三峡库区森林土壤大孔隙特征及对饱和导水率的影响

土壤大孔隙是土体内孔径较大能优先传导水分的根孔、洞穴或裂隙,大孔隙内优先流的产生是土壤水分运动研究由均衡走向非均衡的标志。利用原状土柱的水分穿透试验,对三峡库区山地不同林型覆盖下土壤的大孔隙结构进行了研究,分析了温性阔叶林棕壤、针阔混交林黄棕壤、暖性针叶林黄壤及弃耕草地剖面内大孔隙的剖面分布特征及其对土壤饱和导水率的影响。结果表明:研究区内森林土壤的大孔隙当量孔径在0.3—3 mm之间,占土壤总体积的0.15%—4.72%。大孔隙中孔径0.3—0.6 mm的大孔隙密度最大,占大孔隙总数量的72.2%—90.4%;而孔径1 mm的孔隙仅占大孔隙总数量的1.26%—8.55%。土壤大孔隙密度和大孔隙面积比的顺序为:温性阔叶林棕壤针阔混交林黄棕壤针叶林黄壤弃耕坡地。各孔径段的大孔隙密度在不同样点均呈现A层-B层-C层逐渐减小的趋势,大孔隙密度与有机质含量呈显著正相关关系。土壤饱和导水率与不同孔径大孔隙的密度、面积比均成显著正相关关系,孔径1mm的大孔隙仅占大孔隙总数量的1.26%—8.55%,但决定了饱和导水率84.7%的变异。此外,森林土壤饱和导水率与各土壤层的有机质含量成显著正相关关系,有机质的增多有利于改善土壤的入渗性能。

Characteristic of soil macropores under various types of forest coverage and their influence on saturated hydraulic conductivity in the Three Gorges Reservoir Area

Soil macropores refer to pores with a larger diameter and higher hydraulic conductivity than those of the soil matrix; these macropores include root channels, caves, and fissures. The preferential flow of water to the soil macropores would result in non-uniform soil water transport, this indicates a need to study the mechanisms of soil water movement from homogeneous to heterogeneous fields. Water breakthrough experiments with undisturbed soil samples identified the structure of the macropores of four kinds of mountain soils covered by different forest types in the Three Gorges Reservoir Area (brown earth covered by subalpine temperate deciduous broad-leaved forest, yellow-brown earth covered by deciduous coniferous broad-leaved mixed shrubs, yellow earth covered by low-mountain warm coniferous forests, and abandoned farmland covered by herbaceous plants). The profiled soil macropore characterizations and their effects on saturated hydraulic conductivity were analyzed. The results indicated that the equivalent radii of the soil macropores in the study area ranged from 0.3 to 3.0 mm, accounting for only 0.15%-4.72% of the soil bulk volume. The proportion of macropores with the radii from 0.3 to 0.6 mm was the largest, accounting for 72.2%-90.4% of the total macropore amount, while the proportion of macropores with radii > 1 mm accounted for only 1.26%-8.55%. Soil macropore density and the proportion of the average area of the macropores in different soil types decreased in the following order: brown earth covered by subalpine temperate deciduous broad-leaved forest > yellow-brown earth covered by deciduous coniferous broad-leaved mixed shrubs > yellow earth covered by low-mountain warm coniferous forest > abandoned farmland covered by herbaceous plants. All macropores, regardless of radius, decreased with increasing soil depth, from the humus horizon (layer A) to the leaching illuvial horizon (layer B) to the parent material horizon (layer C), and a significant correlation was observed between soil macropore density and soil organic matter content. Significant correlations were also observed between the following pairs: saturated hydraulic conductivity and soil macropore density, and proportion of the average area of macropores and organic matter content. Macropores with a radius > 1mm, constituting only 1.26%-8.55% of the total amount of macropores, accounted for 84.7% of the variability in saturated soil hydraulic conductivity. Collectively, these results suggest that increased organic matter in forest soil layers can improve its infiltration capability.