Quality Evaluation Of Slope Farmland In Yunnan Province And Soil Erosion/Drought Influencing Mechanisms

Southwest China is the most concentrated area of slope farmland in China,and slope farmland is an important part of local farmland resources.At present,there exist many problems for our farmland utilization,such as quality decrease,space fragmentation,frequent ecological problems,and so on.Thus,our country takes the quantity,quality and ecology"trinity"protection strategy as well as takes the farmland quality protection and improvement as an important strategic fulcrum of"storing grain in land and technology".The slope farmland in Yunnan has their own characteristics of wide distribution area,large slope,serious soil erosion,frequent seasonal drought,low soil quality,etc.Among them,serious water and soil loss as well as seasonal drought are the main factors to restrict the reasonable utilization of slope farmland in Yunnan.Therefore,the scientific evaluation on the slope farmland quality and the analysis for the impact mechanism of soil erosion/drought on the spatial pattern of slope farmland quality are the premise and basis to realize the quantity,quality and ecology"trinity"protection.Based on the spatial and temporal distribution of sloping farmland resources and analysis of evolutionary driving force,this study firstly builds a quality evaluation system for slope farmland at a provincial scale by data collection,GIS spatial modeling and analysis,model calculation,etc.Then,the slope farmland quality is quantitatively evaluated,and the spatial pattern of slope farmland quality is analyzed.From the regional spatial scale,the impact mechanism of soil erosion and drought on slope farmland quality as well as the spatial coupling feature are investigated.Next,the obstacle factors of regional slope farmland quality is diagnosed,and the priority,target and potential of slope farmland quality adjustment are analyzed.The corresponding adjustment measure system and integration mode of sloping farmland quality are built.Finally,this study provides the theoretical and technical support for the quality construction as well as the soil and water ecological environment improvement of slope farmland,and the main conclusions are as follows:?1?Spatial and temporal distribution and evolutionary driving force of slope farmland resourcesThe area of slope farmland is 4.72 million hm²,accounting for 69.79%of the total farmland.And the distribution center trajectory of slope farmland shows a movement trend from the northeast to the southwest.In the recent 35 years,a remarkable dynamic transfer has been occurring among the slope farmland,woodland,grassland,paddy field,etc.,but the general equilibrium is kept between the roll-in and roll-out.The average slope of slope farmland is 15.62°,about 78.96%of the farmland slope is more than 8°,and the proportion of slope farmland with a slope more than 15°reaches 48.54%.During the evolution process of slope grade,there exist different change trends for the dynamic degree of slope farmland with different slope grades,namely,"decrease?increase?decrease"or"decrease?increase?decrease?increase".The slope farmland with a slope less than 15°displays an increase trend,while the slope farmland with a slope more than 15°presents a decrease trend.And the dynamic degree of the slope farmland with a slope more than 25°has a larger fluctuation amplitude.There present four significant distribution belts in the more concentrated area of slope farmland in Yunnan.In recent 35 years,the kernel density distribution of slope farmland changes a little,most of slope farmland distributes in the low-density area?kernel density<12?,and the proportion of high-density area?kernel density>24?is lower.The dominant driving force of the spatial and temporal evolution of slope farmland is the demand for population and economic growth,the demand for increase in the grain yield based on corn and wheat,the demand for growth of agricultural output value and rural per capita net income,and the crop damage caused by drought.Among them,the human factors play a leading role in the spatial and temporal evolution.?2?Evaluation of sloping farmland quality and identification of influence factorsBased on the theoretical framework of"factor-demand-regulation",the evaluation index system of slope farmland in Yunnan are composed of 14 indicators,that is,effective soil layer thickness,thickness of cultivated layer,soil volume-weight,soil texture,pH value of soil,organic matter,available phosphorus,available potassium,10?or higher accumulated temperature,field regularity,continuous degree,rainfall,irrigation assurance rate,field slope.The 30×30 m² grid?pixel?is taken as the evaluation unit.Moreover,the comprehensive weight is taken as the index weight,and the slope farmland quality index?SIFI?is calculated by weight sum method to evaluate the quality variation characteristics of slope farmland.The SIFI of Yunnan is mainly between 0.36 and0.81,and their average value is 0.59.Most unit of SIFI is less than 0.6,and the difference in SIFI of different evaluation unit is significant.The SIFI size for five types of slope farmland from large to small is latosolic red soil,red soil,purple soil,yellow soil,and yellow brown soil.The change of SIFI is related to elevation.Furthermore,the SIFI increases with the elevation increasing in the elevation range of 0¹000m but decreases as the elevation exceeds 1000m.In order to improve the comparability between the evaluation results and the regional farmland quality evaluation results,the slope farmland quality are divided by equidistant five-grade classification method and equidistant ten-grade classification method.As for five-grade classification method,the quality grade of slope farmland in Yunnan is based on"medium"and"higher".And for the ten-grade classification method,the quality grade of slope farmland is based on the six-grade land,five-grade land,seven-grade land,and four-grade land.Lorenz curve of farmland quality grade in different regions presents a“S-type”distribution pattern.Gaussian model can be used to fit the variation function of spatial distribution of slope farmland quality index well.Moreover,the spatial distribution of slope farmland quality index has a medium self-correlation,as well as the structural factors,such as climate condition,soil property,moisture condition,spatial morphology,etc.,play a major role in the spatial heterogeneity of slope farmland.The global spatial self-correlation Moran's I of slope farmland quality grade is0.8489,and its spatial distribution has a significant aggregation characteristics.Among them,the LISA cluster types are based on HH aggregation and LL aggregation.The differences in the cold and hot spots distributions of quality grade of slope farmland are significant.The hot spots mainly distribute in the central region of Yunnan and southern fringe area,while the cold spots mainly distribute in northwest area,northeast area and some regions in southwest area of Yunnan.Moisture condition,solar-thermal condition,soil erosion and soil property are the important factors to affect the quality of slope farmland in Yunnan.Moreover,the soil erosion and drought are the key factors to restrict slope farmland quality improvement of Yunnan.?3?Influence of soil erosion characteristics on slope farmland qualityThe soil erosion amount of slope farmland in Yunnan is 376.57×10⁶ t.a^-1,and the average erosion modulus is 7986.31t/?km².a?.Simultaneously,the proportion of erosion area is 89.37%,and the average soil loss thickness for many years is 7.31mm/a.Soil erosion of slope farmland is mainly in the farmland with 15²5°,more than 25°and 8¹5°slopes.With the increase of slope,the corresponding proportion of erosion area,erosion intensity and erosion amount of slope farmland all present an increase trend.Soil erosion and nutrient loss of slope farmland are the main sources of regional erosion and sediment yield as well as the nutrient loss.Simultaneously,the slope farmland quality index is significantly negative correlated with soil erosion modulus,soil loss thickness and nutrient loss modulus,respectively,in which the two can be fitted well with exponential function.In addition,the soil loss thickness,organic matter loss modulus and soil erosion modulus have the greatest influence on slope farmland quality factor.Among the soil erosion parameters,the soil loss thickness and soil erosion modulus affect the slope farmland quality mainly by the change of parameters such as effective soil thickness,soil volume-weight,and so on.The soil nutrient loss affects the slope farmland quality by the nutrient content change such as organic matter,total nitrogen,available phosphorus,etc.The influence of soil erosion on the slope farmland quality is completed by nine indirect paths,and the total effect of influence is-0.525.The mean of spatial coupling degree between soil erosion and slope farmland quality is 0.821±0.219,generally in a high-level coupling state.Meanwhile,the spatial distribution of slope farmland quality presents a significantly spatial coupling response characteristic on the spatial distribution of soil erosion.Likewise,the mean of coupling coordination degree between soil and water conservation system with the slope farmland quality system is 0.771±0.141,generally in a good coupling coordination state.Thus,there exists a stronger coordinated development relationship between the soil erosion governance and quality improvement of slope farmland.?4?Influence of agricultural drought characteristics on the quality of slope farmlandAverage annual effective rainfall in Yunnan is 941.04mm,which focuses on summer and shows a decreasing trend from southwest to northeast in distribution.In summer,the water deficit in most regions during the crop growth period is more than 0 except for April and May,however,in winter,the water deficit in most regions is less than 0.The agricultural drought in an annual scale is divided into three grades of"medium drought","slight drought"and"normal".Among them,the area of"slight drought"region is the largest,while the area of"medium drought"region is the smallest.Moreover,the seasonal drought is based on the spring drought and winter drought,and the drought level is mainly"severe drought".However,the possibility of agricultural drought in summer and autumn is smaller.The slope farmland quality index has significant positive correlation both with the annual scale and seasonal drought index.Each two can be fitted well by linear function,thus the fitting results show that the higher the drought grade is,the lower the slope farmland quality is.The influence degree of drought in different seasons on the slope farmland quality from large to small is summer,autumn,spring,and winter,in which the influences of drought in summer and autumn are the most significant.Agricultural drought affects the quality of slope farmland manly by the influence of water supply capacity and the change in physical properties of soil of slope farmland.In addition,the influence of drought on the slope farmland quality is finished mainly by four indirect paths with a total effect value of-0.667.The mean of spatial coupling degree between agricultural drought and the slope farmland quality is 0.955±0.091,and most of the evaluation units are in a high-level coupling state.Simultaneously,the spatial distribution of slope farmland quality presents a significantly spatial coupling response characteristic on the spatial distribution of agricultural drought.However,there is a larger difference in the spatial coupling degree between the agricultural drought in different regions and slope farmland quality.The coupling degree is higher in southern fringe area,southwest area,and northeast area of Yunnan,but lower in central area and western area.?5?Diagnosis and regulation of quality obstacle factors of sloping farmlandThe obstacle types of slope farmland quality in Yunnan are mainly erosion degradation,drought and water shortage,organic matter deficiency and nutrient deficiency.The quality obstacle factors combination in different areas and their degrees are different.In the light of the characteristic response time?CRT?of the factor,the slope farmland quality factor is divided into controllable and uncontrollable factors.According to factor obstacle degree?OD?,the priority of factor regulation is graded.Achievable goals for the slope farmland quality regulation include two contextual patterns of ideal state and actual state.Among them,the regulation potential of slope farmland quality in Yunnan under the ideal state is 0.347 with an upgrade from"medium"to"high".As well as the regulation potential of slope farmland quality under the actual state is 0.198 with an upgrade from"medium"to"higher".Hence,the regulation potential in the actual state can be taken as a reference for the regulation of slope farmland quality.The quality regulation measures of slope farmland in Yunnan are composed of cultivation measure,soil fertilizer measure,engineering measure,planting pattern measure,as well as forest grass measures.Likewise,there exist differences in water retention,soil conservation,fertilizer conservation,cultivated-layer structure improvement,and yield increase effect for different regulation measures.Then,the effect value of water retention is between 0.157and 0.521,and the effect value of soil conservation is between 0.199 and 0.984.Furthermore,the effect value of fertilizer conservation is between 0.148 and 0.659,and the yield increase effect is between 0.031 and 0.655.According to differences in the utilization characteristics and the obstacle types of slope farmland in different regions,four kinds of regulation modes are put forward,that is,"soil and water conservation tillage+slope water system+soil fertilization","slope-to-terrace+soil and water conservation tillage+ecological restoration","slope-to-terrace+soil and water conservation tillage+slope water system",and"ecological restoration+slope-to-terrace+soil fertilization".