Abstract:Using the space-for-time substitution experimental design, three apple orchards with different ages but the same species and management (i.e. 5-year old young orchard has not yet borne fruit, 8-year old orchard has borne fruit for two years, and 13-year old mature orchard, respectively) were selected on the Loess Plateau in Changwu County, Shaanxi Province. On July 12th and August 19th, 2015, soil samples were taken at 0.2 m intervals from the surface to 5 m depth of a profile, and the corresponding apple tree branches were taken at each site. The stable oxygen and hydrogen water isotopes of the soil and branch samples were measured and the Bayesian mixing model was used to quantify the contribution of soil layers from different depths to apple water uptake and their responses to precipitation. Results showed that:(1) The depth of the primary water source for apple trees before and after precipitation differed with stand ages. During the dry season, the main water absorbing depth of 13-year old trees was deeper than that of 5- and 8-year old trees. At the peak growing season, rainfall in the rainy season could only increase the soil water storage in 5-year old young orchards that has not borne fruits, thus young orchards were free of deep soil water deficit. For the 8- and 13-year old mature apple orchards that has borne fruits for several years, the deep soil water could not be replenished even under the large precipitation. (2) For 5- and 8-year-old apple trees, the contribution of 0-100 cm shallow soil layer to the plant transpiration was more than 50% during the drought period. However, 50% of tree water came from 100-300 cm depth for the 13-year-old apple orchard. After precipitation, the main water source of trees was 100-300 cm for the 5- and 8-year-old orchards, and the resulting contribution was about 40%. For the 13-year-old apple orchard, the main water contribution layer was 0-100 cm soil layer, with nearly 50% of the total contribution. (3) The contribution of soil water in 300-500 cm layer showed very weak response to precipitation for all the three orchards, maintained around 30% consistently.