Soil geochemistry and digestive solubilization control mercury bioaccumulation in the earthworm Pheretima guillemi

•Soil methylmercury concentrations correlate negatively with total organic carbon.•Total organic carbon and pH determine total mercury and methylmercury bioaccumulation in earthworm from different soil types.•Earthworm digestive fluid is effective in solubilizing soil-associated inorganic mercury and methylmercury.•Soil ingestion plays an important role in methylmercury bioaccumulation in earthworm. Mercury presents a potential risk to soil organisms, yet our understanding of mercury bioaccumulation in soil dwelling organisms is limited. The influence of soil geochemistry and digestive processes on both methylmercury (MeHg) and total mercury (THg) bioavailability to earthworms (Pheretima guillemi) was evaluated in this study. Earthworms were exposed to six mercury-contaminated soils with geochemically contrasting properties for 36 days, and digestive fluid was concurrently collected to solubilize soil-associated mercury. Bioaccumulation factors were 7.5–31.0 and 0.2–0.6 for MeHg and THg, respectively, and MeHg accounted for 17–58% of THg in earthworm. THg and MeHg measured in soils and earthworms were negatively associated with soil total organic carbon (TOC). Earthworm THg and MeHg also increased with increasing soil pH. The proportion of MeHg and THg released into the digestive fluid (digestive solubilizable mercury, DSM) was 8.3–18.1% and 0.4–1.3%, respectively. The greater solubilization of MeHg by digestive fluid than CaCl2, together with a biokinetic model-based estimate of dietary MeHg uptake, indicated the importance of soil ingestion for MeHg bioaccumulation in earthworms.