Water vapor from deep soil reservoirs as a viable water source for plants in sandy soils

In sandy soils, the formation of a dry soil layer creates a capillary barrier that restricts the upward flow of water, thereby limiting its availability to plants. Under such conditions, the water from deeper soil reservoirs may reach the root zone primarily as vapor. It is currently unknown if plants can utilize this water vapor and if there are management possibilities that could enhance the respective water vapor uptake. This study investigates the potential for water vapor uptake in Vigna radiata under controlled drought conditions. Ten-to-fifteen-day old saplings were introduced into columns with sandy soil, that was separated from a water reservoir by a capillary barrier and a root impermeable mesh (50 µm). Treatments included unplanted and planted columns (with and without mulch), with an additional set of planted columns comparing plant survival in the presence or absence of the water vapor source. Cryo-extraction and liquid water isotopic analysis (δ²H) of saplings, soil layers (0–5, 5–10 and 10–15 cm) and vapor condensates revealed differential deuterium enrichment, indicating upward vapor flux and plant uptake of water vapor. Plants with access to water vapor source exhibited an extended survival of ∼ 2.7 days under drought. Mulching further amplified the effectiveness of vapor availability by 38.2 % through reduced surface evaporation, thereby extending the plant survival by ∼4.5 days, compared to planted treatment with no vapor source. These findings provide evidence that plants can access water from a spatially separated water reservoir under conditions permitting only water vapor movement and suggest that agronomic practices such as mulching could enhance this process in semi-arid regions.