Effect of afforestation on soil water dynamics and water uptake under different rainfall types on the Loess hillslope

Knowledge on water uptake is crucial for understanding water transport processes of the soil–vegetation-atmosphere continuum and plant survival strategy in the Loess Plateau. Even though vegetation water sources have been widely explored using hydrogen and oxygen isotopes, the quantification of vegetation water sources after returning farmland to forestland/grassland under different rainfall types, especially extreme rainstorm event, still remains challenging due to difficulties in field monitoring. This study collected precipitation, soil water from 0 to 100 cm soil layer and xylem/stem water and measured their hydrogen (δD) and oxygen (δ18O) isotope compositions in Robinia pseudoacacia forestland (PL), natural forestland (NF) and natural grassland (NG) under four typical rainfall events. The results indicated that among three vegetation types, soil water content was lowest in PL during each rainfall event. In addition, the sensitivity of water-use strategies to precipitation was affected by rainfall types, dry/wet periods and vegetation types. When antecedent soil water was deficit, vegetation water use was more sensitive to precipitation, and the primary water sources after rainfall transferred from deep soil to upper soil. However, when the antecedent soil water was sufficient, water uptake after rainfall tended to be more homogeneous in different soil layers. Among three vegetation types, Robinia pseudoacacia was more sensitive to precipitation. During the extreme rainstorm event II, in PL, the primary water source shifted from the 80–100 cm soil water before rainfall (88.7%) to the 0–10 cm soil water at 1 day after rainfall (64.3%) and the 0–20 cm soil water at 7 day after rainfall (82.8%). Thus, Robinia pseudoacacia can adapt to the environment by switching water-use strategy. The finding can provide a guidance for vegetation rehabilitation in the Loess Plateau.