Zinc isotope fractionation in native and cloned mosses: implications for standardized pollution biomonitoring
Mosses are widely used as biomonitors of atmospheric trace-metal deposition, but metal retention by the moss itself may modify source-related isotope signatures. Here, controlled Zn adsorption experiments, stable Zn isotope measurements, surface acid-base characterization, and Zn K-edge X-ray absorption spectroscopy (XAS) were combined to investigate Zn2+ retention and isotope fractionation in four native moss taxa and in vitro-cultivated Sphagnum palustre clones. Zn2+ adsorption was strongly pH-dependent, consistent with progressive deprotonation of reactive surface sites. Moss-associated Zn2+ was slightly enriched in heavy isotopes relative to the coexisting solution, with {Delta}66Znmoss-solution generally ranging from +0.001 to +0.009 per cent, but no systematic relationship was observed between isotopic fractionation and the fraction of Zn2+ adsorbed. Linear-combination fitting of X-ray Absorption Near-Edge Structure (XANES) indicated predominantly cellulose-like, O-rich coordination environments, with variable phosphate and minor acetate contributions and only small nominal Zn-S fractions. Extended X-ray Absorption Fine Structure (EXAFS) showed first-shell Zn-O distances of approximately 1.96 - 2.05 Angstrom and coordination numbers of about 3.6 - 7.6, confirming substantial structural heterogeneity among samples. Importantly, {Delta}66Znmoss-solution was not correlated with either Zn-O distance or coordination number, indicating that the net isotope effect cannot be described by a single bulk structural parameter. Native and cultivated Sphagnum showed comparable isotopic offsets within analytical uncertainty despite differences in surface properties and adsorption capacity. These results indicate that Zn isotope fractionation in mosses reflects the integrated response of multiple binding environments and support the use of standardized Sphagnum clones in isotope-based biomonitoring.