Land use change, especially mining activities, contributes to anthropic CO2 emissions, leading to decreased carbon (C) storage and loss of biodiversity. Artisanal gold mining associated with the use of mercury (Hg) for amalgamation may change soil organic matter (SOM) contents, and the release of Hg into the environment generates serious environmental problems. Changes in soil biogeochemistry due to C loss and seasonal climate fluctuations affect Hg dynamics and can either increase or decrease its availability. Therefore, our objective was to evaluate the impact of mining on SOM and Hg geochemistry in four Brazilian biomes. We evaluated the dynamics of C and Hg in the dry and rainy seasons of mining and pasture areas by combining spectroscopic, thermogravimetric, and chemical extraction. The critical role of SOM in Hg retention and the influence of climatic seasons on C and nitrogen (N) stocks were highlighted, along with the availability of Hg in solution. Key findings indicated a 50 % reduction in soil C stocks in mined areas, exacerbated during dry seasons, which also saw up to a 70 % increase in bioavailable Hg. SOM played a critical role in Hg retention, with Hg availability closely linked to soil C stability. These results highlight the environmental degradation linked to mining and suggest strategies to mitigate these impacts by increasing SOM and immobilizing Hg. Amalgamation of gold directly into ore, as in the Amazon, has generated great soil Hg stocks, while Hg availability appeared to be governed by soil C stability. This information can serve as a basis for choosing strategies to mitigate environmental degradation caused by changes in land use in mining activities to promote increase in SOM and to immobilize Hg contents.