Soil Colloid Characteristics Of Typical Forest Communities Under Different Vegetation Restoration In Karst Valley Of Southwest China

As one of the three major karst distribution areas in the world,the southwest karst area has fragile ecological environment,low soil-making capacity and barren land.Among them,the trough valley area is also prone to high-level water resource leakage,serious water shortage and frequent drought.At the same time,the contradiction between human and land in the trough valley area is prominent.Excessive land use leads to the destruction of the ecosystem,accelerates local soil erosion and soil structure degradation,and seriously threatens the sustainable development of local agricultural production.As a typical karst area in China,the southwest trough valley area is an important ecologically fragile area and a security barrier area.The effect of vegetation restoration in this area has always been a research hotspot in the field,and soil colloid is an important basis for the formation of soil fertility and ecological functions.Therefore,strengthening the study on the changes of soil colloid characteristics after vegetation restoration in this area is helpful to understand the effect of vegetation restoration in southwest karst trough valley,and has certain practical value and application prospect for ecological management and policy implementation in trough valley area.Based on this,this study selected three land use types of abandoned farmland,natural restoration forest（natural forest）and plantation restoration forest（plantation forest）in the southwest karst trough valley area as the research object.The abandoned farmland was abandoned for half a year,and the natural forest and plantation forest were natural succession after farmland abandonment and plantation restoration for 50-60 years.By analyzing the physical and chemical properties of each soil layer,the particle size distribution of soil colloids,the mineral composition,organic carbon and surface electrochemical characteristics of soil colloids under three land use types,the characteristics of soil colloids under different vegetation restoration methods in the southwest karst trough valley area were revealed.This study mainly draws the following conclusions:（1）After vegetation restoration,the content of sand and clay in woodland soil decreased,and the content of silt increased.There was a significant difference between woodland and abandoned farmland（p<0.05）.After vegetation restoration,the content of>2000 nm particle size soil colloid increased,and the content of 1000-500 nm particle size soil colloid decreased（p>0.05）.>2000 nm particle size soil colloid had the highest proportion,the change range of abandoned farmland was 36.54%-45.53%,the change range of natural forest was 46.26%-60.83%,and the change range of plantation forest was62.78%-73.48%.The proportion of>2000 nm particle size soil colloid was the highest under different land use types.The change range of abandoned farmland was 36.54%-45.53%,the change range of natural forest was 46.26%-60.83%,and the change range of plantation forest was 62.78%-73.48%.The soil minerals under different land use types are mainly illite,cristobalite,quartz,goethite,corundum and aragonite.The proportion of quartz,cristobalite and illite is high,and the proportion of goethite,corundum and aragonite is low.In addition to soil total potassium,the“surface accumulation effect”of soil chemical elements is obvious.The contents of total carbon,organic carbon,ammonium nitrogen and stoichiometric ratio in forest soil were higher than those in abandoned farmland.The contents of organic carbon,soluble organic carbon,total nitrogen,total phosphorus and total potassium in natural forest soil were higher than those in plantation forest.Vegetation restoration can effectively improve soil quality.（2）The soil colloidal clay minerals of abandoned farmland,natural forest and plantation forest were mainly illite,quartz,cristobalite,goethite,corundum and aragonite,and there was no significant difference among different land use types.Natural restoration and plantation afforestation could increase the content of soil colloid organic carbon.Natural forest increased the content of soil colloid organic carbon with particle size of>2000 nm and 2000-1000 nm,and plantation forest increased the content of soil colloid organic carbon with particle size of 2000-1000 nm and 1000-500 nm.There was no significant change in soil organic carbon content under the same land use type.The average content of soil colloid organic carbon in abandoned farmland,natural forest and plantation forest was 25.92 g·kg^-1,26.51 g·kg^-1 and 27.36 g·kg^-1,respectively.Under each land use type,the total organic carbon of soil colloids with particle size>2000 nm was the highest,and the particle size of 1000-500 nm was the lowest.The>2000 nm soil colloid organic carbon in abandoned farmland,natural forest and plantation forest accounted for 53.24%,64.98%and 67.96%of the total colloid organic carbon,respectively.The organic carbon was mainly distributed in the>2000 nm soil colloid.（3）Vegetation restoration significantly improved the electrochemical properties of soil colloids,and the electrochemical properties of soil colloids in different particle sizes of different land use types were different.In the third land use type,the surface potential of>2000 nm particle size soil colloid varies in the range of-107.41--115.26 m V,and the 2000-1000 nm particle size varies in the range of-106.47--114.36 m V.The 1000-500 nm particle size varies in the range of-110.15--115.13 m V.The specific surface area of>2000 nm particle size soil colloid varies in the range of 47.01-55.25 m²·g^-1,the 2000-1000 nm particle size varies in the range of 54.95-58.19 m²·g^-1,and the 1000-500 nm particle size varies in the range of 56.91-59.95 m²·g^-1.The surface charge density of>2000 nm particle size soil colloid varies in the range of 0.33-0.38 C·m^-2,2000-1000 nm particle size varies in the range of 0.39-0.44 C·m^-2,and 1000-500 nm particle size varies in the range of 0.41-0.44 C·m^-2.The surface charge of>2000 nm particle size soil colloid varies in the range of 19.18-20.80 cmol·kg^-1,2000-1000 nm particle size varies in the range of 22.47-25.45 cmol·kg^-1,and 1000-500 nm particle size varies in the range of24.95-25.42 cmol·kg^-1.The surface electric field intensity of>2000 nm particle size soil colloid varies in the range of 4.04-4.63×10⁸ V·m^-1,the 2000-1000 nm particle size ranged from 4.86-5.18×10⁸ V·m^-1,and the 1000-500 nm particle size ranged from 5.17-5.50×10⁸V·m^-1.Vegetation restoration increased the specific surface area,surface charge density,charge number and electric field intensity of soil colloid,and the surface potential of abandoned farmland was higher than that of woodland.There was no significant difference in electrochemical properties between natural forests and plantation forests under different land use types（p>0.05）.Although the surface properties of soil colloids were different among soil layers under the same land use type,there was no obvious change rule.