Abstract: Water is a key factor limiting plant growth in loess regions. Studying water use by different afforestation species under different water conditions is crucial to understand the drought resistance mechanisms of plants in arid and semi-arid regions. We analyzed water use at different depths by the afforestation species Robinia pseudoacacia and Platycladus orientalis under the drought treatment (100% no throughfall, DT) and the natural rainfall (control, CK) by stable isotope (δ¹⁸O, δ²H) technique and explored their drought adaptability. The results showed that R. pseudoacacia mainly absorbed soil water at 0-40 cm soil layer in the wet months (June and September), with a contribution rate of 68.0%±2.4%, and at four layers (0-10, 10-40, 40-60, and 60-120 cm) evenly in the dry months (July and August) in the CK. In contrast, P. orientalis mainly absorbed soil water at 0-40 cm layer in both the wet and dry months, with the contribution rate being 77.0%±2.4% and 57.4%±3.0%, respectively. In the DT, the water-use depths of R. pseudoacacia and P. orientalis tended to move downward with the decreases of soil water content. The water-use depths of R. pseudoacacia changed from 0-40 cm to 60-120 cm, while that of P. orientalis changed from 0-40 cm to the four layers mentioned evenly. R. pseudoacacia and P. orientalis could adjust water-use depths under different water conditions and showed strong drought adaptability, a feature of great significance for evaluating the stress resistance and stability of local plantations.

Key words: water use source, hydrogen and oxygen stable isotopes, drought treatment, Loess Plateau