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

Roots are movers of water in the soil. One method of movement is through hy draulic lift, which occurs when plants extract water from a moist subsoil a nd release it into a dry topsoil. Detection of hydraulic lift has been hamp ered by the lack of instruments sensitive enough to measure the small amoun t 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 re leased by hydraulic lift, using the DPHP technique. Sunflower (Helianthus a nnuus L.) was grown in a column (38 cm height; 25 cm diam.; bulk density = 1.45 Mg/m(3)) packed with a Haynie very fine sandy loam (coarse-silty, mixe d, calcareous, mesic Mollic Udifluvents; FAO-Eutric Fluvisols) with its roo ts divided between a top dry layer and a lower wet layer. Eight DPHP sensor s installed in the soil column were used to monitor soil water content. Dur ing 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 a nd then it was re-watered. At this time, the roots in the upper dry layer r eleased water, increasing the soil water content in the centre of the root mass by 0.019 m(3)/m(3) (increase from 0.121 m(3)/m(3) to 0.140 m(3)/m(3)). The soil-water increase was similar to other values reported in the litera ture and show it to be small.