连栽桉树人工林土壤大孔隙特征及其对饱和导水率的影响

速生人工林多代连栽容易导致土地水源涵养能力下降。土壤大孔隙以优先流的形式补充地下水，是定量研究土壤水分运动的重要指标。以连栽1-4代桉树人工纯林为研究对象（记录为Ⅰ、Ⅱ、Ⅲ、Ⅳ），采用水分穿透曲线法，绘制水分穿透曲线，结合Poiseulle方程计算出大孔隙数量、半径及饱和导水率等指标，对土壤大孔隙特征及其对饱和导水率的影响进行研究。结果表明：（1）桉树人工林土壤的出流速率总体表现先匀速增加后趋于稳定，稳定出流速率总体表现为I > II > III > IV。（2）大孔隙半径范围在0.3-1.5 mm，主要集中于0.4-0.6 mm，随土层深度增加显著减小（P<0.05）。大孔隙数量范围在3.56×10⁴-4.81×10⁵个/m²。随着连栽代次的增加，大孔隙孔径范围变小，同一孔径范围的大孔隙数量减少。土壤容重与大孔隙特征呈极显著负相关关系；有机质含量与大孔隙特征呈极显著正相关关系。（3）各样地土壤饱和导水率范围在0.41-4.50 mm/min，并随着连栽代次增加而降低。将大孔隙的总数量、平均体积与土壤饱和导水率进行线性拟合，拟合方程为：y=ax+b=，（R²>0.66）。综上，随着桉树人工林连栽代次的增加，土壤大孔隙孔径范围缩小、同等半径的大孔隙数量减少，饱和导水率降低，土壤入渗及导水性能减弱，容易造成水土流失。

Characteristics of soil macropores and their influence on saturated hydraulic conductivity of successive Eucalyptus plantation

The multiple generations of fast-growing plantation forests have led to decrease in the water holding capacity of the forest land. Soil macropores replenish groundwater in the form of preferential flow and are an important factor in the quantitative study of soil water movement. A water breakthrough experiment was conducted on the soil taking successive planted 1-4 generations of Eucalyptus plantations as the research object, and the water breakthrough curve was drawn. The number of macropores, radius and saturated hydraulic conductivity were calculated with the combination of Poiseulle equation. The characteristics of soil macropore and the effect on the saturated hydraulic conductivity were investigated. The results show the following:(1) the effluent rate of Eucalyptus plantation soils in general showed a uniform increase and then stabilized. The steady effluent rate showed I > II > III > IV and decreasing with the increase of soil depth. (2) The macropore radius ranged from 0.3 to 1.5 mm, mainly concentrated in 0.4-0.6 mm, which significantly decreased with the deepening of the soil layer (p<0.05). The macropore amount ranged from 3.56×10⁴ to 4.81×10⁵/m². With the increase of successive planting generations, the macropore pore size range became smaller and the amount of macropores in the same pore size range decreased. This was the result of the combined effect of soil physical and chemical properties. Soil bulk density ranged from 1.15 to 1.45 g/cm³ and presented a highly significant negative correlation with macropore characteristics. Saturated water content and total porosity were all significantly and positively correlated with macropore characteristics. The organic matter content ranged from 5.43 to 32.33 g/kg and showed a highly significant positive correlation with the macropore characteristics. (3) The saturated hydraulic conductivity of soil in Eucalyptus plantations ranged from 0.41 to 4.50 mm/min, which decreased with increasing successive planting generations. This showed significant correlation with the basic characteristics of macropore. The total amount and mean volume of macropores showed a significantly positive correlation with the saturated hydraulic conductivity of the soil. The fitting equation was y=ax+b, (R² > 0.66), accounting for 66% and 79% of the variability in the saturated hydraulic conductivity, respectively. As a conclusion, with the increase of successive planting generations of Eucalyptus plantations, the pore size of soil macropores decreased, the amount of macropores of the same radius decreased, and the saturation hydraulic conductivity decreased. The infiltration and water conductivity of the soil under the Eucalyptus plantations are weakened, which is prone to soil erosion.