Change Characteristics And Assessments Of Soil Quality During Old-field Succession In Karst Valley Area

In recent years,with the implementation of various ecological restoration projects and the development of urbanization,a certain amount of abandoned farmland has been produced in karst valley area,which is an area of fragile ecological severely affected by human activities.After the abandonment of farmland,the old-field succession process is no longer directly affected by human activities,and the overlying vegetation changes significantly.The change of environmental factors will inevitably lead to the constant change of soil properties,thus affecting the change of soil quality.However,the previous researches on soil quality in karst areas mainly focused on the comparison of soil quality between different land use patterns and lacked the research on the dynamic changes of soil quality in abandoned farmland in different periods of old-field succession.At the same time,there is still a lack of a unified and widely applicable soil assessment system in karst valley area.Therefore,it is of great reference significance to clarify the change characteristics of soil during the old-field succession of abandoned farmland and establish soil quality assessment method suitable for karst valley area,which has great reference significance for soil amelioration and utilization in karst valley area.This study revealed the response of soil physical,chemical and biological properties to the old-field succession of abandoned farmland through measuring and calculating 37 soil physical,chemical and biological indicators of farmland,grassland,shrubland and forest land in Qingmuguan karst valley,Beibei,Chongqing.In terms of soil quality assessment,20 soil indicators of abandoned farmland sensitive to old-field succession were determined by one-way ANOVA,the minimum data set of soil quality indicators was established by principal component analysis,and all soil indicators were standardized by score function.The standardized soil indicators were given weight and the comprehensive score of soil quality indicators was calculated to quantify the soil quality of abandoned farmland in different succession stages.The influencing factors and mechanism of soil quality change in abandoned farmland during old-field succession were also discussed.Finally,it provided data support and scientific reference for the rational management and protection of soil in the study area.Through investigation and analysis,the main results of this study are as follows:(1)During the old-field succession of abandoned farmland in karst valley,the physical,chemical,and biological properties of soil changed to some extent.(1)In terms of physical properties,with the development of old-field succession,the proportion of macro-aggregates and micro-aggregates in soil increased,while the proportion of silt and clay particles decreased,and the soil aggregate structure became more obvious.The fractal dimension of soil decreased,while the mean weight diameter and geometric mean diameter increased,indicating that soil water stability and erosion resistance were significantly enhanced.(2)In terms of chemical properties,the total nitrogen content in the surface soil of grassland and shrubland increased,but the difference was not significant.The content of available nitrogen in the surface soil of shrubland and forest land increased,and the content of available nitrogen in the surface soil of shrubland reached a significant level.The contents of total phosphorus and available phosphorus in soil decreased significantly.The contents of total potassium and available potassium in soil increased first and then decreased.Soil organic carbon content increased,but the change was not significant.The content of recalcitrant organic carbon in soil decreased while that of labile organic carbon increased,indicating that old-field succession of abandoned farmland in the study area is beneficial to improve soil organic carbon activity and enhance soil fertility.(3)In terms of biological properties,the microbial biomass carbon content in grassland and forest land increased,while the activities of four soil enzymes(sucrase,catalase,alkaline phosphatase and urease)did not change significantly in surface soil.Based on the geometric means of enzyme activities of four kinds of soil enzymes,it was found that the geometric mean of enzyme activity did not change significantly in surface and subsurface soils during natural restoration and old-field succession.(2)Four indicators with the strongest representation and the most significant difference in different soils in karst valley region were determined as the minimum data set for soil quality evaluation,namely,fractal dimension,total potassium content,soil proportion of large aggregates and soil available potassium content.These four indicators can be used as reference indicators for soil quality evaluation in karst valley area.(3)Four soil quality index calculation methods(the linear scoring-additive integration method,the linear scoring-weighted additive integration method,the nonlinear scoring-additive integration methods,and the nonlinear scoring-weighted additive integration method)were compared to find the most suitable index of the soil quality evaluation of karst valley area.It was found that the linear scoring-weighted additive integration method can be used as the future quantitative method to evaluate the soil quality of karst valley area.(4)The established soil quality index method was used to quantitatively evaluate the soil quality at different stages of old-field succession.It was found that the soil quality of grassland,shrubland and forest land was improved compared with that of farmland,indicating that old-field succession was beneficial to the restoration and improvement of soil quality in karst valley area.The natural restoration of abandoned farmland is also a better way of vegetation restoration and ecological reconstruction in karst valley area.