Soil Genetic Horizons Regulate Water Uptake by Picea schrenkiana in Arid Mountain Forests

Mountain forests in arid regions are under long‐term water limitation, and different soil genetic horizons have distinct differences in water storage and root accessibility. Therefore, investigating the relationship between plant water uptake patterns and soil genetic horizons in arid mountain forests can help distinguish the hydrological functions of different soil horizons. In this study, we investigated Picea schrenkiana forests in the Tianshan Mountains at three elevations: 2200, 1800 and 1450 m. During the growing seasons over two consecutive years, soil water from the humus, eluvial and illuvial horizons, together with plant xylem water, was collected and the MixSIAR model was used to quantify the contributions of different soil genetic horizons to water uptake by P. schrenkiana . The results showed that during a wetter year and the early growing season, trees mainly used water from the humus horizon, whereas during a drier year or the late growing season, they increased their uptake of water from the eluvial and illuvial horizons. Along the elevational gradient, P. schrenkiana at 2200 and 1800 m mainly used water from the humus horizon. However, at 1450 m, influenced by root distribution and hydraulic connectivity, P. schrenkiana mainly used water from the eluvial horizon even when the humus horizon had relatively high soil water content. This study indicates that the humus horizon is an important water source for P. schrenkiana , and its role in maintaining forest hydrological health may exceed that of the eluvial and illuvial horizons. Higher soil water content does not necessarily indicate a higher contribution to plant water uptake, as plant water uptake is also related to root distribution, water availability and atmospheric drought stress. This study clarifies the important ecohydrological role of the humus horizon in arid mountain forests and helps improve our understanding of the mechanisms underlying tree water uptake in these ecosystems.