196 - Hg Stocks and Fate in Forest and Cropland Soils from the Eastern Congo Basin

The uptake of gaseous elemental mercury (Hg0) by foliage is an important sink for atmospheric Hg, resulting in a large Hg flux to soils via litterfall. Soils are thus the largest Hg reservoir globally with organic-rich surface soils estimated to contain more than a third of the Hg pool actively cycling between the soil–ocean–atmosphere system. However, little is known about the fate and stability of Hg in this reservoir as only few studies (most of them from the Amazon region) have been conducted to evaluate the impact of land-use change, such as deforestation and agriculture, on Hg cycling. In this study, we report Hg concentrations along various soil profiles and soil compartments (litter, organic and mineral horizons) from study sites located in the eastern-central part of the Congo basin along the Albertine Rift. This tropical region is affected by intense anthropogenic activities, such as deforestation and gold mining, which are expected to increase due to rapid population growth. Soil profiles were selected from various topographic positions (plateaus, slopes, and valleys) and different land covers (forest vs. cropland) to evaluate the effect of topographic gradients and deforestation on Hg concentrations and stocks. Overall, we found that Hg was enriched with soil depth along the profiles, from litter to organic and mineral horizons. Hg concentrations largely differed between the study sites but were generally higher on plateau profiles compared to the valley ones and consistently 2 to 3 times higher in forest soils compared to croplands. Hg losses upon land conversion from forest to croplands were similar for all study sites, ranging from 50 to 65 %. This study brings additional data on Hg cycling in tropical terrestrial ecosystems as well as implications for soil management regarding release of Hg stored in soils.

Authors: Sylvain Bouchet, Tess Giacobbo, Mara Bär, Sebastian Dötterl, Ruben Kretzschmar