Mercury in leaf litter in typical suburban and urban broadleaf forests in China

To study the role of leaf litter in the mercury (Hg) cycle in suburban broadleaf forests and the distribution of Hg in urban forests, we collected leaf litter and soil from suburban evergreen and deciduous broadleaf forests and from urban forests in Beijing. The Hg concentrations in leaf litter from the suburban forests varied from 8.3 to 205.0 ng/g, with an average (avg) of (49.7 ± 36.9) ng/g. The average Hg concentration in evergreen broadleaf forest leaf litter (50.8 ± 39.4) ng/g was higher than that in deciduous broadleaf forest leaf litter (25.8 ± 10.1) ng/g. The estimated Hg fluxes of leaf litter in suburban evergreen and deciduous broadleaf forests were 179.0 and 83.7 mg/(ha·yr), respectively. The Hg concentration in organic horizons (O horizons) ((263.1 ± 237.2) ng/g) was higher than that in eluvial horizons (A horizons) ((83.9 ± 52.0) ng/g). These results indicated that leaf litterfall plays an important role in transporting atmospheric mercury to soil in suburban forests. For urban forests in Beijing, the Hg concentrations in leaf litter ranged from 8.8–119.0 (avg 28.1 ± 16.6) ng/g, with higher concentrations at urban sites than at suburban sites for each tree. The Hg concentrations in surface soil in Beijing were 32.0–25300.0 ng/g and increased from suburban sites to urban sites, with the highest value from Jingshan (JS) Park at the centre of Beijing. Therefore, the distribution of Hg in Beijing urban forests appeared to be strongly influenced by anthropogenic activities.