Regulation of soil water consumption of Robinia pseudoacacia in different stand ages

Large-scale afforestation in China has resulted in widespread soil water deficits. Yet the effects of root water uptake on soil water decline, and how it differs between dry and wet seasons and across different stand ages remain largely unstudied. Using stable water isotopes (δ¹⁸O and δ²H), we investigated water uptake patterns across five Robinia pseudoacacia stand ages (~6, 16, 20, 35, and 45 years) and explored the interactions between soil drying and water-use strategies during the pre-rain and rainy season across two years.  R. pseudoacacia exhibited clear seasonal and age-related differences in water uptake, with contrasting water-use strategies under dry versus normal years. Overall, R. pseudoacacia predominantly relied on shallow soil water (0.67 ± 0.15) during the pre-rain season and shifted to deep soil water uptake (0.73 ± 0.14) in the rainy season. In the drier year of the 2-year observation period, all stands showed similar seasonal water uptake patterns, with a predominant reliance on deeper soil water, whereas in the typical year, water-use strategies differed markedly among stand ages. While middle-aged and old stands (16 to 45 years) accessed water from all soil layers, the younger individuals (6 years) primarily utilized soil water from intermediate and deep layers. Combining information from stable water isotopes and actual evapotranspiration we calculated soil water decline rates for all stands and found that soil water was declining between 14.0% to 24.7% in the 0–60 cm soil layer, 5.2% to 6.9% in the 60–200 cm soil layer, and 3.3% to 4.8% in the 200–500 cm soil layer. During the pre-rain season the deeper soil layers were substantially depleted, especially for young stands and in the drier year, and soil water decline rates were related to age-related differences in soil water content and soil drying patterns. This study presents the first isotope-based quantification of soil water decline across different R. pseudoacacia stand ages, highlighting the starkly different soil drying dynamics.