Xyl(t=2) = f1 * bulksoil(t=1) + f2
* P(t1->t2)
Where bulksoil(t=1) is the isotopic composition
of water in the bulk soil at the previous timestep andP(t1->ts) is the volume weighed
isotopic composition of precipitation that fell between the previous and
most recent xylem sampling dates (typically the last three weeks).
Results and discussion
Isotopic variation of precipitation, soil waters and xylem
water
From April 2020 to March 2022, we observed seasonal cycles in isotope
ratios (shown for δ2H in Figure 2 and for
δ18O in the supplementary material Figure S1) in
incoming precipitation that corresponded with transitions between summer
and winter seasons. Isotope ratios were lighter in winter and heavier in
summer, with volume-weighted annual precipitation δ2H
averaging ‑63.9 (and δ18O averaging -9.5 ‰), as shown
by the dotted lines in Figure 2 b-d. Mixtures of precipitation in soil
or plants that lie above that line (Figure 2) contain more summer
precipitation than annual precipitation does (i.e., summer precipitation
is over-represented in those soil and xylem samples), and mixtures below
that line contain more winter precipitation than annual precipitation
does (i.e., winter precipitation is over-represented in those soil and
xylem samples).
Mobile and bulk soil waters showed a seasonal cycle in
δ18O, similar to that of precipitation but with a
dampened amplitude (Figures 2c&d); however, the beech, spruce and young
spruce samples did not show a clear seasonal cycle (Figure 2b; Figure
S1b). Readers should note that several of the collection dates occurred
during the dormant season for beech, during which some of the highest
and most variable xylem δ2H values were observed.
Overall, most of the bulk soil samples plotted below the mean
precipitation line, indicating that they over-represent winter
precipitation; in contrast, the mobile soil water samples were quite
evenly distributed around the mean precipitation signatures, suggesting
that they, on average, reflect a less directionally biased mixture of
precipitation. The mean mobile soil water δ2H (‑64.8 ±
0.5, mean ± standard error) was similar to the mean annual precipitation
δ2H (‑63.9 ± 2.5), whereas the mean bulk soil water
δ2H was substantially lighter (‑79.2 ± 0.7). Given the
consistency among soil water samplers and close tracking of
precipitation fluctuations, mobile soil waters seemed to reflect recent
precipitation more than bulk soil waters did.