Soil Surface Electrochemical Characteristics And Its Effects On Soil Nutrients In Karst Rocky Desertification Ecosystems,SW China

Karst rocky desertification is a major ecological problem that restricts agricultural production and economic development in karst areas of Southwest China.However,for a long time,there is a lack of understanding of the micro mechanism of soil quality evolution in the process of karst rocky desertification succession,which seriously affects the effectiveness of rocky desertification control.In this study,the typical karst rocky desertification soil in Southwest China was taken as the research object,and the soil surface electrochemical characteristics of four different rocky desertification grades（no,mild,moderate and severe）and their response to the rocky desertification succession stage were studied by using ANOVA and correlation analysis.The soil surface potential,surface charge density,soil surface electrochemical characteristics and their response to the rocky desertification succession stage were discussed by using redundancy analysis（RDA）The coupling relationship between surface electric field intensity,specific surface area,surface charge quantity and environmental factors.The distribution of soil available N and P contents with soil depth in four different land use types（arbor land,shrub land,grassland and abandoned land）and the effects of soil factors on soil available N and P contents were investigated.The main results are as follows:（1）The results showed that the chemical characteristics of soil surface charge were significantly different in different intensity rocky desertification environment.The soil surface potential increased with the increase of rocky desertification level,and the range was-113.74 m V～-115.10 m V,Soil surface charge density,surface electric field intensity,specific surface area and surface charge quantity decreased with the increase of rocky desertification grade,and their variation ranges were0.34 C·m^-2～0.42 C·m^-2,4.85×10⁸ V·m^-1～5.86×10⁸ V·m^-1,47.11m²·g^-1～53.16 m²·g^-1,16.86 cmol·kg^-1～22.82 cmol·kg^-1.In the same rocky desertification intensity,the soil surface electrochemical properties at different soil depths had significant differences,but there was no significant change with soil depth.（2）The clay minerals in the study area are illite,kaolinite,chlorite,illite montmorillonite mixed layer and green montmorillonite mixed layer,and the green montmorillonite mixed layer is the main one;Soil clay,sand,amorphous alumina and humic acid were the main factors affecting the change of soil surface electrochemical properties in karst rocky desertification area,and the interpretation rates were 48.3%,38.1%,13.0%and 12.0%,respectively;The effects of soil particle size composition,organic matter composition and metal oxides on soil surface electrochemical characteristics were as follows:Clay>Sand>silt,humic acid>organic carbon>fulvic acid,amorphous alumina>free iron oxide>amorphous iron oxide.The average values of surface charge density and surface electric field intensity of neutral lime soil in karst area(0.38 C·m^-2,5.37×10⁸ V·m^-1)is lower than that in neutral purple soil(0.69 C·m^-2,9.74×10⁸ V·m^-1),but higher than that in neutral loessal soil(0.21 C·m^-2,2.93×10⁸ V·m^-1)and neutral lateritic soil(0.16 C·m^-2,2.24×10⁸ V·m^-1).The differences of clay mineral types,oxide types and physicochemical properties between calcareous soil and other types of soil are the main reasons for the great differences of soil surface electrochemical properties.（3）In the process of rocky desertification grade rising,the soil loss caused by water and gravity increased,the rock bare rate increased,the soil barren and aboveground biomass decreased,which formed a vicious circle.Compared with the non rocky desertification grade soil,the contents of organic carbon,humic acid and fulvic acid in severe rocky desertification soil decreased by 6.89 g·kg^-1,3.40 g·kg^-1and 3.42 g·kg^-1,respectively.Compared with non rocky desertification area,soil organic carbon in severe rocky desertification area is more difficult to accumulate,and the process of humification is always at a relatively low stage.At the same time,the loss of clay and silt increased with soil erosion,making the soil coarse.Compared with the area without rocky desertification,the sand increased by 6.59%and the clay decreased by 4.49%.The average content of free iron oxide in severe rocky desertification area is 20.70g·kg^-1,which is higher than 15.11 g·kg^-1 in non rocky desertification area.Illite in soil clay minerals is easy to be preserved and accumulated in the environment with weak weathering and leaching,while kaolinite indicates the soil forming environment with strong leaching and strong weathering.The relative content of illite in soil clay minerals in severe rocky desertification area is low,and the relative content of kaolinite is high,which also indicates that the weathering and leaching intensity is high.The above environmental characteristics are the main reasons for the decrease of surface charge density,surface electric field intensity,specific surface area and surface charge quantity with the increase of rocky desertification level.（4）The results showed that the soil total nitrogen,total phosphorus and their availability of arbor land,shrub land,grassland and abandoned land decreased significantly with the increase of soil depth,showing the characteristics of surface aggregation;The contents of soil available N and P were positively correlated with organic carbon components,surface electrochemical properties and amorphous oxides（p<0.05）,but negatively correlated with soil silt and free metal oxides（p<0.05）;Redundancy analysis showed that the influencing factors of soil N and P availability of four land use types were basically the same,in which soil particulate organic carbon,total organic carbon and soil specific surface were the key factors affecting the change of soil available N and P.Due to the strong soil and water conservation capacity and high litter amount of arbor and shrub land,the soil specific surface area and cation exchange capacity of arbor land(60.94 m²·g^-1,14.65 cmol·kg^-1)and shrub land(50.30 m²·g^-1,11.98 cmol·kg^-1)were higher than that of grassland(46.19m²·g^-1,10.95 cmol·kg^-1)and abandoned land(37.36 m²·g^-1,8.82 cmol·kg^-1),The results showed that the adsorption,desorption,retention and interaction ability of nitrogen and phosphorus in different land use types of soil were different,which had a significant impact on soil nitrogen and phosphorus availability.