Abstract Transboundary atmospheric pollution has become a serious threat to ecosystems and human health at the global scale. Lead isotope ratios in mosses are used to evaluate the influence of transboundary atmospheric pollutants owing to the traceability of the origin of these pollutants. However, uncertainties for these ratios remain because uptake of trace metals by mosses depends on the surrounding environment and their absorption processes. This study examined how these factors affect lead isotope ratios in mosses and discussed their usefulness as indicators of transboundary pollutants. Moss samples were collected from thirteen sites across Japan. On the continental side, the moss samples in areas with a high amount of winter precipitation exhibited lead isotope ratios similar to those of transboundary pollutants. These results can be explained by enhanced influence of transboundary pollutants on the mosses through winter precipitation bias, given that transboundary pollutants are primarily carried to Japan during winter and that mosses efficiently take up pollutants through wet deposition. The moss samples on Pb-enriched soil absorbed a large quantity of soil-derived Pb, which was reflected in their Pb isotope ratios. Furthermore, the Pb uptake by mosses can be inhibited by other trace metals that compete with Pb for cation binding sites at the cell wall. This inhibition can alter the contribution of atmospheric Pb deposition to the total Pb content in mosses, thereby affecting the Pb isotope ratios. In conclusion, these results validate the usefulness of Pb isotope ratios in mosses as indicators of transboundary pollutants; however, they also imply the need for meticulous interpretation of moss biomonitoring results. The enhanced influence of transboundary pollutants on mosses due to the winter precipitation bias emphasizes the importance of moss biomonitoring for evaluating ecosystem integrity, as it gives insights on actual changes in the Pb cycling in an ecosystem through Pb accumulation by mosses.

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Highlights • Pb isotope ratios in mosses reflected the influence of transboundary pollutants. • Pb isotope ratios were partially dependent on seasonal variations in precipitation. • Pb isotope ratios were altered by soil-derived Pb in areas with Pb-enriched soil. • Competition among antagonistic cations for Pb uptake affected Pb isotope ratios. • The need for meticulous interpretation of moss biomonitoring results was implied.