植物-土壤系统中水分再分配作用研究进展

Hydraulic redistribution in plant-soil systems

在过去100多年里,植物与土壤之间水分关系的研究多侧重于植物本身的水分利用方式、水分利用效率及其植物根系对水分的吸收等。然而进入20世纪80年代后期,植物生理生态学研究人员开始将注意力转移到植物根系对土壤水分的调节作用,即水分再分配(hydraulicredistribution)作用,具体地讲是在水势差的驱动下水分由根系向土壤中释出的一种双向和被动的水分运转过程,其中既包含水分由深层土壤向表层土壤的释出,也包括由表层土壤向深层土壤的流动,同时还涵盖了水分在水平方向上的侧向运输过程。伴随着研究手段的不断提高和对生态系统水平衡问题的关注,水分再分配逐渐成为近代植物生态学和水文学的交叉学科生态水文学(ecohydrology)的核心研究内容之一。目前该领域的研究已经阐明水分再分配作用在不同程度上对植物个体蒸腾、碳同化速率有很大贡献,有利于提高根系生活力和土壤养分;另外,在不断扩展的生态系统生态学研究中,也加强了对制约水分再分配作用发生的外部因子的认识。回顾和分析了水分再分配的研究历史、生态学意义、影响因素、测定方法等,特别提出阐述浅根系植物对水分再分配作用的依赖性与依赖程度,从植物进化学角度解释水分再分配作用发生的生理学基础和意义及水分再分配作用对土壤微生物活性的影响等方面将是未来研究的几个重点方向。

For much of the past century, researches on water relations of plant-soil systems mostly focued on the mode and efficiency of plant water use and water movement in soils. Although hydraulic redistribution of soil water by plant roots was first reported as early as the late 1920＇ s, the importance of the process in regulating soil water and associated ecological processes was not widely recognized by plant physiologists and ecologists until the late 1980＇ s. The term ＂hydraulic redistribution＂, known also as ＂hydraulic lift＂ as reported in many studies, refers to the phenomenon of soil water redistribution by plant roots through passive processes of water uptake and release driven by water potential gradient in the root-soil interface. However, hydraulic lift has been used more specifically to describe the transportation of soil water from deep wetter layers to shallow drier layers. Recent measurements of sapflow in taproots and lateral roots of trees have demonstrated that roots can also redistribution water either downward or laterally from moist surface soils to drier bottom soil layers. Because of the bidirectional movement of the transported water, ＂ hydraulic redistribution＂ has been proposed as a more comprehensive term than ＂ hydraulic lift＂ to describe the phenomenon. Hydraulic redistribution usually occurs at night when transpiration has diminished sufficiently, or during periods when transpiration is substantially reduced, to allow the water potential of the roots to exceed that of the drier parts of soil layers. To date, much progress has been made in understanding the role of hydraulic redistribution in maintaining water balance of plant community under drought conditions, and in identifying the internal and external factors that regulate the occurrence and magnitude of hydraulic redistribution. Three techniques have been commonly used in the hydraulic redistribution research： the first involves determination of the direction and rate of sapflow in roots using heat pulse or heat dissipation sapflow measurement sytems; the second is to measure the deuterium content of neighbor species when deuterated water is supply to deep roots of the target trees to trace the movement of water to shallow roots of grasses ; and the third is to measure changes in soil water potential and/or soil water content. There have been more than 60 cases of hydraulic redistribution reported for plant species across diverse habitats, and it is expected that the hydraulic redistribution is widespread wherever conditions are conductive to its occurrence. Nevertheless, more efforts are needed in exploring the ecological function and significance of the hydraulic redistribution, especially its significance in ecosystem restoration of degrading ecosystems in arid and semi-arid environments. In this article we reviewed our state of knowledge on hydraulic redistribution, and examined the significance of the process in water balance of plant-soil systems. Based on our review of literature and synthesis of existing information, we stressed the need for further research to evaluate the contribution of hydraulic redistribution to the water balance of plant community and ecosystem restoration in arid and semi-arid areas or in regions experiencing the stage of frequent droughts. Specific questions that should be addressed in future research may include, but not limited to： （1） How dependent are the plants with shallow roots on hydraulic redistribution by plants with deep roots in arid ecosystems？ （2） What are the biological basis and significance of hydraulic redistribution from perspective of plant evolution？ （3） What is the significance of hydraulic redistribution to maintaining soil microbial activities？ （4） Do the mode and magnitude of hydraulic redistribution differ among different typos of vegetation？ Moreover, quantitative information on the threshold of soil water potential and/or soil water content for inducing hydraulic redistribution in different ecosystems would be useful in elucidating the plant water relations in arid and semi-arid regions.