Partial soil wetting and drying regulate root water uptake in deep-rooted apple orchard

Abstract

The drought-prone Chinese Loess Plateau (CLP) apple trees have roots that extend deep into the soil. Even when the upper soil layers (0–10 m) are above the permanent wilting point, deep (>10 m) root water uptake (RWU) occurs; the reasons for the deep RWU remain unknown. This study aimed to investigate the RWU mechanism and quantify fine root growth across soil moisture gradients in three deep-rooted apple orchards (A1994, A2001, and A2008). The volumetric soil water content, δ 2 H and δ 18 O isotopes, and fine root traits were sampled from the three orchards from 2016 to 2020, following the space-for-time substitution approach. A Bayesian mixing model (MixSIAR) was implemented to estimate the soil moisture contributions from different layers to RWU. Among the three apple orchards, persistent dryness prevailed in the 5–10 m soil zone as trees aged, whereas the 0–5 m soil zone was influenced by precipitation. At soil depths of 10–20 m, water accumulated over decades acts as a legacy reservoir for fine root growth. Fine root growth stabilized in the dry mid-layer (5–10 m) but increased significantly in the deep, moist soil (10–20 m), thereby enhancing resilience to moisture extremes. Results from MixSIAR revealed that the dry mid-layer soil contributed 14%, while the shallow (0–5 m) and deep (10–20 m) moist layers contributed 58% and 28%, respectively. Evidence of the partial wetting and drying RWU mechanism underscores the adaptive plasticity of deep-rooted apple trees and provides a physiological basis for developing precision irrigation strategies in semi-arid orchard systems.