Multi-component tree biomass approach to estimate litterfall Hg deposition in a warm-temperate coniferous forest in...

Terrestrial ecosystems are a key component in the biogeochemical cycle of Hg. About 50 % of atmospheric Hg is captured in the system because of the ability of vegetation to retain and subsequently transfer it to the soil surface through litterfall. In a stand dominated by Scots pine (Pinus sylvestris), the widest spatially distributed tree species in the northern hemisphere and the second worldwide, this two-year study evaluated monthly the litterfall Hg deposition fluxes (FHg_Ta) through all litterfall fractions involved (needles, twigs, bark, miscellaneous, and male inflorescences). The range of annual fluxes of litterfall (LF_Ta) was 504-512 g m^-2, although needles were the dominant fraction (275-315 g m^-2), the non-foliar tissues contribution reached 41 % of LF_Ta. Average of total Hg concentrations in litterfall fractions were branches (60-61 μg kg^-1) > miscellaneous (52-49 μg kg^-1) ≈ needles (51-49 μg kg^-1) > bark (45-43 μg kg^-1) > male inflorescences (24-20 μg kg^-1). Annual fluxes of litterfall Hg deposition (FHg_Ta) accounted for 25.4 and 23.1 μg m^-2, and needles, being the largest contributor (59-67 %), especially during the growing season (from the end of spring to early autumn). Nevertheless, during the dormant season, non-foliar tissues ruled the Hg deposition, indicating mechanisms of atmospheric Hg retention other than foliar uptake are also involved, as the notable influence of litterfall biomass (LF_Ta) on FHg_Ta also indicated. The monthly sampling monitoring strategy and the Hg determination in individual litterfall fractions served as an accurate annual record of FHg_Ta that allows for anticipating high Hg deposition periods that could threaten soil biota and freshwater ecosystems.