Study Soil Anti-scourability And It's Influencing Factors In Purple Soil Hilly Region

Because of the influencing of natural factor and human activities, the ecological environment of the Three Gorges Reservoir Area is very frail. In this area the land resources is short of and soil and water loss is serious. Purple soil land is the important agricultural resource of the Three Gorges Reservoir Area, at the same time purple soil is the primary source of soil and water loss. After the construction of the Three Gorges Reservior, the conflict between human and land resources will be more outstanding and soil and water loss will be more serious, too. So researching on the erosion mechanism and regulation of purple soil has important significance for soil and water conservation and ecological environment construction in this area. The erosion process is the interactional course between erosion motivity and erosion object. Soil as the erosion object, it's anti-erosion capability is the inherent factor influencing the process of soil erosion. Soil anti-scourability is the capability that soil counteracts the mechanical destroy and pushing power of runoff, and it is the very important index to evaluate the soil anti-erosion capability , and it also can be the index for evaluating the soil anti-erosion capability under different areas,different underlayer condition and different land use types. Study on soil anti-scourability is very significative for the further studies of soil erosion mechanism and also can provide academic warranty for small watershed treatment. This paper studied on the characteristic and influencing factors of soil anti-scourability of different land use types in purple soil hilly region by used the methods of undisturbed soil sample washing and soil physical and chemical properties analysis indoor. The main results are as follows:1. The dynamic change regulation of the sediment concentration and the index of soil anti-scourability in the process of scouring under different land use types. During the process of scouring, the sediment concentration of all land use types was high at the beginning, then decreased gradually along with the time. The sediment concentration of tree-crop intercropping land, orchard and bamboo forest land had steady trend after decreased, while the sediment concentration of traditional farmland had a increasing trend when it decreased to a certain extent. This can indicate that the surface layer soil loss of traditional farmland will further develop and have latent danger. During the scouring process, the sediment concentration were traditional farmland (0.81g/L) > tree-crop intercropping land(0.73g/L)> orchard (0.69 g/L)>bamboo forest land(0.58 g/L). The index of soil anti- scourability increased gradually along with the time. The index of soil anti- scourability ANS had remarkable quadratic correlation with time t. The change regulation of the index of soil anti-scourability along with the time obeyed the quadratic equation: ANS=at²+bt+c.2. The soil anti- scourability of different land use types in purple soil hilly region. The index of soil anti-scourability were bamboo forest land (1.37 min/g)> orchard(0.99 min/g)> tree-crop intercropping land(0.92 min/g)> traditional farmland(0.70 min/g). Forestland and agroforest compound models are efficacious for enhancing the soil anti- scourability. In purple soil hilly region changing the traditional planting models, bringing conversion cropland to forest project into effect and extending agroforest compound models is the effective measures to protect soil resource, reduce sediment source of Yangtse Rive and pledge the ecological security of the Three Gorges Reservoir Area.3. Soil physical properties influencing the soil anti- scourability. Soil bulk density and soil porosity had remarkable correlation with the index of soil anti-scourability, and the respective correlation coefficient is 0.552 and -0.553. The coarse sand (1～0.25mm) content had great remarkable positive correlation with the index of soil anti-scourability and the correlation coefficient is 0.615. The >0.25mm water stable aggregates content had great remarkable positive correlation with the index of soil anti-scourability and the correlation coefficient is 0.572. The initial water infiltration rate had remarkable positive correlation with the index of soil anti-scourability and the correlation coefficient is 0.517.4. Soil chemical properties influencing the soil anti- scourability. The correlation between soil organic matter content, total nitrogen content, total phosphorus content and total potassium content and the index of soil anti-scourability is not remarkable. This showed that soil chemical properties have little effect on the soil anti- scourability.5. The linear regression equation of the index of soil anti-scourability establishing. Based on analysis of the correlation between soil physical and chemical properties and the index of soil anti-scourability, we chose soil porosity (X₁) , coarse sand (1～0.25mm) content (X₂) , >0.25mm water stable aggregates content (X3) and initial water infiltration rate (X4) to carry on the regression analysis, and established the regression model: ANS=2.847-0.058X₁+0.003X₂+0.018X₃+0.136X₄.6. The characteristic of root distribution. The average root density is 60.85 roots/100cm².The root length of different root diameter decreased in the following order: <0.5mm root >0.5～1mm root>1～3mm root> >3mm root, and the length of <0.5mm root took the 92.52% of the total length of root. The root biomass of different root diameter was <0.5mm roots >1～3mm roots> >3mm root X).5～1mm root, and The root biomass of <0.5mm root took the 42.42% of the total root biomass. <0.5mm root is the main part of all root diameter in purple soil.7. The root characteristic under different land use types. The average root density of different land use types decreased in the following order: bamboo forest land > orchard> tree-crop intercropping land> traditional farmland. The length of<0.5mm root was in order of bamboo forest land (1732.21cm) > tree-crop intercropping land (394.97cm)>orchard(281.55cm) > traditional farmland(248.16 cm). The length of<1mm root decreased in the following order: bamboo forest land > tree-crop intercropping land >orchard > traditional farmland. The length of<3mm root had the same trend that bamboo forest land (1874.91cm) > tree-crop intercropping land (413.53cm) >orchard (306.15cm) > traditional farmland (271.68 cm) . The root biomass of <0.5mm root was in order of bamboo forest land > tree-crop intercropping land > traditional farmland > orchard. The root biomass of <1mm root decreased in the following order: bamboo forest land (0.7568g) > tree-crop intercropping land (0.1238g) > orchard (0.0774g) > traditional farmland (0.0631g) .And the root biomass of<3mm root was in order of bamboo forest land > orchard > tree-crop intercropping land > traditional farmland.8. The increased effect of soil anti-scourability by root system and the quantitative relationship between the increased effect and the root system under different land use types. The intensified values of soil anti-scourability by root system (ΔANS) were bamboo forest land (0.86 min/g)> orchard(0.48min/g)> tree-crop intercropping land(0.41 min/g)> traditional farmland(0.19 min/g).And the intensified percentage had the same trend that bamboo forest land (167.84%)> orchard(94.51%)> tree-crop intercropping land(79.61%)> traditional farmland(37.26%). The intensified values of soil anti-scourability (ΔANS) had remarkable positive power function correlation with the root biomass in different diameter (<0.5mm,<1mm,<3mm).The correlation coefficient between the root biomass of <1mm root system and the increased effect of soil anti-scourability (ΔANS) was the highest which indicate that <1mm root system was the main factor influencing soil anti-scourability. The correlation between the root length in different diameter and root density and the intensified values of soil anti-scourability is not remarkable.