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

Antía Gómez Armesto,Claudia Campillo-Cora,Lucía Bibián-Núñez,Juan Carlos Nóvoa-Muñoz,Manuel Arias-Estévez,Xabier Pontevedra-Pombal

doi:10.3232/sjss.2018.v8.n1.05

Antía Gómez Armesto, Claudia Campillo-Cora + Show 4 more

https://doi.org/10.3232/sjss.2018.v8.n1.05

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Abstract

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) &gt; fine silt (130 ng g-1) &gt; coarse silt (80 ng g-1) &gt; fine sand (32 ng g-1) &gt; 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 (&lt; 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 &lt; 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 &lt; 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 &gt; 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.

Highlights

Mercury is considered an element of concern due to its accumulation in natural ecosystems that results in harmful effects to wildlife and humans (Driscoll et al 2013), and is considered to be a global pollutant (Richardson et al 2013).Recent estimates consider that approximately 45% of the atmospheric Hg is deposited on terrestrial ecosystems (Driscoll et al 2013), where soils represent its largest reservoir accounting for up to 75% of the Hg stored in the biosphere (Mason and Sheu 2002)
As there are few works focused on the Hg distribution in aggregate size fractions of soils with background Hg concentrations (< 100 ng g-1), the main aim of this study was to assess the distribution of total Hg among aggregate size fractions in a representative podzol from a temperate forest area
Higher Hg enrichment factors (HgEF) levels in coarser fractions than in finer fractions disagree with the results reported by Acosta et al (2009) and Yutong et al (2016), who found the greater enrichment factor (EF) values for heavy metals in aggregate fractions of silt and clay sizes

Summary

Introduction

Mercury is considered an element of concern due to its accumulation in natural ecosystems that results in harmful effects to wildlife and humans (Driscoll et al 2013), and is considered to be a global pollutant (Richardson et al 2013).Recent estimates consider that approximately 45% of the atmospheric Hg is deposited on terrestrial ecosystems (Driscoll et al 2013), where soils represent its largest reservoir accounting for up to 75% of the Hg stored in the biosphere (Mason and Sheu 2002). Podzolization is considered a crucial process in the mobilization of Hg from surface horizons, together with dissolved organic matter (Schlüter 1997), toward deeper soil layers where it is accumulated due to its adsorption by Fe and Al oxyhydroxides and metal (Al, Fe)humus complexes (Schuster 1991; Roulet and Lucotte 1995; Roulet et al 1998; Schlüter 1997; Guedron et al 2009; Peña-Rodríguez et al 2014). The different horizons of podzols would play a decisive role in Hg mobilization in case of an increase in Hg availability due to a higher rate of organic matter mineralization as consequence of the global warming (SmithDowney et al 2010)

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