Is Hg uniformly distributed among soil aggregate size fractions?. A case study in spodic horizons in sub-Antarctic forest soils of Tierra del Fuego (Argentina).

GÓMEZ-ARMESTO, A.; PÉREZ-MILLS, M.; PONTEVEDRA-POMBAL, X.; PÉREZ-RODRÍGUEZ, P.; ARIAS-ESTÉVEZ, M.; DIODATO, S.; MORETTO, A.; GARCÍA-RODEJA GAYOSO, E. ; NÓVOA-MUÑOZ, J. C.

Barcelona

Congreso; XXV SETAC Europe Annual Meeting; 2015

SETAC-

Although the atmosphere plays a key role in the global Hg cycle, this metal has a limited time of residence before its deposition in the soil surface. Soils are recognized as the largest Hg sink, accounting up to 75% of the Hg present in the biosphere. The highest Hg concentrations are usually found in the uppermost soils layers, mainly due to Hg affinity for soil organic matter. However, it was also reported the presence of large concentrations of Hg in deeper soils layers, as occurs in podzols where spodic horizons (Bh, Bhs, Bs) become the last barrier against the mobilization of Hg towards groundwater and surface waters, which could increase notably the risk of environmental toxicity. The immobilization of Hg in podzol B horizons depends on the interactions between Hg and soil organic matter, metal-humus complexes or Al and Fe oxyhydroxides, but it is also closely related to soil aggregate size fractions. This study is focused in the distribution of total Hg (Hg T) in coarse sand, fine sand, coarse silt, fine silt and clay fractions of five Bs horizons of sub-Antartic forest soils from Tierra del Fuego (Argentina). The total Hg content in the fine earth fraction (< 2 mm) ranged between 33 and 98 μg kg-1, whereas the clay size fraction showed the highest average value of total Hg T (140 μg kg-1; range 94-183 μg kg-1) followed by the fine silt fraction which had a range of Hg T between 45 and 109 μg kg-1. In spite of this, clay and fine silt fractions only account on average for 32% of all Hg T, mainly due to the low amount of these aggregate soil fractions in the studied horizons. On the contrary, both sand fractions showed a moderate content of Hg T, with means of 79 and 84 μg kg-1 for coarse and fine sand, respectively. This implies that 56% of Hg T in these soils is associated to soil aggregates of sand size, whose mobilization will be easier after erosion events that could destroy the uppermost A and E horizons, leaving exposed the Bs horizons. This process can be expected in forest soils that undergo changes in its land use such us logging activities, which are spreading rapidly in some areas of Tierra del Fuego. However, in the spodic horizons HgT seems to be strongly bound to organic C, metal-humus complexes and Al and Fe oxyhydroxides, decreasing the risk of Hg mobilization towards the adjacent water courses and thus its potential transformation in methylated mercury species whose toxicity for the biota is widely known.