| Slope farmland is the main land type of karst area in southwest China. Because of its special topography and geological structure, rock desertification and soil erosion problem is serious in slope farmland area, which caused barren soil nutrients and low land quality. Therefore, study of the spatial variability of the soil properties in slope farmland at karst rocky desertification area is meaningful, and is an important basis for soil erosion and rock desertification evaluation of the slope farmland in karst area, which also play an important role for utilization of sloping farmland.By the methods of geo-statistics, this paper studied the spatial variability of soil properties in the slope land of karst rock desertification area in Zhong Liang Mountain, Chongqing. Statistical analysis and spatial interpolation were used for the soil physical and chemical properties mapping, which is useful to understand the spatial variability of soil properties. This paper also analyzed the impact of environmental factors on soil properties, to provide a reference for the rational development and utilization of the slope farmland in rocky desertification area, as well as the prevention of rocky desertification governance.After field investigation in Zhong Liang Mountain, two different slope farmlands were chosen as research plots. Based on the purpose, two plots seized100×100m were installed (plot A and plot B), then soil sampling and data analysis implemented in these two plots. The results are as follows:The classical statistical analysis of soil physical properties showed that:The variation coefficient value of surface soil bulk density, porosity and silt in plot A were less than0.1, which presented weak variability; the variation coefficient value of soil moisture, topsoil depth, sand and clay in plot A were between0.1and1, which presented moderate variability. The surface soil bulk density, moisture, porosity and silt’s variation coefficient in plot B were less than0.1, showed weak variability; while the variation coefficient of the topsoil depth, sand and clay were between0.1and1, showed moderate variability. The classical statistical analysis of soil chemical properties showed that the surface soil organic matter, total nitrogen, total potassium, hydrolytic Nitrogen and available potassium’s variation coefficient in plot A and plot B were between0.1and0.3, performed moderate variability; the variation coefficient of the soil pH in both plots were less than0.1, showed weak variability; the available phosphorus variation coefficient in plot B was1.08, which is the maximum value of all the chemical variation coefficient, expressed a strong variability; the available phosphorus variation o coefficient in plot A was0.82, also showed strong variability. Overall, the variability of surface soil chemical properties in plot B is stronger than that in plot A.The reasonable number of samples required for small scale spatial variability analysis showed that:The reasonable number of sampling varies greatly, and is mainly related to the variation coefficient value of the current sample data. The larger the variation coefficient, the greater the reasonable sampling number required. At the significance level for90%, and accuracy error for10%, the reasonable sampling number required for most of the soil properties are less than25, which proved that the actual soil sampling number in this study can meet the requirements for analysis of the spatial variability of soil properties.The Geo-statistical analysis showed that:Generally, the surface soil physical properties in plot A has a strong spatial correlation, the structural factors (including weather patterns, soil parent material, topographic features, rock properties, soil type and other natural factors) impact it strongly. Spatial distribution of the surface soil physical properties in plot B are both affected by structural factors (natural factors) and random factors (human factors). Geo-statistical analysis of the surface soil chemical properties of the study area showed that vast majority of the soil chemical properties has strong space correlation, structural factors, such as slope, slope position, aspect and rock properties have an important impact on the spatial variability of soil chemical properties.The analysis of the spatial distribution characteristics showed that:The surface soil physical properties in of plot A and plot B showed different spatial distribution pattern from each other. For plot A, the moisture content of the surface soil and topsoil thickness showed a similar space distribution characteristics, from the top of the slope to the bottom of the slope, its content increased gradually, with sloping contours distribution; bulk density, porosity, sand, silt, clay spatial distribution characteristics showed a plaque, might be associated with different local terrain. For plot B, the surface soil moisture content, porosity and thickness of topsoil showed similar distribution characteristics, followed by rising from the top of the hill to the bottom of the slope; bulk density, on the contrary, from the top of the slope to the bottom of the slope, the content reduced; silt and clay performed a more uniform spatial distribution, the spatial variability is relatively weak. Same with the surface soil physical properties, the surface soil chemical properties also showed different spatial distribution characteristics. For plot A, the spatial distribution of pH performed patchy; From the top of the slope to the bottom of the slope, roughly along the north-south direction, the soil organic matter content decreased; The spatial distribution of total nitrogen and hydrolytic nitrogen is more uniform, performs weak variability; The total Potassium and available potassium showed a strip from the top of the slope to the bottom of the slope, and its content decreased gradually; The spatial distribution of total phosphorus and available phosphorus is similar. For plot B, the pH and soil organic matter increased gradually from the upper-left corner to the right corner at the base slope, performed a banded structure along the east-west direction; The content of total nitrogen and total potassium showed a patchy distribution; The distribution characteristics of total phosphorus and available phosphorus is contrary to that of pH and soil organic matter; From the upper left corner to the right corner at the base slope, the total phosphorus and available phosphorus decreased gradually; The Spatial distribution of hydrolyzed nitrogen is homogeneous; And the potassium gradually reduced from the top slope to the bottom slope. Overall, the spatial variability of surface soil chemical properties in plot A and plot B are relatively strong, varies from each other, and is mainly affected by structural factors(natural factors), and affected by human factors in some degree.The Multivariate statistical analysis between environmental factors and soil properties showed that:Environmental factors take significant effect on the spatial distribution of soil physical and chemical properties. For plot A, the incidence of different environmental factors are sequenced as follows:slope position> rock strike and dip> aspect> vegetation coverage> bedrock bare rate> slope shape> rock inclination> vegetation and crop type> slope> farming level. Slope position is the constraint factor for the spatial variability of surface soil properties in plot A. For plot B, the incidence of different environmental factors are sequenced as follows:slope position> slope shape> farming level> vegetation and crop type> aspect> vegetation coverage> slope> rock inclination> rock strike and dip>bedrock bare rate. Overall, slope position and slope shape are the constraint factors for the spatial variability of surface soil properties in plot B. From the RDA three-dimensional ordination diagram, the results showed that slope position is the most important environmental constraint factor for the spatial variation of the slope farmland in rocky desertification area at small scale. |
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