Inferring the seasonal origins of the plant (Cinnamomum camphora) and soil water from precipitation by stable isotope techniques in the East Asian monsoon region

From 2017 to 2019, xylem and leaf samples of Cinnamomum camphora were sampled and soil water samples at 0–100 cm depth were sampled 65 times simultaneously in a typical East Asian monsoon region. The seasonal origin of plant and soil water from precipitation was inferred based on the stable isotope techniques, including the evaporation line slope (SEL) estimations and the seasonal origin index (SOI) calculation. The regression SEL of leaf water was close to the theoretical SELs estimated based on the Craig–Gordon model, but xylem water and soil water showed higher regression SELs than the theoretical SELs, this may be due to the seasonality of the precipitation isotopes and evaporative fractionation. The fractionation-compensated isotopic values calculated based on the theoretical SELs of different water types were close, with the differences within 2.4‰ for δ18O and 20.0‰ for δ2H of each other, and the uncertainty of the fractionation-compensated isotopic values was low enough in the calculation of SOI. The SOI results showed that summer precipitation was more prevalent in plant and soil water, and more winter precipitation may recharge runoff when evapotranspiration demand is weak. Overall, the leaf sampling and the theoretical method have the potential to infer the seasonal water origin over a relatively long period.