High mercury stocks in eastern Congo basin soils and their substantial mobilization after deforestation

Tropical forested ecosystems are of utmost importance for the global Hg cycling as they take up large amounts of atmospheric Hg, which are largely stored in underlying soils. Tropical soils are therefore considered a major Hg reservoir, which can turn into a source upon e.g. land-use change. However, data on Hg in vegetation and soils from tropical African forests is critically lacking. Here, we report for the first time Hg concentrations in fresh leaves, litterfall and along soil profiles (down to 1 m) selected from various topographic positions (plateaus, slopes and valleys) and different land covers (forest vs cropland) from 3 study sites in the eastern part of the Congo basin. We show that Hg concentrations in leaves and litterfall are similar across the 3 sites (median 28 and 42 ng g^-1, respectively) and comparable to values previously reported for remote deciduous forests. The yearly average litterfall flux (43 ± 13 µg m^-2 y^-1) confirm the efficient uptake of atmospheric Hg by the vegetation of these tropical forests. The accumulation of Hg in the soil mineral horizons is mostly controlled by Fe pedogenic oxides and among the highest values reported to date for natural soils (median 179 and up to 925 ng g^-1). Hg losses upon deforestation were large and similar for all study sites, from about 50 to 65%. They were linearly related to the amount of Hg stored in the mineral horizons, which will help constrain terrestrial Hg losses under land-use changes in such ecosystems.