Root water uptake model shows age-related water uptake patterns of apple trees on the Chinese Loess Plateau

Changwu Tableland, Chinese Loess Plateau Soil water status and water demand are two important factors to influence root water uptake (RWU) patterns, but the mechanisms involved in deep-rooted plants remain unknown. This study addresses this knowledge gap by investigating diurnal variations in RWU patterns for two different-aged apple orchards (7 vs. 19 years) on the Chinese Loess Plateau (CLP). We achieved this by inputting measured plant and soil information into a physical-based numerical RWU model. The results showed that the 19-year-old orchard exhibits distinct characteristics compared to the 7-year-old orchard. Specifically, the 19-year-old orchard had less deep soil water content, resulting in lower matric potential and hydraulic conductance. In contrast to cropland soil water, the cumulative deep soil water (>2 m) deficits were 17.6 and 1108.3 mm in the 7-year-old and 19-year-old orchards, respectively. Despite the 19-year-old orchard having 67% of its fine roots below 5 m soil depth, their contribution ratio to total RWU was only 30–40%, indicating less available deep soil water for the older orchard. Sap flow and leaf water potential had bell-shaped diurnal variations, leading to shifting soil water contributions to RWU in the 7-year-old and 19-year-old orchards. These changes were particularly evident when soil water potential in the root zone was heterogeneous, resulting in contrasting RWU patterns during different water demand periods. Notably, the 7-year-old orchard exhibited less water contribution from deep 2–5 m layer and enriched xylem water isotopes at midday with high water demand, while the 19-year-old orchard had more water contribution from deep 5–21.6 m layer and depleted xylem water isotopes. These contrasting RWU variations shed light on the profound impact of orchard age on deep RWU patterns, especially under the varying water demand conditions after CLP afforestation.