Hydraulic lift: consequences of water efflux from the roots of plants

Abstract

Hydraulic lift is the passive movement of water from roots into soil layers with lower water potential, while other parts of the root system in moister soil layers, usually at depth, are absorbing water. Here, we review the brief history of laboratory and field evidence supporting this phenomenon and discuss some of the consequences of this below-ground behavior for the ecology of plants. Hydraulic lift has been shown in a relatively small number of species (27 species of herbs, grasses, shrubs, and trees), but there is no fundamental reason why it should not be more common as long as active root systems are spanning a gradient in soil water potential (Ψ) and that the resistance to water loss from roots is low. While the majority of documented cases of hydraulic lift in the field are for semiarid and arid land species inhabiting desert and steppe environments, recent studies indicate that hydraulic lift is not restricted to these species or regions. Large quantities of water, amounting to an appreciable fraction of daily transpiration, are lifted at night. This temporary partial rehydration of upper soil layers provides a source of water, along with soil moisture deeper in the profile, for transpiration the following day and, under conditions of high atmospheric demand, can substantially facilitate water movement through the soil-plant-atmosphere system. Release of water into the upper soil layers has been shown to afford the opportunity for neighboring plants to utilize this source of water. Also, because soils tend to dry from the surface downward and nutrients are usually most plentiful in the upper soil layers, lifted water may provide moisture that facilitates favorable biogeochemical conditions for enhancing mineral nutrient availability, microbial processes, and the acquisition of nutrients by roots. Hydraulic lift may also prolong or enhance fine-root activity by keeping them hydrated. Such indirect benefits of hydraulic lift may have been the primary selective force in the evolution of this process. Alternatively, hydraulic lift may simply be the consequence of roots not possessing true rectifying properties (i.e., roots are leaky to water). Finally, the direction of water movement may also be downward or horizontal if the prevailing Ψ gradient so dictates, i.e., inverse, or lateral, hydraulic lift. Such downward movement through the root system may allow growth of roots in otherwise dry soil at depth, permitting the establishment of many phreatophytic species.

A Mathematical Model of Water and Nutrient Transport in Xylem Vessels of a Wheat Plant

“…that an increase in p 0 (and p ext ), which occurs when transpiration is reduced, results in MDF in the root zone and consequently water e✏ux into the soil. The traditional case of hydraulic lift discussed by Caldwell et al (1998) is characterised by outwards radial flow from the roots to the soil in the upper soil layers and inwards radial flow at the lower soil layers, which is opposite to the profile found in this paper for root MDF. This traditional profile explains why the process is called hydraulic lift, as water is transferred from lower moist soil to higher dry soil.…”

“…This traditional profile explains why the process is called hydraulic lift, as water is transferred from lower moist soil to higher dry soil. However, the opposite flow profile (inwards radial flow at the upper soil layers and outwards radial flow in the lower soil layers) which is called inverse hydraulic lift is also found to occur, but usually for roots grown in dry soils (Caldwell et al, 1998). Inverse hydraulic lift displays the same radial water profile as the root MDF seen in this paper, although the process may be di↵erent, since inverse hydraulic lift involves the transfer of water downwards which is not seen in the divergent flow of the root UDF.…”

“…This regime seems somewhat physical as there is evidence that similar flows occur; for example passive water e✏ux from roots to soil of lower water potential has been observed and is known as hydraulic lift (Caldwell et al, 1998). Water is released from roots when transpiration is low, which is usually at night, and then reabsorbed the next Appendix B: Range of p 0 that would give rise to unidirectional flow for p 0 6 = p ext , and G = 0…”

Multiple cropping systems as drivers for providing multiple ecosystem services: from concepts to design

“…For instance, the facilitation of water acquisition in drought conditions by hydraulic lift has been reported for species with different root system architectures (Caldwell et al 1998 and references therein; Pang et al 2013;Sekiya et al 2011). Isotope labeling methods demonstrated that species with deep root systems transfer water from deeper wet soil layers to shallow dry layers, which benefits neighboring plants (in particular species with shallow root systems) (Dawson 1993;Sekiya and Yano 2004).…”

Fine-scale spatial patterns of the Tertiary relict Zelkova abelicea (Ulmaceae) indicate possible processes contributing to its persistence to climate changes

“…During drought periods, Acer saccharum demonstrates ''hydraulic lift,'' a nocturnal uptake of water by roots from deep soil layers that is released from shallow roots into upper soil layers (Dawson 1993), so that neighboring plants can use 3-60 % of the hydraulically lifted water supplied by Acer saccharum trees. Overall, hydraulically redistributed water has been shown to increase annual water use in A. saccharum forest by 19-40 % (Caldwell et al 1998).…”