紫色土坡耕地耕层质量影响因素及其敏感性分析

坡耕地耕层质量变化特征由降雨侵蚀、耕作活动交互作用影响。该文以紫色土坡耕地为研究对象,结合主成分分析与评价模型对耕层质量特征进行分析,并解析了海拔、坡度、有效土层厚度和坡位4个地形因子对坡耕地耕层质量的影响及敏感性。结果表明:1)小流域坡耕地耕层厚度、土壤有机质、土壤容重均处于适宜性区间内,各土壤养分指标总体处于中低水平;土壤容重(变异系数CV为7.97%)和总孔隙度(CV为8.36%)变异系数相对较低,处于弱分异(CV<10%)水平,其余土壤指标均属中等分异水平。2)紫色土坡耕地耕层质量评价最小数据集由容重、贯入阻力、有机质、土壤黏粒含量、耕层厚度、抗剪强度、饱和导水率和有效磷8个指标组成。基于MDS评价表明,紫色土坡耕地不同坡位耕层质量指数为下坡(0.458)>中坡(0.443)>上坡(0.417),下坡较中坡和上坡分别提升3.39%和9.83%。3)紫色土坡耕地耕层质量影响因素可分为4类,Ⅰ类坡度限制型、Ⅱ类坡位限制型、Ⅲ类有效土层厚度限制型、Ⅳ类海拔限制型,4种耕层类型的样本数分占坡耕地耕层样本总数的38.89%、22.22%、14.81%和24.08%,主要影响因素是坡度。4)坡耕地耕层质量与有效土层厚度、坡位为正相关,与海拔、坡度表现为负相关,地形因子对耕层质量敏感程度为海拔(-0.399)>坡位(0.192)>坡度(-0.112)>有效土层厚度(0.110),海拔敏感程度为有效土层厚度的3.56倍。研究结果可为紫色土坡耕地耕层质量评价及有效调控提供理论依据,有利于紫色土坡耕地资源可持续利用。

Influencing factors and sensitivity analysis of cultivated-layer quality of purple soil slope farmland

The change process of cultivated layer quality for slope farmland at plot scale is influenced by the interactive effects between erosive rainfall and artificial farming activities at different temporal and spatial scales. For a slope surface, there exists a successive erosion process of rainfall-runoff-infiltration and the consequence is first reflected in the quality of cultivated layer. Among the factors induced the erosion process at plot scale, terrain is an important factor because it could redistribute the ratio of rainfall--infiltration, which directly cause a different erosion process for an erosive rainfall and the different degradation degree during a rainy season and a growth season. Taking purple soil slope farmland as an example, the paper firstly analyzed the varying characteristics of cultivated layer quality of slope farmland, then quantitatively evaluated the effects of four terrain factors, altitude, slope angle, effective soil thickness and slope position on cultivated layer quality and their sensitivity of impact, adopted principal component analysis and evaluation model. The results indicated: 1) Such soil properties as cultivated layer thickness, soil organic matter and soil bulk density of cultivated layer were all within the suitable scope and soil nutrient properties were generally at middle & low levels. The variation coefficients of soil bulk density and soil total porosity showed weak variability (CV<10%), presenting 7.97% and 8.36% respectively and the other soil properties showed moderate variability. 2)Minimum data set for cultivated layer quality evaluation of purple soil slope farmland consisted of eight soil parameters, bulk density, penetration resistance, soil organic matter, clay content, cultivated layer thickness, soil shear strength, saturated water conductivity and bioavailable phosphorus. Soil physical properties are more important than chemical properties except soil organic matter for erosion control and crop growth of slope farmland. Evaluation based on the MDS revealed that the cultivated layer quality varied spatially over the slope, being 0.458 in the low part of the slope, 0.443 in the middle part of the slope and 0.417 in the upper part of the slope and the quality value of the lower slope was 3.39% and 9.83% higher than that of the middle slope and upper slope positions, respectively. 3) The factors affecting cultivated layer quality divided into four types: slope-angle impacted type, slope location impacted type, effective soil layer thickness impacted type and altitude impacted type and they accounted for 38.89%, 22.22%, 14.81% and 24.08% of the total samples taken from slope farmland respectively. The slope angle was the main determinant of cultivated layer quality because it could induce directly many ecological processes at plot scale. The slope angle and effective soil layer thickness could be modified by such soil and water conservation measures as terrace, biological ridge and soil improvement methods. But the slope location and altitude could not be modified easily only by selecting suitable crop system. 4) The quality of cultivated layer had positive correlation with its thickness and location in the slope, and negative correlation with altitude and slope angle. Sensitivity analysis revealed that the determinants were ranked in the order of altitude (-0.399) > location in the slope (0.192) > slope angle (-0.112) > effective soil layer thickness (0.110) based on their impact on cultivated lay quality. These results could provide some useful evaluation parameters and its regulating direction for cultivated layer quality, which contributed to the sustainable utilization of purple soil slope farmland. Furthermore, other samples of slope farmland should be studied in order to find out the critical values of MDS according to 2 functions of cultivated layer, erosion control and soil productivity.