Using species-specific enriched stable isotopes to study the effect of fresh mercury inputs in soil-earthworm systems
Abstract The fate of mercury (Hg) in the soil-earthworm system is still far from being fully understood, especially regarding recurrent and challenging questions about the importance of the reactivity of exogenous Hg species. Thus, to predict the potential effect of Hg inputs in terrestrial ecosystems, it is necessary to evaluate separately the reactivity of the endogenous and exogenous Hg species and, for this purpose, the use of enriched stable isotope tracers is a promising tool. In the present work, earthworms ( Lumbricus terrestris ) were exposed to historically Hg contaminated soils from the AlmadEn mining district, Spain. The soils were either non-spiked, which contain only endogenous or native Hg naturally occurring in the soil, or spiked with isotopically enriched inorganic Hg ( 199 IHg), representing exogenous or spiked Hg apart from the native one. The differential reactivity of endogenous and exogenous Hg in the soil conditioned the processes of methylation, mobilization, and assimilation of inorganic Hg by earthworms. Both endogenous and exogenous Hg species also behave distinctly regarding their bioaccumulation in earthworms, as suggested by the bioaccumulation factors, being the endogenous methylmercury (MeHg) the species more readily bioaccumulated by earthworms and in a higher extent. To the best of our knowledge, this work demonstrates for the first time the potential of enriched stable isotopes to study the effects of fresh Hg inputs in soil-earthworm systems. The findings of this work can be taken as a case study on the dynamics of Hg species in complex terrestrial systems and open a new door for future experiments. Highlights Enriched stable isotopes can be used to study the effect of Hg inputs in soil-earthworm systems. Addition of enriched inorganic Hg does not alter the behaviour of endogenous Hg species in the soil. Methylation process is more efficient for isotope tracers used in this study than for endogenous Hg. Endogenous MeHg has been bioaccumulated by earthworms in a higher extent that the exogenous form. The inherent geochemistry of the studied site is critical to select the most suitable chemical form of Hg spike. Graphical abstract [DISPLAY OMISSION]
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