The mean depth of soil water uptake by two temperate grassland species over time subjected to mild soil water deficit and competitive association

Little is known concerning the soil water use dynamics of white clover (WC) and ryegrass (RG) grown in mixtures. A greenhouse study, on a deep soil, was conducted to determine the mean depth of soil water uptake of WC and RG plants grown in a competitive association and subjected to a moderate soil water deficit. Plant growth period simulated that experienced by newly sown grassland in temperate regions. Three irrigation solutions, each containing a different hydrogen isotope (deuterium) concentration, expressed as delta notation (), were provided at three different soil depths through specially constructed tubes and containers (0.50 m diameter, 1 m depth) in order to create a soil deuterium profile gradient. Young leaves and not the entire plant were harvested in order to preserve the competitive plant association over time. Patterns of leaf D value were constant for both WC and RG. Lower leaf D values in RG compared to WC was attributed to RG more efficient stomatal control. Increases in the mean depth of soil water uptake as soil water deficit increased was similar between plants. The mean depth of soil water uptake of WC was at all times greater than that of RG. After 3 months of competitive growth, WC roots obtained water from a soil depth 30% greater than that of RG. In our experimental conditions, the ability of WC to obtain water from substantially lower soil depths may give it a competitive advantage over RG during the period subsequent to pasture sowing if surface soil water deficits are experienced and deeper soil layers contain water.