Degree of subsoil rock weathering alters soil water storage and tree water uses in irrigated apple orchards

Water stored in the weathered rock layers beneath shallow soils is essential for deep-rooted plants, particularly under limited precipitation and irrigation, and becomes even critical during drought. However, the influence of weathering degree on water supply capacity and tree transpiration remains poorly understood. We selected three apple orchards with different weathering degrees (WWP: weaker; SWP: stronger; FWP: fully weathered) and monitored soil moisture, tree transpiration, and root water uptake sources over three years using thermal diffusion probes, TDR, and stable isotopes. Results indicated that bulk density in the weathered layers exceeded 1.8 g cm −3 for WWP and SWP, significantly higher than FWP (1.4 g cm −3 ). WWP exhibited higher gravel content (over 30 %) and lower clay (below 10 %) compared to SWP and FWP, resulting in greater hydraulic conductivity but reduced water retention. Consequently, water storage in WWP profiles (0–2 m depth) was markedly lower than in SWP and FWP. Groundwater levels under WWP responded faster to water inputs, indicating rapid percolation. Tree transpiration rates followed the order SWP, FWP, WWP, highlighting differences in water availability across profiles. Stable isotope analyses revealed that trees in FWP orchards utilized water flexibly across layers. Notably, the average utilization rate of weathered rock water (80–200 cm) in SWP was only 16.62 %, while WWP showed a significantly higher utilization rate of 36.50 %. These findings suggest that orchards on weakly weathered rock with poor surface water-holding capacity should adopt reduced irrigation volumes with increased frequency to improve water use efficiency in shallow-soil hilly areas. • Weaker weathered orchards exhibit elevated gravel content with reduced clay. • Stronger weathered orchards demonstrate superior water retention capacity. • Weaker weathered orchards show heightened susceptibility to water stress. • Weaker weathered orchards exhibit greater reliance on rock-regolith water sources.