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

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.