Xylem and soil water extraction via centrifugation in a sub-tropical
urban green space
Abstract
Extracting water from discrete xylem and soil samples and continuous
(long-term) monitoring of water vapor across the soil-plant-atmosphere
continuum remains challenging and under a large debate. Here, we present
a detailed one-year study in the Fort Worth Botanic Garden
(north-central Texas) to evaluate the analytical robustness of the
centrifugation extraction method and understand water sourcing from
three common urban tree species (Elderberry, Sambucus canadensis;
Cherry Laurel, Prunus caroliniana; and Boxelder Maple, Acer
negundo). Xylem (N=110) isotope ratios (δ 18O and δ
2H) are compared to local precipitation (N=498),
throughfall (N=33), and soil water (N=105) at different depths (0-38
cm). Complementary soil water samples were obtained from cup suction
lysimeters (N=42) (0-38 cm). Soil and xylem water extraction volumes
ranged from 100 µL to 7.5 mL in plant samples and from 100 µL to 10.5 mL
in soil samples. Extraction success rates were 68.8% and 75.2% for
xylem and soil samples, respectively. The minimum sample total water
content for effective extractions was determined as 10.6% (soil) and
17.8% (xylem). Xylem mean narrowband and broadband (proxy for organic
contamination) were 0.23±0.40 (-) and 1.00±0.01 (-), respectively. These
values agree with mean narrowband and broadband metrics from throughfall
and soils, which highlight the non-invasive nature of centrifugated
extractions. Annual mean soil δ 18O compositions
(-3.6±1.7‰) corresponded with the throughfall input (-3.6±2.4‰). Xylem δ
18O compositions exhibited a strong temporal
enrichment trend at the end of the winter, summer, and fall seasons.
Mean spring xylem δ 18O (-2.85‰) was less variable and
close to soil mean compositions (-2.82‰). For this season, Bayesian
mixing analysis showed source water contributions from distinct soil
depths: 0 cm to 12.7 cm for Boxelder Maple, 12.7 cm to 25.4 cm for
Cherry Laurel, and 12.7 cm to 38.1 cm for Elderberry. Our results offer
a standardized and effective protocol for centrifugation extractions and
reveal plant water uptake preferences in a highly altered urban green
space during an unprecedented warm year.