Variability in precipitation influences the water sourcing and adaptive strategies of key plant species within the desert steppe ecosystem

The composition and species diversity of desert plant communities exhibit high sensitivity to changes in precipitation. From the perspective of plant community dynamics, alterations in precipitation levels result in increased complexity in water absorption and utilization strategies, consequently leading to diversified adaptation strategies. Focusing on the desert steppe within the transitional zone between agriculture and animal husbandry in northern China, this study investigates the mechanisms through which variations in precipitation impact plant species' water absorption and utilization strategies in desert steppes. In this study, a controlled precipitation experiment was conducted on site in a natural desert steppe setting with three precipitation treatments: 50 % reduction in precipitation (PE), normal precipitation (CK), and 50 % increase in precipitation (PD). By analysing the δ2H and δ18O values of plants, soil, and precipitation, as well as indicators such as plant species composition, biomass, specific leaf area (SLA), and δ13C values, the changes in plant community species and the main plant species' water sources and strategies under different precipitation conditions were studied. The results show that (1) Within each treatment, the variability of soil δ2H and δ18O in the 0–20 cm soil layer was greater than that in the 20–60 cm soil layer. The slope of the soil δ2H and δ18O fitting line in the PE treatment was larger than that in the PD and CK treatments. (2) Except for Leymus secalinus, the dominant plant species, including Lespedeza potaninii, Polygala tenuifolia, Convolvulus ammannii, Kali collinum, and Stipa breviflora, exhibited an increase in water absorption from deeper soil layers as precipitation decreased. The diversity in water sources among the dominant plant species decreased with increasing precipitation, while the overlap index of soil moisture utilization across different soil layers initially increased and then decreased with increasing precipitation levels. (3) The root-to-shoot ratio (RS) of L. potaninii and P. sibirica shows an extremely significant negative correlation with SWC, while the RS of C. ammannii and K. collinum exhibits a significant negative correlation with SWC. The δ13C values of S. breviflora demonstrate an extremely significant negative correlation with SWC, while those of L. secalinus, C. ammannii, and S. breviflora show a significant negative correlation. In contrast, the δ13C values of K. collinum exhibit an extremely significant positive correlation with SWC. The SLA of all six plants exhibits an extremely significant positive correlation with SWC. The water source diversity of L. potaninii, C. ammannii, K. collinum, and S. breviflora demonstrates an extremely significant positive correlation with SWC, while that of P. sibirica and L. secalinus shows a significant positive correlation with SWC. Conclusion: As precipitation decreases, soil water evaporation becomes more pronounced. The primary plant species within the community exhibit flexibility in absorbing moisture from different soil layers in response to changes in precipitation levels. Differential water utilization strategies among different plants have led to hydrological niche separation, intensifying competition for soil moisture, particularly during extremely arid conditions. Plant water utilization strategies tend to become more conservative as precipitation decreases, and they demonstrate the ability to adapt by altering their water use strategies in response to varying precipitation levels.