黄土高原土石山区土壤密度的坡面变化——以六盘山香水河小流域为例
Variation of Soil Bulk Density on Slopes in the Rocky Mountainous Areas of Loess Plateau, Northwest China: A Case Study of Xiangshuihe Small Watershed in Liupan Mountains
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摘要: ［目的］ 土壤密度是最基本的森林土壤水文物理性质参数，为了定量描述土壤密度随坡位的变化和影响因素， 研究不同坡面土壤密度的空间异质性。［方法］ 在宁夏六盘山半湿润区香水河小流域，选择华北落叶松人工林和天然灌丛2个典型坡面，按坡位从上到下设置连续样地。2014年进行了森林土壤调查，在样地内掘土壤剖面，采用环刀法，分层测定土壤密度。［结果］ 在1 m深土层内，土壤密度（Y，g.cm-3）随土层（X，cm）加深呈线性增大，拟合关系式：华北落叶松坡面为Y1=0.755 5+0.007 7X（R2=0.99），天然灌丛坡面为Y2=0.919 5+0.004 7X（R2=0.98），其原因包括植被根系活动、土壤生物活动、砾石含量等的土层差异；在2个坡面上，0100 cm土层平均土壤密度均有明显坡位差异，且2个坡面的沿坡整体变化趋势不相同，华北落叶松坡面从坡顶到坡脚是先逐渐增大，后趋于稳定；天然灌丛坡面从坡顶到坡脚是"增大-减小-增大"的趋势，不同土层土壤密度的坡面变化也各不相同。在华北落叶松林坡面引起土壤密度坡位差异的主要原因是海拔，而在天然灌丛坡面是植物生长状况；2个坡面的土壤密度平均值均出现在相对坡长的0.40.5处。坡面上各样地的平均土壤密度和坡面平均值的差值（Z，g.cm-3）随相对水平坡长（样地离开坡顶水平距离和整个坡面水平坡长的比）（X，m）变化的回归关系式分别为：华北落叶松坡面Z1=-0.139X2+0.25X-0.080 1（R2=0.93），天然灌丛坡面Z2=0.494 4X3-0.767 2X2+0.340 3X-0.040 5（R2=0.95）。［结论］ 华北落叶松林坡面和天然灌丛坡面的土壤密度都存在明显的坡位变化，但变化格局和主要影响因素不同。基于土壤密度坡面变化的拟合关系，可实现从特定坡位测定值推算坡面平均值的尺度转换。Abstract: [Objective] Soil bulk density is the most basic parameters of the forest soil physical properties. To researched the spatial heterogeneity of soil bulk density on different slopes to quantitatively describe the variation of soil bulk density on slopes and the influencing factors. [Method] To selected 2 representative slopes, one covered by plantation of Larix principis-rupprechtii and another by natural shrubs, in the sub-humid small watershed of Xiangshuihe of the Liupan Mountains. Sample plots were set up continuously from the slope top downwards to slope foot. The soil physical properties including soil bulk density were investigated in 2014, through the ring core samples from different soil layers within the soil profiles of each plot. [Result] The mean soil bulk density (Y, g·cm-3) increased linearly with increasing soil depths (X, cm) within the range of 0-100 cm, with the fitted relation of Y1=0.755 5+0.007 7X (R2=0.99) for the plantation slope, and Y2=0.919 5+0.004 7X (R2=0.98) for the shrub slope. This vertical variation is influenced by the soil depth differences of vegetation root activity, soil biological activity and gravel content, etc. On the two slopes studied, the 0-100 cm average of soil bulk density showed a clear difference among slope positions, and the overall trend along the slope among the two slopes were also different. On the slope of L. principis-rupprechtii plantation, the soil bulk density firstly increased gradually from slope top downwards and then stabilized; while it showed a variation trend of "increase-decrease-increase" on the shrub slope. The variation of soil bulk density in each soil layer differed from each other among the 2 slopes, too. The main cause of such bulk density variation is the elevation difference for the plantation slope, but the plant growth for the shrub slope. The whole-slope average of soil bulk density appeared at the slope position with the relative slope length of 0.4-0.5. The variation of soil bulk density difference between certain slope position and the whole-slope average (Z, g·cm-3) with increasing relative slope length (the ratio between the horizontal distance of one plot from slope top to the horizontal length of whole slope) (X, m) was describe by a fitted relation: Z1=-0.139X2+0.25X-0.080 1 (R2=0.93) for the plantation slope, and Z2=0.494 4X3-0.767 2X2+0.340 3X-0.040 5 (R2=0.95) for the shrub slope. Whereby the whole-slope average of soil bulk density can be estimated from the bulk density measured at a certain slope position, so that up-scaling of soil bulk density can be realized within the spatial range from plot to slope. [Conclusion] An obvious variation of soil bulk density along slope position exists on both plantation slopes and shrub slope, but with different variation pattern and main influencing factor. Using the fitted relation describing the slope variation of soil bulk density, the up-scaling of soil bulk density from the measured value at certain slope position to the average of whole slope can be realized.
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Key words:
- soil bulk density
- / slope variation
- / spatial heterogeneity
- / Liupan Mountains
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