火烧迹地根系分布对浅层含根土土水特征及渗透特性的影响

Influences of root distribution on the soil water characteristics and permeability characteristics of shallow root-bearing soil in the burned areas

火灾会造成森林浅层土水力特性发生改变,植物根系可以改变土壤土水特征和渗透特性。以重庆市北碚区缙云山虎头村的火烧迹地为研究对象,采用平衡锥仪法、烘干法等测定不同火灾程度下马尾松林和毛竹林土壤的液限、密度、饱和含水率等基本物理性质,通过变水头渗透试验测定浅层含根土的饱和渗透系数k s,采用激光粒度仪法测定颗粒级配曲线并基于van-Genuchten(VG)模型拟合土-水特征曲线。结果表明:1)森林火灾导致其浅层土壤密度降低,饱和含水率、液限、孔隙度等显著升高;2)火烧迹地根系数量减少,其中0~<2 mm径级的细根数量变化最大,减少约30%~50%;3)对于轻、中和重3种火烧程度,马尾松林火烧后其饱和渗透系数升高,尤其在重度火烧区表层土相比未遭受火烧浅层土的渗透系数提高58.26倍,毛竹林导水率变化规律与马尾松林相似且渗透系数的改变程度更大,其重度火烧区表层土的渗透系数提高162.24倍;4)火烧迹地浅表层含根土土-水特征曲线进气值参数α随火烧程度的加剧而降低3.0%~6.8%,其孔径分布参数n随火烧程度的加剧而增加6%~30%。研究结果进一步量化森林火灾对浅表层含根土饱和渗透特性和土水特征的影响,为火烧迹地边坡水力特性研究及边坡稳定分析提供科学依据。

[Background]A forest fire occurred in Hutou village,Xiema street,Beibei district,Jinyun Mountain National Nature Reserve,Chongqing.In order to further study the harm caused by forest fires and promote their post-disaster reconstruction,the author carried out an investigation in the burned area of Jinyun Mountain Nature Reserve,collected soil samples to determine the saturated permeability coefficient and soil-water characteristic curve of soil,and studied the response mechanism of fire process to soil hydraulic characteristics,in order to provide reference for the prevention and control of geological disasters in the burned areas.[Methods]The basic physical properties such as liquid limit,bulk density and saturated water content under different fire degrees were measured by equilibrium cone method and ring knife method.The saturated permeability coefficient k s of shallow root soil was measured by soil variable head permeability test.The particle size distribution curve was measured by laser particle size analyzer and the soil water characteristic curve was fitted based on van-Genuchten(VG)model.[Results]1)After the forest fire,the bulk density,saturated water content,liquid limit,porosity and other physical properties of the shallow surface soil increased,and the number of roots in the burned area decreased.Among them,the number of fine roots 0-2 mm changed the most,which decreased by about 2-3 times.2)For the three degrees of mild,moderate and severe fire,the saturated permeability coefficient of Pinus massoniana forest and Phyllostachys pubescens forest increased after fire.In particular,the permeability coefficient of the surface soil in the severely burned area of P.massoniana forest increased by 58.26 times compared with that of the unburned shallow soil,and the permeability coefficient of the surface soil in the severely burned area of P.pubescens forest increased by 162.24 times.3)The results showed that the soil particle size in the fire area was smaller than that in the non-fire area,and the air intake value parameterαand the pore size distribution parameter n of the soil water characteristic curve of the shallow root soil in the fire area decreased with the increase of the degree of fire,and the pore size distribution parameter n increased with the increase of the degree of fire.[Conclusions]Forest fires lead to a decrease in the number of fine roots in the surface soil and an uneven distribution in the soil layer,and affect the physical properties of the surface soil,so that the pore size distribution changes,which further leads to the increase of the saturated permeability coefficient and the weakening of the water holding capacity.In the post-disaster recovery of geological disasters in burned areas,the response mechanism of the above soil hydraulic characteristics should be considered.