A δ2H offset correction method for quantifying root water uptake of riparian trees

Root water uptake plays an important role in water cycle in Groundwater-Soil-Plant-Atmosphere-Continuum. Stable isotopes (delta H-2 and delta O-18) are effective tools to quantify the use of different water sources by plant roots. However, the widespread delta H-2 offsets of stem water from its potential sources due to delta H-2 fractionation during root water uptake result in conflicting interpretations of water utilization using stable isotopes. In this study, a potential water source line (PWL), i.e., a linear regression line between delta O-18 and delta H-2 data of both soil water at different depths and groundwater, was proposed to correct delta H-2 offsets of stem water. The PWL-corrected delta H-2 was determined by subtracting the deviation between delta H-2 in stem water and PWL from the original value. The MixSIAR model coupled with seven types of input data (i.e. various combinations of single or dual isotopes with uncorrected or corrected delta H-2 data by PWL or soil water line (SWL)) were used to determine seasonal variations in water uptake patterns of riparian tree of Salix babylonica (L.) along the Jian and Chaobai River in Beijing, China. These methods were evaluated via three criteria including Akaike Information Criterion (AIC), Bayesian Information Criterion (BIC) and root mean square error (RMSE). Results showed that different types of input data led to considerable differences in the contributions of soil water at upper 30 cm (9.9-57.6%) and below 80 cm depths (29.0-76.4%). Seasonal water uptake patterns were significantly different especially when delta H-2 offset was pronounced (p < 0.05). The dual-isotopes method with uncorrected delta H-2 underestimated the contributions of soil water in the 0-30 cm layer (by 30.4%) and groundwater (by 56.3%) compared to that with PWL-corrected delta H-2. The PWL correction method obtained a higher groundwater contribution (mean of 29.5%) than that estimated by the SWL correction method (mean of 24.5%). The MixSIAR model using dual-isotopes with PWL-corrected delta H-2 produced the smallest AIC (94.1), BIC (91.9) and RMSE values (4.8%) than other methods (94.9-101.7, 92.6-99.5 and 5.3-12.4%, respectively), which underlined the best performance of PWL correction method. The present study provides crucial insights into quantifying accurate root water uptake sources even if delta H-2 offset exists.

Automated summary

Root water uptake is essential in the water cycle, and stable isotopes are effective tools for studying the sources of water uptake in plants. The study introduces the PWL method to correct for delta H-2 offsets and improve interpretations of water utilization. Results show that different input data methods can significantly impact the understanding of seasonal water uptake patterns in riparian trees.