Evaluating the bias effects of rooting depth and cryogenic vacuum extraction to quantify root water uptake patterns in deep-rooted apple trees

Accurately assessing isotope information on water sources and mixtures is essential for determining root water uptake (RWU) patterns. This study investigates the impact of rooting depth and cryogenic vacuum extraction (CVE) on the isotopic composition of deep soil water and RWU patterns in a 19-year-old apple orchard on China’s Loess Plateau. We used a Bayesian mixing model (MixSIAR) to analyze corrected isotopic data from soil and xylem water samples collected at different depths, ranging from 3 m to 21.6 m. Our findings reveal a progressive decline in the isotopic composition of deep soil water by 0.63‰ m–1, which decreased deep soil water contributions to transpiration by 1–12% at different rooting depths. The xylem and soil water isotopes became more enriched after correction, with xylem water isotopes closer to the soil water isotope line and water isotopes enriched in the shallow soil layer. Consequently, the contributions of shallow soil water to transpiration increased after xylem water isotope correction but decreased after soil water isotope correction. Monthly averages of these ratios decreased by 12.8% or increased by 8.4%, respectively. After correcting for both soil and xylem water isotopes, the average contributions of shallow soil water increased by 7.4% each month, while those of deep soil water were not consistent. Our findings suggest that rooting depth and CVE have a greater impact on seasonal contribution ratios but a comparatively milder influence on seasonal patterns. This delineation is important for accurately quantifying plant water use strategies.