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

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 (FHgTa) through all litterfall fractions involved (needles, twigs, bark, miscellaneous, and male inflorescences). The range of annual fluxes of litterfall (LFTa) 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 LFTa. 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 (FHgTa) accounted for 25.4 and 23.1 μg·m-2 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 (LFTa) on FHgTa also indicated. The monthly sampling monitoring strategy and the Hg determination in individual litterfall fractions served as an accurate annual record of FHgTa that allows for anticipating high Hg deposition periods that could threaten soil biota and freshwater ecosystems.