Abstract This study evaluates the response of the bacterial communities to different mercury (Hg) amendments in temperate forest soils. Seven soils were spiked with increasing amounts of Hg [(0, 0.032, 0.32, 3.2 and 32 μg Hg(II) g−1 dry soil]. After 30 days, we examined the bioavailable Hg using bacterial biosenors (mer-lux), basal respiration, bacterial community structures and identified indicator OTUs which were responsive to Hg. In soils treated with at least 3.2 μg Hg g−1 dry soil, resulting in bioavailable Hg higher than 0.004 μg Hg g−1 dry soil, the basal respiration was strongly affected. High bioavailable Hg also caused significant changes in the bacterial T-RFLP profiles. Members of the Alphaproteobacteria (Rhodospirillales) and Betaproteobacteria (Burkholderiales) were found to be Hg-tolerant. Here, we propose a critical limit concentration for soluble Hg of 0.004 μg Hg g−1 soil.