Abstract. Increasing numbers of field studies have detected
isotopic mismatches between plant trunk water and its potential sources.
However, the cause of these isotopic offsets is not clear, and it is
uncertain whether they occur during root water uptake or during water
transmission from root to trunk. Thus, we measured the specific isotopic
composition (δ2H and δ18O) of each component
(e.g. bulk soil water, mobile water, groundwater, trunk water and root
water of Salix matsudana Koidz trees) in the
soil–root–trunk continuum with a resolution of about 3 days. We report three main findings. First, we detected
a clear separation between the isotopic compositions of mobile water and bulk soil water, but the distinction between mobile water and bulk soil water
gradually decreased with increasing soil depth. Second, root water composition deviated
from bulk soil water isotopic composition but overlapped with the
composition derived for less mobile water. The maximum differences in
δ2H and δ18O between bulk soil water and root
water were −8.6 ‰ and −1.8 ‰, respectively. Third,
trunk water was only isotopically similar to root water at 100–160 cm depths, and it remained stable during the experimental period, suggesting
that the trees consistently used the stable deep water source. In
conclusion, the isotopic offset between bulk soil water and trunk water of
S. matsudana reflected an isotopic mismatch between root water and bulk soil water
associated with the heterogeneity of the soil water. Our results illuminate
relationships between the isotopic compositions of soil waters of various
mobilities, root water and trunk water that may be useful for advancing our
understanding of root water uptake and transport.