The response of Evapotranspiration to osmotic potential in small-scale lab lysimeters

Drought and climatic change are among the main environmental stressors for the water and soil qualities. Soil water potential is the major soil-related factor controlling water availability to plants and their evapotranspiration. It consists of two main components: matric and osmotic potential. Although the effect of matric potential on plant evapotranspiration has been extensively studied under various conditions, there is still a lack of quantitative studies on the effects of osmotic potential on evapotranspiration.

In our study, we investigated the influence of soil osmotic potential on the evapotranspiration rate and cumulative evapotranspiration of grass planted in small laboratory lysimeters. A sandy loam soil material was packed in four lysimeters with a volume of 6000 cm³ and equal bulk density. The soil material was air dried, freed from roots and passed through a 2 mm sieve. Each lysimeter was equipped with soil sensors at two different depths to monitor soil moisture, bulk electrical conductivity, temperature, and matric potential. To obtain continuous mass balance measurements, each lysimeter was placed on a balance connected to the computer. Grass seeds were planted in each lysimeter at the same density and irrigated with distilled water until plant height was 12 cm. Irrigation water of two different qualities (EC= 0 and 4.79 dS/m) was then applied to produce different levels (0 and -0.17 MPa) of osmotic potential. The volumetric water content was adjusted to a value between 15 and 20 % in each lysimeter during the grass growth period. When the volumetric water content reached 15 %, irrigation water was added to the lysimeters to increase it to 20 %. Data were collected to calculate changes in osmotic potential relative to changes in total soil water potential. In addition, the relationship between osmotic potential and evapotranspiration rate during the growing season was determined.

Our results indicate a controlling role of soil osmotic potential on total soil water potential. This role results a significant reductions in evapotranspiration in response to increases in osmotic potential, in addition to effects on plant health. Osmotic potential has a significant function on total soil water potential when the soil becomes dry and poor water qualities are used in irrigation.