The suitability of isotopic methods in identifying water sources of a shallow-rooted herbaceous plant in a desert steppe

Isotopic methodologies have gained prominence in investigating the composition of plant water sources; however, concerns regarding their suitability and reliability in diverse environments have emerged in recent years. This study presents a comparative analysis of root, soil, and liquid water (precipitation, dew, and groundwater) samples obtained from a desert steppe using isotope ratio infrared spectrometry (IRIS) and isotope ratio mass spectrometry (IRMS). The objective was to evaluate the applicability of these techniques in discerning the water sources of Stipa breviflora, a shallow-rooted herbaceous plant species. Additionally, we explored the root water uptake characteristics and water use strategy of S. breviflora. Our findings indicate that the IRIS method had more enriched values of D compared to the IRMS method across all samples, while no discernible pattern was observed for 18O. Notably, the differences observed among all samples exceeded the instruments' accuracies. Moreover, an unexpected occurrence was noted, whereby both D and 18O values in the root water were more enriched than in any of the considered water sources, rendering identification of the plant water sources unattainable. By conducting a re-analysis of more refined soil layer samples, we discovered that S. breviflora exhibits the ability to absorb and utilize water sources in close proximity to the soil surface. It further suggested that the shallow-rooted herbaceous plants in desert steppes can exploit small rainfalls, frequently overlooked in their ecological importance. Considering the distinctive soil and plant characteristics of desert steppes, we recommend adopting IRMS methods in conjunction with refined surface soil sampling for isotopic analysis aiming to identify water sources of shallow-rooted herbaceous plants. This study provides novel insights into assessing the suitability of isotopic techniques for analyzing plant water sources, while enhancing our understanding of water use strategies and environmental adaptation mechanisms employed by shallow-rooted herbaceous plants within xerophytic grassland ecosystems.