秦岭火地塘林区土壤大孔隙分布特征及对导水性能的影响

大孔隙广泛分布于森林土壤中,是定量研究与土壤水分运动有关的重要因素,其研究可深化森林涵养水源机理的认识。基于田间持水量到饱和含水量之间的土壤孔隙作为大孔隙的标准,利用土壤水分穿透曲线和Poiseulle方程研究了秦岭火地塘林区森林土壤大孔隙分布特征及其对土壤饱和导水率的影响。结果表明,林区土壤大孔隙当量孔径主要分布在0.3—3.8 mm之间;当量孔径1.5 mm的大孔隙密度较小,其数量仅占大孔隙总数量的5.37%;各当量孔径的大孔隙密度随土层分布基本呈现为上层大、下层小的特点,且垂直分布差异显著,其与有机质含量分布有极显著的相关性。0—60 cm土层大孔隙平均面积比顺序为:针阔混交林油松林落叶阔叶林华山松林。不同当量孔径的大孔隙密度与饱和导水率呈显著正相关关系,当量孔径大于1.5 mm的大孔隙密度决定了饱和导水率84%的变异;大孔隙率平均在1.6%—13.3%之间,当其小于5%时,饱和导水率随着大孔隙率增大而增大。

Distribution of soil macropores and their influence on saturated hydraulic conductivity in the Huoditang forest region of the Qinling Mountains

Macroporosity is an important factor related to soil water movement. Relatively little is known abut soil macroporosity in the Huoditang forest region of the Qinling Mountains. Although they contribute relatively little to total soil porosity, macropores have a very important influence on saturated hydraulic conductivity and on subsurface flow. The study of macropores is important for understanding the characteristics of soil water movement and the mechanism of water conservation under forest vegetation. At the microscopic scale, the flow of water in all soil macropores is related to the size of the void. Furthermore, water flow rates are controlled by the smallest void in any single continuous flow path (i.e., pore neck). The objectives of this experiment were to study the distribution of soil macropores in the Huoditang Forest region and to determine the influence of these macropores on saturated hydraulic conductivity. The volume of soil macropores is equivalent to the volume of water in a soil at saturation minus the volume of water in the same soil at field capacity. This definition, along with water breakthrough curves and the Poiseulle formula were used in this study. The results showed that the equivalent radii of soil macropores in the Huoditang Forest ranged from 0.3 to 3.8 mm. Macropores with equivalent radii > 1.5 mm made up only 5.37% of the total macropore content. The order of average area proportion of 0-60 cm layer was mixed forest > Pinus tabulaeformis forest > deciduous broadleaf forest > Pinus armandii forest. Heavy clay may restrict the development of macropores in the P. armandii forest soil. Macropore density was significantly greater in the upper soil layers than in the lower soil layers. There was significant correlation between macropore density and the soil organic matter content. Macropore distribution in the root zone differed significantly among the different forest types. The content of rock fragments in deeper soil layers also affected macropore distribution. The average densities of macropores with different equivalent radii were positively related with saturated hydraulic conductivity. The density of macropores with radii > 1.5 mm accounted for 84% of the variability in saturated hydraulic conductivity. Soil porosity ranged from 1.6 to 13.3%. The saturated hydraulic conductivity increased with porosity when porosity was less than 5%. This is because the possibility of such discontinuous behavior increased both as the size and connectivity of the macropores increased and as the effect of capillary tension within the macropores became smaller. Pore structure is of crucial importance. In general, macropores, especially those with radii >1.5 mm, play a critical role in conserving water in forest soils.