The biogeochemical cycling of zinc in terrestrial ecosystems: Insights from zinc isotopes

Zinc (Zn) isotope geochemistry is a powerful tool for tracing Zn biogeochemical cycling in terrestrial ecosystems. This review comprehensively synthesizes Zn isotope signatures and fractionation mechanisms in atmospheric, soil, aquatic, and plant systems, integrating laboratory experiments and field observations. Laboratory studies have identified key drivers of Zn isotope fractionation, including mineral dissolution-precipitation, adsorption onto mineral surfaces, and organic complexation. Field studies show that interactions, particularly the competition between OM and (hydr)oxides, dominate Zn isotope compositions, with OM-rich soils exhibiting relatively lower δ⁶⁶Zn values. Anthropogenic activities significantly influence Zn isotope signatures. Agricultural practices exert an insignificant effect on natural δ⁶⁶Zn; traffic emissions exhibit slightly light Zn isotope compositions; urban emissions (e.g., electroplating) and industrial processes (e.g., smelting) are the predominant Zn sources, inducing significant Zn isotopic offsets. Plants preferentially uptake ⁶⁴Zn, while selective retention of ⁶⁶Zn in root cell walls may cover the uptake preference. Furthermore, translocation within plants occurs through low-affinity (diffusion-driven) and high-affinity (carrier protein-mediated) pathways, favoring ⁶⁴Zn and ⁶⁶Zn, respectively. These mechanisms collectively govern Zn isotopic variations. Despite progress, key knowledge gaps remain, particularly in quantifying intracellular isotope fractionation and disentangling multi-process interactions under natural conditions. Future research should integrate multi-isotope tracers (e.g., Zn-Cd-Pb), spatially resolved microanalytical techniques, and predictive modeling to refine pollution tracing and advance mechanistic understanding of Zn cycling. By bridging laboratory insights with ecosystem-scale observations, this review highlights the potential of Zn isotopes to address environmental challenges, from tracing metal pollution to guiding sustainable resource management in the Anthropocene.