| Slope farmland of red soil is the important area for agricultural production.and soil erosion region in southern hilly region and southwest Yunnan-guizhou plateau area.The soil with a weak anti-erosion ability is loose,where rainfall displays an uneven characteristic and fertilization and tillage are inappropriate,binging a series of soil quality degradation problems,such as serious soil and water loss,nutrient deficiency and shallow soil layer.The cultivated-layer with fertile soil and loose texture is most easily eroded by rainfall,runoff,sheet flood and valley flow.Therefore,it is of great practical significance and strategic consideration to prevent soil erosion,improve soil quality,and achieve the maximum coordinated development of ecological and economic benefits.This paper analyzed physical and chemical properties and shear strength characteristics of l of slope farmland,influence factors of shear strength by principal component analysis and correlation analysis,the response characteristics of the shear strength to the soil moisture content,magnitude of dry-wet cycles,the dry-wet cycles,the soil bulk density and particle size distribution using red soil in Jiangxi,red soil in Guangxi and red soil in Yunnan.(1)Soil bulk density of cultivated-layers in three different types of red soil was different signifanctly,JR(red soil in Jiangxi)> GR(red soil in Guangxi)> YR(red soil in Yunnan)..Bulk density of cultivated-layers decreased with the increase of soil depth,0—20cm(1.16/ g·cm-3)> 20—40(1.23 g·cm-3)>20—40(1.31 g·cm-3).The total porosity of different cultivated layer on slope farmland was not different significantly.However capillary porosity was different significantly,with significant difference between 40—60 cm(GR)and 0—20 cm(GR),20—40 cm(GR),and with significant difference between 40—60 cm(YR)and 20—40 cm(YR).Initial infiltration rate and stable initial infiltration rate of 40—60 cm(JR)was 1.86 times and 2.89 times higher when compared with 20-40 cm(JR),Initial infiltration rate and stable initial infiltration rate of 40—60 cm(JR).Initial infiltration rate and stable initial infiltration rate of 40—60 cm(GR)was 2.60 times and 2.35 times higher when compared with 20-40 cm(GR),Initial infiltration rate and stable initial infiltration rate of 40—60 cm(JR).It suggested that there should be obvious plow bottom in the plough layer.Soil particle composition of cultivated-layers in three different types of red soil was dominated in soil silt and soil sand,soil clay was less than 30%.Soil mechanical stable aggregates of >5 mm and 5-2mm in in three different types of red soil was different significantly in the order as JR(45.30%)>YR(35.41%)>GR(33.75%),GR(45.30%)>YR(35.41%)>JR(33.75%),respectively..Water-stable soil aggregates of >5mm and >1mm decreased with soil depths in the order as 0—20 cm> 20—40 cm> 40—60 cm.Soil micro aggregates in three different types of red soil was dominated in the fractions of 0.25~0.05 mm and 0.05 ~ 0.01 mm.(2)The content of soil organic matter,total nitrogen,total phosphorus,available nitrogen and available phosphorus of cultivated-layers in three different types of red soil increased significantly with soil depths.Soil organic matter :0—20cm(13.67 ~ 32.65 g/kg)> 20—40cm(6.49 ~ 22.77 g/kg)>40—60cm(3.23 ~ 10.75 g/kg),total nitrogen:0—20cm(1.0~1.6 g/kg)> 20—40cm(0.5 ~ 1.0 g/kg)>40—60cm(0.5 ~ 0.7 g/kg),total phosphorus:0—20cm(0.5 ~ 1.2 g/kg)> 20—40cm(0.2 ~ 0.8 g/kg)>40—60cm(0.2 ~ 0.6 g/kg),available nitrogen:0—20cm(90.5~ 173.0 mg/kg)> 20—40cm(46.3 ~ 151.45 mg/kg)>40—60cm(32.7 ~ 63.5 mg/kg),available phosphorus:0—20cm(19.5 ~ 37.5 mg/kg)> 20—40cm(3.1 ~ 6.0 mg/kg)>40—60cm(2.9 ~ 3.5mg/kg).(3)Soil shear strength of cultivated-layers in three different types of red soil increased periodically,it increased at first then keet stable and decreaced rapidly when the soil depth was greater than 0-40 cm.The shear strength increased linearly with depth in stage I(JR,YR :0-40 cm)varing from 3.58 to 5.45 kPa and vaying from 3.50 to 5.55 kPa,respectively.The shear strength increased stabilised in stage ?(JR,YR : 40-60 cm)varing from 5.01 to 5.78 k Pa and changing from 5.55 to 5.99 kPa,respectively.The shear strength increased linearly with depth in stage I(GR :0-30 cm)varing from 3.58 to 5.45 kPa and vaying from 3.58 to 5.54 kPa,respectively.The shear strength increased stabilised in stage ?(GR : 30-60 cm).Based on Pearson correlation analysis,the result showed that there was a significant positive correlation between shear strength and bulk density(0.73),with a significant correlation between shear strength and total nitrogen(-0.72),total phosphorus(-0.71),available phosphorus(-0.79)with regard to JR.There was a significant correlation between shear strength and bulk density(0.73)and moisture conte(0.77),with a significant correlation between shear strength and organic matter(-0.92),total nitrogen(-0.91),total phosphorus(-0.93),available phosphorus(-0.86),alkali hydrolyzable nitrogen(-0.90)with regard to GR.There was a significant negtive correlation between shear strength and MWD(-0.72),moisture conte(-0.78)and organic matter(-0.69)with regard to GR.Soil bulk density,water content,soil organic matter,total nitrogen,total phosphorus and available phosphorus were the main factors affecting the shear strength of red soil.Based on the principal component analysis,the results showed that the influencing factors of shear strength of red soil were soil organic matter,total nitrogen,alkali hydrolyzable nitrogen,total phosphorus,available phosphorus,physical factor dispersion coefficient,water content,physical clay,MWD of wet sieve.(4)The shear strength of JR decreased nonlinearly with the water content.The shear strength increased slightly and then decreased to a stable value under the pressures of 100 kPa and 200 kPa;the shear strength decreased nonlinearly with moisture content,it tended to be stable in stage?(11% ~ 19%)and decreased rapidly in stage?(300 kPa:19%~21%,400 kPa:19%~23%),it kept stable in stage ?(300 kPa:21% ~ 27%,400kPa:23%~27%).The shear strength of GR decreased nonlinearly with the water content.It fluctuated with the water content under the pressures of 100 kPa.It fluctuated with the water content in stage?(11%~ 17%)and decreased rapidly in stage?(17%~ 27%),then it stablized when moisture content was more than 25%.The cohesive force of cultivated-layers in three different types of red soil increased first and then decreased with the increase of water content,.the peak values of the shear strength were reached when water content lied between 11%~13%.Cohesive force of cultivated-layers in JR increased rapidly when water content varying from 11% to 21% and it gradually decreased when water content exceded 21%.Cohesive force of cultivated-layers in GR and YR increased rapidly when water content varying from 11% to 23% and it gradually decreased when water content exceded 23%.The internal friction angle of cultivated-layers in three different types of red soil decreases nonlinearly with the increase of water content.The internal friction angle decayed linearly when water content of JR,GR and YR was lower than 19%,17% and 15%,respectively.When water content was higher than the turning point(19% in JR,17% in GR and 15% in YR),the internal friction angle of JR and GR decreased with high volatility,however the internal friction angle of YR stablized.(5)The cohesive force decayed periodically with cycles of drying and wetting in two stages,decaying rapidly in stage?(0 ~ 3,4)and tending towards stability in stage?(3,4 ~ 5).The cohesive force of specimens on the fifth-cycle drying and wetting was 1.55 to 2.33 kPa,which was 20.0% to 29.8% times of specimens on initial cycle.Interal friction angle decreased with fluctuations with cycles of drying and wetting.In terms of interaction effect between number of cycles and cyclic amplitude,cohesive force increaseed slightly and then decreases with the cyclic amplitude,and it peaking when cyclic amplitude was 15%.And contribution of number of cycles to cohesive force was higher than that of cyclic amplitude.Interal friction angle decreased with fluctuations with cycles of drying and wetting,which was 87.2% to 94.0% times of cohesive force of specimens on the initial cycle after fifth-cycle drying and wetting.The internal friction angle varied violently with cyclic amplitude,in a sense it was not impaced by the cyclic amplitude.cohesive force of YR increased slightly and then decreases with the cyclic amplitude,and it peaking when cyclic amplitude was 15%.And contribution of number of cycles to cohesive force was higher than that of cyclic amplitude.Interal friction angle decreased with fluctuations with cycles of drying and wetting,which was 87.2% to 94.0% times of cohesive force of specimens on the initial cycle after fifth-cycle drying and wetting.The internal friction angle varied violently with cyclic amplitude,in a sense it was not impaced by the cyclic amplitude.(6)The cohesive force of cultivated-layers in three different types of red soil increased gradually with soil bulk density increasing.When bulk density was low(JR:0.9~1.1g/cm3,GR:0.9~1.0g/cm3),cohesive force increased gradually.However when bulk density was high(JR: 1.1~1.3g/cm3,GR: 0.9~1.1g/cm3),cohesive force increased rapidly.The internal friction angle of cultivated-layers in three different types of red soil increased with the increase of bulk density,the trends of internal friction angle in three different types of red soil showed: ?(YR)> ?(JR)>(GR).The internal friction angle of JR,YR and GR varied among 7.45°~ to 8.21°?8.45° ~ 9.74°6.46° ~ 7.96°.The particle size distribution has no significant effect on the cohesive force,however has no significant effect on the internal friction angle.The cohesive force of cultivated-layers in three different types of red soil varied in the order as A > C > D > B(JR),B > A > D > C(GR),A > B > D > C(YR),with the cohesive force ranging among 5.57 ~ 5.78 kPa?3.67 ~ 4.10 kPa?3.40 ~ 4.33 kPa,respectively.The internal friction angle increased with the particle size distribution and the internal friction angle of GR and JR was more susceptible to particle size distribution than than of YR..The the internal friction angle of cultivated-layers in three different types of red soil varied in the order as D(8.67°)> B(8.30°)> C(7.86°)> A(7.57°)(JR),D(8.67°)> C(8.45°)> B(8.22°)> A(7.38°)(GR),D(9.33°)> B(9.29°)> C(8.99°)> A(8.97°)(YR)?... |
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