Soil Moisture Movement And Hydrochemical Changes In Slope Land Of Karst Trough Valley And Its Influencing Factors

Soil is an important water storage in Karst ecosystem,and soil water is the main limiting factor of karst ecological environment,which is closely related to social and economic development.However,because of its special hydrogeological background,the soil in karst area is significantly different from that in non-karst area.For example,the soil layer is thin and discontinuous,the soil is easy to lose,the nutrients are easy to lose,and the soil water retention ability is poor.The characteristics of soil water movement may be different from that in non-karst area.Therefore,the study on the mechanism of soil water movement in karst areas is conducive to provide reference for the rational utilization of soil water resources in karst trough valley areas,and also provides scientific basis for the prevention and control of groundwater pollution in karst areas.In this study,Longfeng karst valley in Chongqing was selected as the research area.Eight sample points were selected on slope lands on both sides of the valley and at the bottom of the valley(upper slope position of the bedding slope V₁ and V_shi,lower slope position of the bedding slope V₂,bottom of the valley V₃,lower slope position of the inverse slope V₄ and V₅,upper slope position of the inverse slope V₆ and V₇).On the basis of detailed investigation of the basic conditions of each sample point?geological background,topography,vegetation and soil physical and chemical properties,etc.?,the synchronous hydrogen and oxygen isotope monitoring of atmospheric precipitation and soil water as well as the hydrogen and oxygen isotope monitoring of soil water and soil moisture content at each sample point after rain were carried out.Based on the method of hydrogen and oxygen isotopic tracer and traditional hydrological observation,the law of soil water movement in karst valley region was explored through the comparative analysis of atmospheric precipitation and soil water,as well as the temporal change of soil water hydrogen and oxygen isotopes and water content after rain.The process of soil water movement in karst valley region was revealed by the distribution characteristics of soil water hydrogen and oxygen isotopes at different slope positions,and based on the characteristics of soil water chemistry and soil water ion leaching at different slope positions,the process of soil water movement on slope land was discussed.It was found that:?1?Spatial and temporal change of soil water?¹⁸O and its influencing factorsIn terms of time,the seasonal change of soil water?¹⁸O was slightly positive in the early rainy season and the late dry season,and negative in the late rainy season and the early dry season,which was related to the negative seasonal change in the rainy season and positive seasonal change in the dry season.However,the seasonal dynamics of soil water?¹⁸O lagged behind the precipitation for 1 to 2 months,and the lag time increased with the increase of soil depth.In terms of space,soil water?¹⁸O had the spatial change characteristics of gradual enrichment from lower slope position to upper slope position.Because the speed of soil water movement in the upper slope position was faster than that in the lower slope position,and the soil water content in the upper slope position was lower,so the retained water in the soil was mainly the part of the soil that is difficult to movement,and soil water?¹⁸O was enriched by evaporation for a long time.Soil water?¹⁸O in the inverse slope was richer than that in the bedding slope.Because of the low soil water content in the inverse slope,Pinus massoniana Lamb was the main vegetation.Because the ability of vegetation to intercept precipitation was strong,the penetrating precipitation?¹⁸O of soil water supplied by the inverse slope might be more enrichment,resulting in stable isotopes of soil water are more abundant than those of bedding slope.?2?The characteristics of soil water movementOn the seasonal scale,there were differences between 0-10 cm soil water?¹⁸O and lower soil water?¹⁸O at V₁,V_shi,V₂,V₃,V₄ and V₅ sample points,and the difference increased with the increase of soil depth.In addition,the temporal change trend of lower soil water lagged behind that of upper soil,and the vertical profile of 0-50 cm soil water?¹⁸O was consistent with the temporal change trend of precipitation?¹⁸O before sampling.The mechanism of soil water movement was piston flow.The temporal change trend of lower soil water?¹⁸O in V₆ and V₇ was prior to or synchronized with that in upper soil water?¹⁸O,and the vertical profile did not simulate the temporal change characteristics of precipitation?¹⁸O in previous period,which proved that the mechanism of soil water movement was preferential flow.In a short time scale?after rain?,during the recession of soil water,the upper soil water rapidly receded in V₁,V_shi,V₂,V₃,V₄?0-20 cm or 0-30 cm?,while the lower soil water?20-50 cm or 30-50 cm or 40-50 cm?received the recharge of the upper soil water during the recession,which made the soil water content stable or slightly increased.The changes of soil water content were more complex in V₆ and V₇ layers,which was determined by the soil profile structure of each sample point.The return water range of the upper slope position was larger than that of the lower slope position,which indicated that the retreat of soil water reflected the slope position effect.?3?Hydrochemical and ion leaching characteristics of soil waterBecause of the influence of geological background and topographic factors,the soil hydrochemical characteristics of the bedding slope and the reverse slope were obviously different.Ca²⁺,Mg²⁺and HCO₃^-are the main components of the soil water in the bedding slope,which showed neutral to weak alkaline.The hydrochemical type of the soil was mainly Ca-HCO₃ type.Due to the difference of geological background?sandstone of xujiahe formation?and the influence of topographic factors,soil and soil water were acidic on the reverse slope,and the hydrochemical type of soil water in karst area was Ca-SO₄-HCO₃ type.The ion concentration and amount of soil water in bedding slope had the characteristics of accumulation from the upper slope position to the lower slope position,and the total annual leaching of soil ions also had the same change characteristics.The total annual leaching of soil water ions of the upper slope position and the lower slope position was 22.69 g/?m²?a?and 75.64 g/?m²?a?in the bedding slope,respectively.On the contrary,the concentration of soil water ions in the upper slope position was higher than that in the lower slope position.However,the annual total leaching of soil water ions?36.13 g/?m²?a??in the lower slope position was higher than that in the upper slope position?19.75 g/?m²?a??due to the large amount of soil water in the lower slope position.In vertical direction,the total amount of water ion leaching in upper soil was higher than that in lower soil.?4?Soil water movement process on slope landThere are great differences in soil water movement speed between bedding slopes and reverse slopes of karst trough valley and at the bottom of valley.The average velocity of soil water movement on the upper slope position and lower slope position of the bedding slope were 13.33 cm/d and 3.33 cm/d,respectively.The average velocity of soil water movement on the upper slope position and lower slope position of the bedding slope were 20.00 cm/d and 10.00 cm/d,respectively.The velocity of soil water movement might be slower in the later stage for the lower slope position of the bedding slope,but it could not be calculated because there was no obvious isotope peak in the vertical profile of soil water?¹⁸O.In addition,the average velocity of soil water movement was 4.00 cm/d at the bottom of the valley.On the whole,the velocity of soil water movement was faster in the reverse slope than in the bedding slope,which was obvious in the upper slope position.According to the temporal change of soil water?¹⁸O at different sample sites after rain,there was lateral transportation of soil water in slope land.Combined with the spatial change characteristics of the main ion(SO₄^2-)of soil water in the upper slope position of the inverse slope decreased from the upper slope position to the lower slope position,soil water accumulation on lower slope position of the bedding slope was lower than that on lower slope position of the inverse slope,and the physical and chemical properties of the two sample sites were similar.It was speculated that the lateral recharge of soil water in the bedding slope was larger than that in the reverse slope,and the output of soil water in the reverse slope was large in the vertical direction.