Quantitative partitioning of temporal origin of transpiration into pre- and post-plantation under deep-rooted vegetation on the Loess Plateau of China

Deep-rooted vegetation transpires a considerable amount of deep soil water with different ages in the unsaturated zone. However, the tradeoffs between new water of transpiration (temporally originating from post-planting precipitation) and old water of transpiration (temporally originating from pre-planting precipitation) across the vegetation lifespan are poorly understood. In this study, we collected soil samples from beyond 28 m soil depth on the Loess Plateau of China to investigate the influence of deep-rooted vegetation on the age of soil water and analyze the proportion of new and old water of transpiration in the unsaturated zone under grassland, 22-year-old apple orchard, and 17-year-old peach orchard. Water isotopes (²H, ¹⁸O, and ³H), solutes (chloride, nitrate, sulfate), and soil water content were used to identify the critical water ages in the unsaturated zone (one-year water age, water age corresponding to stand age, and the maximum water age of transpiration), and to determine soil water deficit, soil evaporation loss fraction, and potential groundwater recharge. The results showed that soil water mainly moved as piston flow in these soil profiles, and deep soil water largely came from heavy precipitation. Deep-rooted vegetation restrained new pore water velocity and potential groundwater recharge. New pore water velocity declined from 0.40 m yr^-1 to 0.14 m yr^-1 and 0.34 m yr^-1 for apple and peach, respectively. Deep-rooted vegetation decreased groundwater recharge by 9.46 % for apple and 7.04 % for peach, compared to grassland. Over the vegetation lifespan, annual average transpiration was 500.56 mm yr^-1 and 468.89 mm yr^-1 with maximum water age of 63 years and 45 years for apple and peach, respectively. The transpiration of deep-rooted vegetation mainly used new water (94.97 % for apple and 97.47 % for peach). The total old water of transpiration was 553 mm for apple and 209 mm for peach. Our results identify the temporal sources of vegetation water use, offering new insights into the transpiration process of deep-rooted vegetation.