森林生态系统林木根系对优先流的影响

Effects of plant root systems on preferential flow in forest ecosystems

土壤水和溶质运移是土壤学和环境科学研究的难点和热点,优先流是一种常见的土壤溶质运移形式,绕过土壤基质而优先运移至地下水源,造成土壤养分的流失和水质的恶化。林木根系是土壤层的重要部分,其结构形态影响着优先流过程,为量化林木根系结构对土壤优先流的影响,以首都圈森林生态系统鹫峰定位监测站为研究区域,利用野外染色示踪与室内分析相结合的方法,定量分析根长密度和根系生物量在优先流区和基质流区的变化。结果表明:1)随着土层深度的增加,根长密度表现为减小的趋势,对径级d<1 mm,1<d<3 mm和3<d<5 mm根系而言,根长密度在优先流区大于基质流区发生概率分别为66.7%,88.9%和83.3%;2)根系d<1 mm对优先流贡献度最大,均值为94.8%,1<d<3 mm和3<d<5 mm根系对优先流贡献度较小,均值分别为4.3%和0.9%;3)研究点根系生物量进行统计,66.7%优先流区根系生物量大于基质流区根系生物量。开展根系对优先流的影响研究,有助于探明土壤水分运移规律,分析地表地下水质恶化根源,为生态环境安全提供理论指导和技术支持。

The study of the transportation of soil water and solutes,especially preferential flow,is a hot topic in pedology,ecology,and related environmental fields. Preferential flow,now recognized as a common pedological phenomenon,generally occurs without reaching equilibrium or as a non-uniform,random,highly variable process both spatially and temporally and is essentially unpredictable. Preferential flow and soil matrix flow are two typical permeating patterns of water flow and solute transport. Flow in soil matrix pores at the pedon scale results from spatial variation in soil texture and bulk density; it is affected by the presence of stones or rock fragments and by the ability of some substrates to repel water.Flow in large,continuous pores or macropores at the pore scale arises from different soil interactions such as biopores formed by root systems and macrofauna,cracks formed by freeze and thaw or shrink / swell circles,as well as voids formed by irrigation,cultivation,and tillage. Consequently,preferential flow results in complex flow patterns that bypass the normal soil matrix patterns; this increases the risk that pollutants（ e. g. heavy metals,radionuclides） will reach greater soil depths than would otherwise occur or that this also may allow pollutants to even reach the groundwater. Many factors control preferential flow including soil bulk density,soil structure,soil moisture content,rainfall intensity and tillage systems.Plant roots and other abiotic factors are some of the most important factors influencing preferential flow. In particular,the growth of plant roots influences the process of preferential flow. Obviously,root growth is a dynamic process and involvesthe formation of a continuous macropore network; plant roots may also create a series of well-connected pores that enhance preferential flow. Plant roots are widely known to play a key role in the development of preferential flow at the plot scale,but a quantitative description of the interaction between plant roots,the development of micropores and preferential flow is still a major challenge in preferential flow studies. This study investigated a forest ecosystemLocated in Jiufeng National Forest Park,Beijing,China,with the goal of characterizing and quantifying the effects of plant roots on preferential flow and the related complex interactions. Field dye tracing methods and laboratory experiments were applied to confirm and evaluate changes in root length density and root biomass along preferential pathways as well as in the soil matrix. Results indicated that 1） root length density,in general,decreased with an increase in soil depth. Root length density was larger in preferential pathways than in the surrounding soil matrix and was 66. 7%,88. 9% and 88. 3% for plant roots with a diameter（ d） of 〈1 mm,1〈 d 〈3 mm and 3 〈d 〈5 mm respectively. 2） The contribution of plant roots d〈 1 mm to preferential flow was almost 94. 8% for all experimental plots,while it was 4. 3% and 0. 9% for plant roots 1 〈d 〈3 mm and 3 〈d 〈5 mm,respectively. 3） The proportion of root biomass in the preferential pathways larger than in the soil matrix was 66. 7% for all experimental plots. Evaluating the effects of plant roots on preferential flow will help researchers to better understand the factors controlling soil water and solute transportation and may prevent subsurface flow and groundwater being polluted for forest ecosystems containing relatively more plant roots and stones,especially allowing for protection of the environment from pollution.