Characteristics Of Soil Magnetic Susceptibility And Its Environmental Significance Of Typical Dolomite Catchment In Subtropical Area

Guizhou Shibing Karst is included in the World Natural Heritage List for its unique tropical-subtropical dolomite karst characteristics.It is probably the world’s most outstanding example of humid subtropical karst in dolomite.The soil that is easily lost in karst areas can easily lead to rocky desertification and make the soil more barren.Therefore,the study of soil in dolomite karst areas is of significant significance.Soil magnetism is a new field of soil science,which can reflect comprehensive information such as parent material,climate,vegetation,hydrology and human activities.In this study,the Huangzhouhe Basin of Shibing Dolomite Karst World Heritage Site in Guizhou Province was used as the research area.Through measurement and analysis of surface and profile soil magnetic susceptibility characteristics and soil nutrient characteristics,the spatial distribution characteristics of soil magnetic susceptibility in the dolomite area and the environmental indication significance were discussed.Enriching the relevant research on the soil resources of the Shibing Karst World Natural Heritage Site,providing a basis for monitoring the process of soil migration and environmental evolution in the heritage site,and providing the follow-up and efficient application of soil magnetic technology in the fields of soil erosion and theoretical reference rocky desertification prevention in karst areas.The main research conclusions are:（1）The soil nutrients in the dolomite karst area present the following characteristics:in the surface soil,the average soil p H is 6.92;hydrolyzed nitrogen is the most unstable,and total nitrogen is the most stable;the average content of total nitrogen,total phosphorus,and total potassium are 1.61,0.70,20.36 g·kg^-1;the average contents of hydrolyzed nitrogen,available phosphorus,and available potassium are:59.79,8.90,108.75mg·kg^-1;the ratios of C:N,C:P,and N:P are 18.14,23.87,2.11,respectively,lower than the national level.In the soil profile,the average p H is 6.48,and the p H of the soil near the bedrock layer is the lowest,and the regularity is not obvious,and the overall change is not significant;the soil organic carbon of the paddy field shows the change characteristics of gradually decreasing from the surface layer,but the forest land and the The opposite is true in dry land;the change of total nitrogen content is highly consistent with the change of organic carbon;total phosphorus and available phosphorus content show a decreasing trend with the increase of soil depth;total potassium and available potassium maintain a stable trend after the 40 cm soil layer.Except for the soil P content in the 0-10cm soil layer and N:P,there is a significant quadratic function relationship between the soil C,N,P content and the stoichiometric ratio of the other soil depths.There is a certain correlation between various soil nutrients.The organic carbon content is much higher than the national surface soil,the organic carbon and total nitrogen content are rich,and the total phosphorus content is the most suitable.Slope,aspect,climatic conditions and human activities are the main factors affecting soil nutrients,among which human activities profoundly affect the accumulation of surface soil nutrients.（2）The magnetic susceptibility of the surface soil in the dolomite karst area presents the following characteristics:the range ofχ_lf is6～69.04×10^-8·m³·kg^-1,the average is 26.03×10^-8·m³·kg^-1,compared with other regions lower,the topsoil magnetic susceptibility is dominated by low-frequency magnetic susceptibility;χ_fd from 0.5 to 13.69%,most of the topsoil is dominated by SP particles,but 6%of the topsoil basically does not contain SP particles;bothχ_lf andχ_fd medium variation intensity,the coefficient of variation is 57.40%,43.32%.The spatial distribution ofχ_lf andχ_fd are divided into four levels,which are related to land use type,human activity range,and elevation.The main land use in the area with the highestχ_lf andχ_fd distribution is forest land,and the lowest is paddy field.There is a positive correlation between soil p H andχ_lf,and a negative correlation withχ_fd,but both reach a significant level;organic carbon has a positive correlation withχ_lf andχ_fd.It is preliminarily judged that the soil magnetic susceptibilityχ_lf can be used as the 0-10cm soil in the study area.A proxy indicator of organic carbon;there is a certain correlation between total nitrogen,hydrolyzed nitrogen,total potassium,total phosphorus,available phosphorus,quick-acting potassium andχ_lf,χ_fd,which can indicate that there is a corresponding relationship between these nutrient contents and ferromagnetic minerals.As a result,the magnetic susceptibility value becomes higher or lower,reflecting a certain relationship between ferrous minerals and minerals that are stored in soil nutrients.（3）The soil magnetic susceptibility of the profile in the dolomite karst area presents the following characteristics:theχ_lf of the topsoil is larger than theχ_lf of the bottom;theχ_lf of the topsoil with the soil profile of woodland and dry land is enhanced,and the main factors are soil formation and farming methods.The soil layer dominated by the parent material layer of forest land has strong soil-forming effect,and the content of ultrafine magnetic minerals in most of the soil is>75%;dryland section soil contains both ultrafine magnetic minerals and coarser non-ultrafine magnetic minerals;Paddy soil contains SP particles in four soil layers of 0-10 cm,50-60 cm,70-80 cm,and 80-90 cm.The content of SP particles is more than 75%.The other soil layers contain SP particles and coarser particles,non-superfine magnetic minerals.In general,χ_lfandχ_fd of dryland soil profiles are negatively correlated with p H,organic carbon,total nitrogen,and total potassium;χ_lf andχ_fd of paddy soil profiles are positively correlated with organic carbon,and other nutrient parameters are positively correlated withχ_lf The relationship is negatively correlated withχ_fd,but none of them reach a significant level;there is a significant positive correlation betweenχ_lf and available phosphorus,total potassium,available potassium,and organic carbon in the soil profile of forest land,and a significant negative correlation betweenχ_fd and organic carbon.（4）Environmental indicator significance of magnetization of dolomite karst soil.The magnetic susceptibility of the surface soil in the study area is dominated by low-frequency magnetic susceptibility,andχ_lfis low compared with other areas.Theχ_fd range is 0.5～13.69%.At first,it is believed that the soil in the study area is highly developed,and most of the topsoil is dominated by SP particles,but 6%of the topsoil basically does not contain SP particles;the soil in the study area gradually becomes soil.With the progress of weathering,the magnetic particles formed due to soil formation gradually increase,resulting in an increase in the magnetic susceptibility of the soil.From the perspective of the characteristics of soil magnetic susceptibility changes,as the depth of the soil layer increases,the soil magnetic susceptibilityχ_lf of dry land,paddy field,and forested land shows a gradually decreasing trend,and the soilχ_lf is higher in the 0～30 cm cultivated layer,which further indicates The0～30 cm farming layer is frequently affected by human agricultural activities,resulting in a significant increase in soil magnetic susceptibility,while the soil under the farming layer is less affected by human activities.The magnetic minerals produced by the soil below 30 cm are relative to the magnetic minerals produced by human activities.The soil formation caused by chemical factors has a more significant impact on soil magnetism,and it is in line with the general law of the process of soil iron aluminization in the development of weak magnetic bedrock.The soil magnetism is mainly contributed by maghemite.