Xylem and Soil Water Extraction via Centrifugation in a Subtropical Urban Green Space

Abstract

Extracting water from discrete xylem and soil samples for δ¹⁸O and δ²H analysis remains a central challenge in ecohydrology. This study evaluates the applicability of the centrifugation method in extracting (a) mobile water with (b) no induced fractionation and (c) low organic contamination. Three common urban tree species (elderberry, Sambucus canadensis; cherry laurel, Prunus caroliniana; and boxelder maple, Acer negundo) were studied for 1 year in the Fort Worth Botanic Garden (north-central Texas, United States). Xylem (N = 110) isotope values were compared alongside throughfall (N = 33), soil water (N = 79) and lysimeter water (N = 42) at different depths (0–38 cm). Xylem and soil water extraction volumes ranged from 0.1 to 7.5 mL and 0.1 to 10.5 mL, respectively. Extracted water volumes exhibited no strong correlations with isotopic values. Water was extracted from soil and xylem samples with gravimetric water contents as low as 10.6% and 17.8%, respectively. Although elderberry and cherry laurel exhibited narrowband and broadband values (indicators for organic contamination) similar to throughfall, soil water and lysimeter water, boxelder maple reported higher values in summer and fall. Annual δ¹⁸O mean values in soil water (−3.2‰ ± 1.7‰) and lysimeter water (−3.3‰ ± 1.1‰) corresponded with the throughfall input (−3.1‰ ± 2.0‰). Xylem δ¹⁸O exhibited higher values in winter, summer and fall than throughfall, soil water or lysimeter water. However, during spring, xylem mean δ¹⁸O values were less variable and close to soil and lysimeter δ¹⁸O values: elderberry (−3.4‰ ± 0.5‰), cherry laurel (2.9‰ ± 0.5‰) and boxelder (−2.3‰ ± 0.4‰). Our results highlight the applicability of the centrifugation method in extracting mobile water, unbiased by low extraction volumes, temperature-induced fractionation or high organic contamination. Low xylem and soil water contents limit the extraction capacity of centrifugation and increase the concentration of organics, particularly during summer and fall seasons.