Species-specific and seasonally dynamic δD offsets in a walnut-alfalfa intercropping system: unraveling the role of climatic and soil moisture drivers

Growing evidence has revealed δD offsets in soil-plant water systems, contradicting the long-held hypothesis of isotopic non-fractionation during plant water absorption. We aimed to determine whether δD offsets exist for plants in walnut (Juglans regia L.) -alfalfa (Medicago sativa L.) intercropping system and to explore the effects of intercropping and plant species on δD offsets. We conducted an analysis of the isotopic signatures of soil and plant water, calculated δD offsets utilizing the soil water excess (SW-excess) equation across varying seasons from 2021 to 2023 in walnut-alfalfa intercropping, monocropping walnut, and monocropping alfalfa systems. The findings revealed that all plant systems exhibited δD offsets with distinct seasonal variations. Specifically, walnuts and alfalfa displayed noteworthy positive δD offsets during dry-1 season (1.43 and 2.98 ‰) and substantial negative δD offsets during the rainy season (-8.80 and -12.25 ‰). Additionally, walnut exhibited negative δD offsets (-7.53 ‰) in dry-2 season, while alfalfa showed no δD offsets. The δD offset patterns in intercropped and monocropped plants remained consistent across different seasons, with no significant disparities between them. Correlation analyses indicated that walnut δD offsets were negatively associated with air humidity and soil moisture content, yet positively correlated with saturated vapor pressure difference; whereas, alfalfa δD offsets were notably negatively correlated with air temperature and relative humidity. Our findings suggest that δD offset may be a prevalent phenomenon, unaffected by intercropping patterns but significantly influenced by plant species. Compared to walnuts, alfalfa displayed higher sensitivity to climatic factors, resulting in a more pronounced offset magnitude.