Evaluation of sampling and sample preparation methodologies for determination of mercury concentrations and stable...

Globally, foliar uptake of atmospheric mercury (Hg) is a key deposition pathway for terrestrial surfaces. However, diverse sampling and pre-treatment methods across studies can influence the comparability of foliar Hg concentrations and isotopic measurements, leading to biases that impact our understanding of Hg inter-compartmental exchanges at regional and global scale. In this study, we tested the effect of different sample drying and rinsing methods on foliage samples collected from three distinct locations on the tree crown of a European hornbeam (Carpinus Betulus L.). Samples were taken from Ljubljana, an urban area, and Idrija, a historically Hg-contaminated mining site, representing contrasting environments. Vertical crown stratification significantly influenced Hg concentrations, with inner canopy foliage in Ljubljana containing 46-50% higher Hg than outer and upper positions due to reduced solar exposure and enhanced throughfall deposition. Drying methods did not alter Hg concentrations (p > 0.05), validating cross-study comparability for low-Hg environments. Conversely, rinsing removed 24-60% of Hg in Idrija samples (p < 0.01), attributable to particulate-bound Hg (PBM) enriched in heavier isotopes (δ²⁰²Hg: -1.9 ± 1.5‰ unrinsed vs. -3.2 ± 1.0‰ rinsed). We recommend that rinsing be applied when studying internal plant Hg cycling, but omitted when total foliage-associated Hg, including surface-bound fractions, is of interest. Future studies should investigate inter-species differences, seasonal variability in foliar Hg dynamics, and the effects of drying methods on Hg isotopic fractionation to improve methodological consistency and understanding of Hg cycling.