condition of 10°showed that the roots of different vegetation types can enhance the soil anti-scourability,and the enhancement effect showed that Robinia pseudoacacia forest and Pinus tabulaeformis forest were stronger,and the enhancement values of soil anti-scourability were 4.16 and 3.44 respectively.The enhancement effect of Armeniaca sibirica forest,Vitex negundo land were weaker and the difference between them was not significant,which showed that 2.18 and 2.25 respectively.There was significant or extremely significant positive correlation between the root length density,root surface area density and root volume density ofroots with diameter less than 1 mm and the enhancement effects of soil anti-scourability.(5)The differences of physical and chemical properties of surface soil under different vegetation types and slope conditions are shown as follows:the soil bulk density of Pinus tabulaeformis and Robinia pseudoacacia forest land were smaller,the porosity were larger,the soil bulk density of Armeniaca sibirica forest,Vitex negundo land were larger,the porosity were smaller,and grassland were the largest and the smallest respectively,which reflects that the soil of Pinus tabulaeformis and Robinia pseudoacacia forest land has better aeration and permeability compared with other 3 vegetation types.The content of water stable aggregates in the surface soil of Pinus tabulaeformis,Robinia pseudoacacia and Armeniaca sibirica forest were higher than that of Vitex negundo land and grassland,especially under the steep slopes of 25°and 30°.The content of water stable aggregates showed that the steep slopes(25 °,30 °)were lower than that of gentle slope(10° ? 15° ? 20°).The index of soil mechanical composition of different vegetation types showed that the sand content of Pinus tabulaeformis,Robinia pseudoacacia and Vitex negundo land were lower and the content of silt and clay were higher than that of Armeniaca sibirica forest and grassland.The chemical properties of Pinus tabulaeformis,Robinia pseudoacacia and apricot were higher.(6)The results of SEM structural equation showed that the factors affecting the surface soil anti-scourability are followed by:external conditions,plant roots and soil properties.In the external conditions,different vegetation types had a great impact on the soil anti-scourability,which showed that the soil anti-scourability of Pinus tabulaeformis and Robinia pseudoacacia forest land were better.The soil anti-scourability of gentle slope was stronger than that of steep slope.The uniformity,richness and litter thickness of undergrowth plants have positive effects on soil anti-scourability.In the plant root index,the weight of each factor was in order of < 1mm root surface area density(RSAD),< 1mm root length density(RLD),total root biomass density(RWD),total root volume density(RVD).In the index of soil property,the influence weight is total porosity,water stable aggregate content,bulk density,organic matter,clay content and sand content.The results provide scientific basis for the improvement of soil and water conservation function of the vegetation root soil layer in Loess Plateau,and have practical and theoretical significance for the construction of local ecological environment. |
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