Water vapor movement and utilization with condensation in the upper layers of a sandy soil column

In semi-arid and arid regions, prolonged dry spells lead to a significant reduction in topsoil moisture, forming a dry soil layer, where water can only move as vapor. Such conditions hinder young crops with weakly developed root systems to directly reach deep water reservoir. Since there is no indication of gaseous water uptake from soil pores, plants might be able to acquire water from these deep sources when water vapor condensates at night or at a vapor barrier (mulch). To trace potential water vapor uptake by plants, we developed a sand column experiment using deuterium labeled water. The water source column was separated by a glass bead layer and a root barrier (50µm mesh) from the soil to ensure that there was no capillary rise or root uptake of liquid water. Four treatments with three replicates, including planted (Pl), unplanted (Un), mulch variation (Pl+M; Un+M) and an additional unlabeled control column, were installed in a climatic chamber. Vigna radiata a moderately drought resistant plant species was selected for this experiment. Constant day-night cycle with stable temperature and light conditions were maintained for a period of seven days without irrigation. Afterwards, soil samples were collected at 0-5, 5-10 and 10-15 cm depths. Vigna radiata saplings and condensed water samples were collected separately. Cryo-extraction of water from the samples and liquid isotopic water analysis revealed substantial results for the uptake of water vapor by young saplings. Evaporation from the water source into the column was recorded to be 0.7-2.1 ml. The δD/H ratios were analysed using Triple Isotope Water Analyser (Los Gatos Research). The relative potential uptake of water vapor by young saplings was recorded to be as high as 0.56ml for the 'Pl' and 0.35ml for the 'Pl+M' treatment. The utilization of water vapor by young plants in the upper soil layer could prolong plant life during dry spells. Although, it remains uncertain to what extent the prolongation could be maintained.

Keywords: Deuterium Labeled Water, Plant Vitality, Water Conductivity