Abstract: The mean transit time (MTT) is a good indicator of water cycle processes. We know little about the MTT of different water bodies within the soil-plant-atmosphere continuum (SPAC) in the subtropical monsoon region. We estimated the MTT of stratified soil water at different depths as well as the xylem water and leaf water in typical Cinnamomum camphora woodland located in Changsha City from March 2017 to October 2019. The main methods used in this study included the stable isotope technology, the linear mixed model and the sine wave fitting method. The results showed that the stable isotopes were more depleted in summer and enriched in winter for different water bodies within the SPAC. The δ²H values of soil water gradually decreased as depth increased. The δ²H values of xylem water closely resembled those of soil water, but the δ²H values of leaf water were more positive and exhibited larger variation. Results of the linear mixed model indicated that the lower MTT values of soil water and plant water occurred between June and September, while the higher values were often observed around January and from April to May. The precipitation replenishment exhibited a significant negative correlation with the MTT. The MTT of soil water generally increased with depth, although preferential flow could enhance the replenishment of deeper soil water and subsequently reduce the MTT. The mean MTT values of xylem water and leaf water were similar. Results of the sine wave fitting method showed that the young water fraction (F_yw) of soil water gradually decreased as depth increased, while the MTT of soil water gradually increased as depth increased. The F_yw and MTT of xylem water were lower and higher than those of leaf water, respectively. Both the mean MTT values of soil water based on the linear mixed model or the sine wave fitting method increased from the surface to the deeper soil layers. The former exhibited a smaller variation range and the latter showed a larger variation range. The mean MTT value of xylem water based on the linear mixed model was 2.4 days less than that of leaf water, while the MTT value of xylem water in the sine wave fitting method was 87.4 days higher than that of leaf water. These differences may be due to the parameterization of “new/young water”, the uncertainty of results, and the effect of evaporative fractionation. This study contributes to a better understanding of water transport and consumption processes within the SPAC and provides valuable insights for agricultural production and water resources management in the subtropical monsoon region.

Key words: soil-plant-atmosphere continuum, stable isotope, mean transit time, young water fraction, precipitation replenishment