Technical note: Method effects on isotope-based inference of apple tree water sources

Abstract. Stable hydrogen and oxygen isotopes are widely used to trace plant water sources, but extraction and analytical choices can bias interpretation. Using an apple orchard on the Shandong Peninsula as a field example, we compared cryogenic vacuum extraction (CVE) with centrifugation, laser spectroscopy with isotope-ratio mass spectrometry (IRMS), and tested how apparent xylem δ²H offsets affect MixSIAR source apportionment. Across plant organs, water became progressively enriched from roots and branches to leaves and fruit. In paired branch samples measured by IRMS, centrifugation yielded δ²H values about 10‰ higher than CVE, whereas δ¹⁸O differences were small (≈1–2‰). Laser spectroscopy produced systematically higher δ¹⁸O than IRMS for plant extracts (typically 1–3‰), while non-plant source waters agreed closely between instruments. A δ²H offset correction shifted inferred uptake from shallow to deeper soil water. These results show that methodological choices can alter isotope-based inference of plant water sources and should be explicitly evaluated and reported.