Abstract
AbstractWe compared stable isotopes of water in plant stem (xylem) water and soil collected over a complete growing season from five well‐known long‐term study sites in northern/cold regions. These spanned a decreasing temperature gradient from Bruntland Burn (Scotland), Dorset (Canadian Shield), Dry Creek (USA), Krycklan (Sweden), to Wolf Creek (northern Canada). Xylem water was isotopically depleted compared to soil waters, most notably for deuterium. The degree to which potential soil water sources could explain the isotopic composition of xylem water was assessed quantitatively using overlapping polygons to enclose respective data sets when plotted in dual isotope space. At most sites isotopes in xylem water from angiosperms showed a strong overlap with soil water; this was not the case for gymnosperms. In most cases, xylem water composition on a given sampling day could be better explained if soil water composition was considered over longer antecedent periods spanning many months. Xylem water at most sites was usually most dissimilar to soil water in drier summer months, although sites differed in the sequence of change. Open questions remain on why a significant proportion of isotopically depleted water in plant xylem cannot be explained by soil water sources, particularly for gymnosperms. It is recommended that future research focuses on the potential for fractionation to affect water uptake at the soil‐root interface, both through effects of exchange between the vapour and liquid phases of soil water and the effects of mycorrhizal interactions. Additionally, in cold regions, evaporation and diffusion of xylem water in winter may be an important process.
Highlights
The growth of the “critical zone” paradigm has added impetus to closer investigation of soil–plant-atmosphere interactions in ecohydrology (Grant & Dietrich, 2017)
Direct hypothesis testing of potential processes that may explain the difference between the isotopic composition of xylem water and that of potential water sources has been advanced by detailed experiments in controlled environments, often involving the use of Bayesian mixing models which assume all potential plant water sources have been sampled (e.g., Stock et al, 2018)
As field data become increasingly available from critical zone studies, more exploratory, inferential approaches can be insightful in terms of quantifying the degree to which xylem water isotopes can or cannot be attributed to measured soil water sources (Amin et al, 2020)
Summary
The growth of the “critical zone” paradigm has added impetus to closer investigation of soil–plant-atmosphere interactions in ecohydrology (Grant & Dietrich, 2017). Soil waters held under different tensions may have different isotopic characteristics: for example, freely moving (low tension) water sampled by suction lysimeters often shows a much less marked evaporative fractionation signal than bulk soil waters dominated by less mobile (high tension) storage extracted by cryogenic or equilibration methods (Sprenger, Tetzlaff, Buttle, Laudon, Leistert, et al, 2018; Vargas et al, 2017). Low elevation sites had no trees, but a variety of additional shrubs including Bitterbrush (Purshia tridentata), Chokecherry (Ericameria nauseosa), Yellow willow (Salix lucida) and Water birch (Betula occidentalis) Rooting depths at Wolf Creek and Bruntland Burn are largely within the top 30 cm with smaller fractions to 50 cm
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