Study On Nutrient Loss Of Fissure Water And Soil In Karst Rocky Desertification Area

The erosion of karst fissure ground water and soil seriously restricts sustainable development of karst ecology,economy and society in Southwest China,and abstracts more attention and study from the worldwide.As a unique form of karst soil erosion,groundwater soil loss has an important impact on evolution of rocky desertification,quality of underground river water,and drought and flood disasters.At present,the research on karst underground leakage mainly focuses on qualitative description and simulation test phase of the phenomenon.In this study,the three typical rocky desertification areas in Guizhou were Bijie Salaxi Research Area,Guanling-Zhenfeng Huajiang Research Area,and Shibing Karst Research Area as the research units.Firstly,we investigated fissure shapes and soil physio-chemical properties of research area.Based on these investigations,we designed simulation tests of karst fissure leakage and nutrient loss to reveal coupling mechanism of fissure water and soil loss and nutrient loss.This study is based on geology,pedology,hydrology,ecology,environmental science and other disciplines as the theoretical basis.A total of 43 fissure soils were sampled from the Bijie Salaxi Research Area,70 from the Guanling-Zhenfeng Huajiang Research Area,and 47 from Shi Bingkarst research areas.Through a four-month rainfall monitoring?from June to September in 2019?in Guanling-Zhenfeng Huajiang Research Area,we collected 26 occurrences of runoff and 20 occurrences of sand production and analyzed nutrient experiment of these samples.At the same time,rainfall monitoring was conducted to study the effects of fissure runoff and sediment production under different rain intensities,and impacts of fissure shapes on runoff and sediment production.This study also discussed the response of ground water and soil loss to rainfall and its environmental impacts.Furthermore,we investigated the coupling mechanism of soil loss and nutrient loss in cracked groundwater and the environmental effects of rocky desertification,which can provide theoretical support and scientific basis for ecological restoration of rocky desertification management.The results and conclusions are as follows:?1?The fissure trace length and opening value are the largest in Guanling-Zhenfeng Huajiang study area.With the increase of fissure depth,the fissure soil bulk density and soil clay content gradually increase,and the total soil porosity,sand content and silt content gradually decrease.The content of nutrients in fissured soil is shown;the contents of soil organic carbon,dissolved organic carbon,Cl^-and SO₄^2-decrease gradually with the increase of fissure depth,the surface content of Ca²⁺and Mg²⁺are high,and there is no obvious change with the increase of fissure depth.Through the comparison of research areas,it is found that the deeper the degree of rock desertification,the higher the degree of fissure development.The soil organic carbon content and dissolved organic carbon content in the Shibing Karst research area were higher than those in the Bijie Salaxi and Guanling-Zhenfeng Huajiang research areas.The content of Mg²⁺in the fissure soil in the Shibing Karst research area dominated by dolomite is higher than that of the Bijiesalaxi and Guanling-Zhenfeng Huajiang research areas dominated by limestone.Overall,the deeper the rocky desertification,the stronger the fissure development,the worse the soil compaction,the better the total porosity,the poorer the erosion resistance,the higher the risk of underground loss,and the lower the nutrient content.Fissure morphology,soil organic carbon,soluble organic carbon dissolved organic carbon,Cl^-and SO₄^2-content have no obvious correlation with bedrock lithology.?2?The changes in the moisture and electrical conductivity of fissured soil rise and fall with the increase and decrease of rainfall.LX1?leakage simulation test No.1 fissure?shows that the lower layer of soil has a higher moisture content than the surface layer.The surface layer is more sensitive to rainfall and fluctuates greatly with the rainfall,while the lower layer shows obvious stability.LX2?leakage simulation test No.2 fissure?is greatly affected by rainfall,the soil moisture content in the 70 cm layer is the lowest,and the soil moisture changes are large.The impact shows a clear dominant flow.The conductivity of soil water fluctuates with the change of rainfall.Generally,as the rainfall continues,the conductivity value shows a downward trend,and the peak conductivity value has a lag compared with the peak rainfall value.The water dynamics and electrical conductivity of fissured soil increase and decrease with the increase and decrease of rainfall.There is soil infiltration and dominant flow in the water loss of fissured soil,and the trend of soil water solute decreases.?3?The greater the rainfall and the opening of the fissure,the greater the runoff and sand production in the fissure underground.By comparing the effects of runoff and sediment production in fissures under different rainfall types,it was found that the value of runoff and sediment production under the heavy rainstorm type had the largest change,while the light rain and moderate rain had the smallest change in rain intensity.Under light rain conditions,the runoff and sediment production of the fissures are the smallest,and heavy rainstorms have the largest impact on the runoff and sediment production and the highest contribution rate.Under light rain conditions,except for LX2 and LX6?leakage simulation test No.6?,the rest of the fractures do not produce sand.Rain intensity on the runoff and sediment production of the fissures shows that the greater the rainfall intensity,the higher the runoff and sediment production.Under the same rainfall conditions and the underground connectivity of the fissures,the larger the fissure opening,the greater the underground runoff and sediment production,and the larger the fissure soil volume,the smaller the runoff and sediment yield.Fissure water and soil loss are affected by rainfall,fissure opening area,fissure soil volume and connectivity.?4?The higher the soil nutrient content,the higher the soil nutrient concentration in the fissures;the loss of solute nutrients as the rainfall monitoring continues,the nutrient concentration shows a decreasing trend,the greater the rainfall,the more nutrient loss.Comparative analysis of total nutrient loss revealed that soil organic carbon was mainly lost in the form of soil,while dissolved organic carbon,Ca²⁺,Mg²⁺,Cl^-and SO₄^2-were mainly lost in the form of solute.Under rainfall conditions,the fissure shape has a significant effect on nutrient loss.The larger the fissure opening,the higher the concentration of solute nutrient loss,and the larger the volume of fissure soil,the lower the concentration of solute nutrient loss.In terms of total nutrients,the larger the crack opening area,the more nutrients are lost,and there are significant differences in nutrient loss between different fissures.The influence of rainfall on the solute nutrient loss of fissures can be fitted with a linear function.The solute nutrient loss and crack opening area are fitted with a quadratic function,and Ca²⁺is best fitted with a linear function.