Post-exploitation fate of alluvial goldmines in the Amazon: soil erosion, metal(loid) remobilisation and evolution of ecological functions during restoration.

Artisanal and small-scale gold mining is responsible for deforestation and habitat degradation in Amazonian ecosystems due to soil reworking and increased turbidity in surrounding rivers. In French Guiana, legal mines are required to perform rehabilitation after gold extraction. To assess the successfulness of this rehabilitation, the temporal evolution of particles and metal(loid)s (mercury, lead, arsenic) export was measured, as well as several parameters linked to soil functions (soil carbon and nitrogen, microbial diversity, enzymatic activities). Results show a rapid decrease in particle export (i.e., 3-fold reduction within the first 8 months, and up to 2000-fold reduction after 4 years), linked to an increase in spontaneous colonisation by herbaceous plants. This decrease in particle export induced an efficient decrease in metal(loid)s (Hg, Pb, As) export predominantly associated to the particulate fraction. The parameters associated with ecological functions did not show any significant increase within 5 years after rehabilitation, possibly due to depletion of nitrogen stocks by the herbaceous growth inducing a competition for resources with soil microbial communities. Microbial communities on the mining site displayed significant evolutions during the 5 years after restoration, with no visible convergence with forest-soil communities, but underlining the adaptability to climatic conditions and a changing environment. These results highlight that current rehabilitation practices are efficient in limiting particle and major contaminants export while more attention needs to be addressed to the recovery of soil functions and biological activities.