沙丘多枝柽柳灌丛根层土壤含水量变化特征与根系水力提升证据

 分析干旱区深根型荒漠植物的根层土壤水分是揭示荒漠植物与土壤水分关系机理的重要方面。在黑河中游一片风沙侵蚀区域的多枝柽柳(Tamarix ramosissima)人工林地中, 对表层0.3 m到3 m深的土壤不同深度的含水量进行了连续的动态观测。结果显示, 多枝柽柳根系层土壤含水量可以分为明显不同的3层: 浅层(0.2–1.7 m深)相对湿润层、中间(1.7–2.7 m深)相对干层和深层(2.7 m以下)有效含水层。在多枝柽柳生长盛期, 浅层相对湿润层土壤含水量呈现明显的昼夜变化特征, 同时, 在晚上植物根系与浅层土壤之间存在正水势梯度, 这说明存在根系水力提升现象。水力提升是干旱气候下根层浅层土壤含水量保持相对湿润的主要原因, 并因此维系浅层根系的发育, 也为多枝柽柳具备的防风固沙功能提供了可能的解释。据初步估算, 多枝柽柳根系水力提升占每天耗水量的5%–8%, 耗水的主要水分来源仍然是充足的土壤深层有效含水层。

Change characteristics in soil water content in root zone and evidence of root hydraulic lift in Tamarix ramosissima thickets on sand dunes

Aims Soil water in the root zone is a direct water source for desert phreatophytes; however, the significance of soil water for desert phreatophytes has been ignored. Instead, research has focused on the relationship between groundwater and desert phreatophytes. Our objectives were to explore spatial and temporal changes in soil water content in the root zone of Tamarix ramosissima, analyze the significance of soil water to the shrub and reveal root hydraulic lift phenomenon and its ecological effects.
Methods Soil volumetric water content was measured every half hour during the booming growth period of T. ramosissima by frequency domain capacitance sensors. The sensors were located at soil depths of 0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 2.1, 2.4, 2.7 and 3.0 m.
Important findings The soil water profile can be divided into three layers: shallow relatively wet layer (0.2–1.7 m), middle relatively dry layer (1.7–2.7 m), and deep available water layer. In the shallow relatively wet layer, the soil water content showed obvious diurnal variation, decreased in daytime and increased at night. At the same time, no similar variation of soil water content in the other two layers was observed. Concurrent measurement and comparison of the plant stem water potential and the soil water potential in the shallow relatively wet layer suggested that a positive water potential gradient in root-soil interface would exist at night, which is the physical basis of water efflux from root to soil. The root hydraulic lift of T. ramosissima is the main reason that the shallow soil layer was relatively wet under the arid climate. Further root sampling determined that fine roots in the shallow soil layer were greatly developed, which indicated that the hydraulically lifted water maintained the development of shallow fine roots and ensured root activity. The great development of shallow roots is a probable explanation for the sand-fixation function of T. ramosissima. It was estimated that the percentage of hydraulically lifted water to the daily evapotranspiration in T. ramosissima stand was about 5%–8%. Water from the deep layer accounted for most water consumption of T. ramosissima, which is jointly controlled by soil texture, root uptake and the
groundwater table.