Variation of the stable isotopes of water in the soil-plant-atmosphere continuum of a Cinnamomum camphora woodland in the East Asian monsoon region

Abstract The hydrogen and oxygen stable isotopic (2H and 18O) tracer method has become an important means of studying water transfer between the atmosphere, rivers and lakes, soils, plants, and aquifers. We conducted a study of the variations of the stable isotopes of water, and the seasonal variations in water use of Cinnamomum camphora, in the soil-plant-atmosphere continuum in a C. camphora woodland in Changsha, central-southern China, which lies within the monsoon region of East Asia. Stable isotopes were monitored in precipitation, soil water (0-130-cm depth), groundwater, twig xylem water of C. camphora, and at the same time relevant environmental variables (soil water content, and meteorological variables) were measured, from March 2017 to August 2019. The monitoring data were collated and analyzed to determine the variations of stable isotopes in different water pools and mutual exchanges. The results indicate that daily precipitation (P) larger than 3.7 mm was regarded as effective precipitation for soil water replenishment. Isotopic compositions of soil water and twig xylem water fell below the LMWLP>3.7 (the local meteoric water line eliminated rainfall events with P ≤ 3.7 mm). Isotopic compositions of shallow soil water (0-60 cm) and twig xylem water were positively correlated with that of cumulative precipitation within two months. These patterns of precipitation isotopic signal propagation delineate a clear pathway of water transfer in this soil and plant continuum. The slope of the soil water evaporation line (SEL) in the study area was higher than that of the LMWLP>3.7 because of the situation that precipitation in warm season with a larger evaporative demand is more depleted with heavy isotopes (i.e., more negative delta values) than that in cold season when the evaporative demand is low. Therefore, it was not feasible to determine the strength of isotopic fractionation based on the isotopic water line method. According to the MixSIR model, C. camphora mainly uptake shallow soil water during the observation period. However, in July-September when soil is dry, deep soil water (60-130 cm) contributes to C. camphora water uptake for ∼50%. These findings improve our understanding of the water cycle of the soil-plant-atmosphere continuum in the East Asian monsoon region.