Root-zone hydraulic lift evaluated with the dual-probe heat-pulse technique

Abstract

Roots are movers of water in the soil. One method of movement is through hydraulic lift, which occurs when plants extract water from a moist subsoil and release it into a dry topsoil. Detection of hydraulic lift has been hampered by the lack of instruments sensitive enough to measure the small amount of water moved. Recently, the dual-probe heat-pulse (DPHP) technique has been used to monitor with fine spatial resolution the soil water content in root-zones. The objective of this research was to determine if water is released by hydraulic lift, using the DPHP technique. Sunflower (Helianthus annuus L.) was grown in a column (38 cm height; 25 cm diam.; bulk density = 1.45 Mg/m³) packed with a Haynie very fine sandy loam (coarse-silty, mixed, calcareous, mesic Mollic Udifluvents; FAO-Eutric Fluvisols) with its roots divided between a top dry layer and a lower wet layer. Eight DPHP sensors installed in the soil column were used to monitor soil water content. During 24 measurement days, hydraulic lift was evident only when the plant was wilted. This occurred when the lower ‘wet’ layer had been allowed to dry and then it was re-watered. At this time, the roots in the upper dry layer released water, increasing the soil water content in the centre of the root mass by 0.019 m³/m³ (increase from 0.121 m³/m³ to 0.140 m³/m³). The soil-water increase was similar to other values reported in the literature and show it to be small.