| Collapsing gully is a serious and specific soil erosion phenomenon that the slope surface collapse under the effect of common hydraulic and gravity,mainly occurs in tropical and subtropical granite hilly region of South China.Collapsing gully has many characteristics,such as large quantity,wide distribution,rapid development and large erosion,damages land resource,deteriorates ecological environment.Alluvial fan is the component parts of collapsing gully,a large amount of alluvial material moving into the farmland caused serious farmland desertification and the loss of tilth.With our state pay more and more attention to ecological civilization construction,the prevention study of collapsing gully alluvial is in a hurry.The study on spatial variability of soil properties and fertility evaluation of collapsing gully alluvial fan has special practical significance.It can enrich theoretical basis of effective management of alluvial fan,and provide scientific practice basis for fertilization,tillage,classification,restoration,improvement and sustainable use of the alluvial soil.This paper focused on the typically collapsing gully alluvial fan in Wuhua County,Guangdong Province.Through the field investigation and measurement combined with the indoor experiment and analysis,based on GIS,geostatistical analysis method,fractal theory,analytic hierarchy process(AHP)and any other related software,we study the physical and chemical character,fractal dimension characteristic and spatial variation of fertility quality of collapsing gully alluvial fan,modeling the nutrient transport rule.The main conclusions are as follows:(1)In this alluvial fan,the sand average content of the soil particle composition is the highest,clay content is the lowest,and the soil nutrient status is also very low.Compared with the unaffected farmland,soil texture become coarse,saturation moisture content and capillary porosity decrease,aeration porosity and saturated hydraulic conductivity increase,the content of soil nutrients decreased significantly,soil pH value decrease.In terms of variability,the variation coefficient of soil pH is weak variability,while other variation coefficient of soil properties values are medium variation.In terms of relevance,the soil sand and clay shows very significant correlation with all the other soil properties,soil organic matter shows very significant positive correlation with other nutrients,and there is also a significant positive correlation between soil nutrients.There is a significant correlation between soil pH and sand content and cation exchange capacity,and soil pHalso shows relevance with other soil properties.(2)According to the principle of minimum error of cross validation method,theoretical semivariogram models of the content of clay,silt,sand,saturated moisture content,aeration porosity,soil hydraulic conductivity,total nitrogen,total phosphorus,total potassium and cation exchange capacity are best described with a Gaussian model,capillary porosity use exponential model,whereas organic matter,available nitrogen,available phosphorus,available potassium and pH are best fitted with a spherical model,and the determination coefficient is the highest,and the fitting effect is the best.All the soil physical and chemical properties of this alluvial fan are characterized by strong spatial correlation,and the variability of soil properties is mainly affected by structural factors,such as parent material,topography,rainfall,etc.All the spatial variation ranged from 32.94 to 61.00 m,so the alluvial fan soil property variation is obvious.According to the result of ordinary kriging interpolation,the soil properties of the alluvial fan show strong spatial distribution regularity,which is consistent with the results of correlation analysis.From the top to edge of alluvial fan,the distribution of clay content,silt content,capillary porosity and saturated water content increase convex to the fan edge strip shaped regularly;the distribution of sand content,ventilation porosity and saturated hydraulic conductivity decrease in strip form;Soil pH changes from strong acid to acid;The content of soil nutrient increase gradually,organic matter,total phosphorus,available potassium and cation exchange capacity present a regular spatial distribution pattern of increasing convex to the fan edge arc shaped,while the distribution trend of total nitrogen,total potassium,available nitrogen,available phosphorus content is irregular,that shows scattered distribution of plaque shaped.(3)The fractal dimension of this alluvial fan ranged from 2.496 to 2.825,the average value is 2.669,the coefficient of nugget is much less than 10%,variable range is 44.444 m.There is a significant linear positive correlation between fractal dimension and clay,silt and nutrient content,while fractal dimension present a significant linear negative correlation with sand content.Among them,clay content was the most influential factor to fractal dimension,and the coefficient correlation was up to 0.969,so that soil fractal dimension can effectively reflect the soil texture level and fertility quality.The spatial variation rule of fractal dimension is similar to clay,silt and nutrient indexes,contrary to sand.The fractal dimension is the smallest in the vicinity of the fan apex,and the maximum is near the fan edge,from the top to the edge of the alluvial fan,fractal dimension presents a increasing trend of convex to the fan edge strip shaped regularly.(4)The soil fertility quality is poor in this alluvial fan,the IFI ranges from 0.089 to0.637 with the average value of 0.443,and from the top to the edge of the alluvial fan the IFI increase gradually in stripe form obviously.Soils fertility was divided into level five,level four and level three.The area of level five is the largest,and area accounts for55.98%.The area accounts of level four and level three are 33.84% and 10.18%.The poorest soils(IFI<0.2)are located in the top of the alluvial fan,while the richest(IFI>0.6),in the edge of the alluvial fan.In other words: the farther the distance to the top of the alluvial fan,the richer the soil.(5)The elevation of this alluvial fan ranges from 207.039 to 208.548,which is the highest at the top of the fan,and the lowest near the fan edge,from the top to edge of the alluvial fan,the elevation presents decreasing trend of convex to fan edge arc shaped.The relative elevation and distance to top of the alluvial fan shows significant correlation with soil nutrient content,with the increase of elevation,the nutrient content decreased,and the nutrient content increased with the increase of the distance from the top of the fan.In this alluvial fan range,it is different that the effect of terrain and runoff on nutrient,but the trend of global radiation is obvious.The nutrient prediction equation is obtained by taking the distance from the top of the fan,the relative elevation and the fractal dimension as variables,and the fitting coefficients are up to 0.663,the error is less than 8.09%,therefore this equation can be used to quantitatively describe the soil nutrient distribution and transportation rules of alluvial fan. |
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