Total mercury distribution among soil aggregate size fractions in a temperate forest podzol

This study determined the distribution of total Hg (HgT) among aggregate size fractions in the A, E, Bh and Bs horizons of a representative temperate forest podzol. The aggregate distribution was dominated by the coarse sand size fraction (average of 55%) followed by fine sand (29%), fine silt (10%), coarse silt (4%) and clay (2%). In general, HgT mean values increased as the aggregate size become smaller: clay (170 ng g-1) > fine silt (130 ng g-1) > coarse silt (80 ng g-1) > fine sand (32 ng g-1) > coarse sand (14 ng g-1). Total Hg enrichment in clay-sized aggregates ranged from 2 to 11 times higher than the values shown by the bulk soil (< 2 mm). The accumulation of HgT in the finer size aggregates was closely related to total organic C, Na-pyrophosphate extracted C, metal (Al, Fe)-humus complexes and Al and Fe oxyhydroxides. Indeed, these parameters varied significantly (p < 0.05) with the aggregate size and their highest values were found in the finer fractions. This suggested the role of these soil compounds in the increase of the specific surface area per mass unit and negative charges in the smallest aggregates, favouring Hg retention. Mercury accumulation factor (HgAF) values reached up to 10.8 in the clay size aggregates, being close to 1 in sand size fractions. Regarding Hg enrichment factors (HgEF), they were < 4 (“moderate pollution” category) in most of the horizons and aggregate sizes. Grain size mass loading (GSFHg) revealed that finer fractions had a higher Hg loading than their mass fractions, with a notable contribution of fine silt which made up > 50% of HgT in Bh and Bs horizons. The potential ecological risk index (PERIHg) increased as the aggregate size decreased, with the highest values in the illuvial horizons (45-903) and lowest in the E horizon (3-363). Heterogeneous distribution of Hg in the soil aggregate size fractions must be considered for Hg determination for purposes such as critical loads, background values or environmental risk indices. In addition, Hg accumulation in finer aggregates could be of concern due to its potential mobility in forest soils, either transferred by leaching to groundwater and freshwaters or mobilized by runoff in surface horizons.