Effects of sulfur-rich biochar amendment on microbial methylation of mercury in rhizosphere paddy soil and methylmercury accumulation in rice.

Biochar amendment has the potential to reduce methylmercury (MeHg) uptake by rice grains in soil-rice ecosystem. Considering that sulfur can strongly bind Hg and thus reduce its bioavailability, S-modified biochar has been used to immobilize Hg in soils. However, whether natural S-enriched biochar can further reduce Hg and MeHg phytoavailability remains unknown. Moreover, the rhizosphere is one of the most important microbial hotspots regulating the pollutant dynamics in terrestrial ecosystems. Therefore, it is of greater practical significance to examine the impact of biochar amendment on MeHg production and phytoavailability in the rhizosphere versus nonrhizosphere. Here, by conducting a pot experiment, we evaluated the efficacy of biochar derived from sulfur-enriched oilseed rape straw to reduce MeHg accumulation in rice. The results demonstrated that: (1) biochar-induced enhancement of chloride ion and sulfate levels in the overlying water and pore water facilitate microbial methylation of Hg and thus MeHg production in rhizosphere soil. (2) biochar amendment increased rhizosphere soil sulfur content and humic acid-like substances, strengthening MeHg binding to soil, and thus reducing grain MeHg levels by 47%-75%. Our results highlight the necessity to applying natural sulfur-rich biochar accompanied with exogenous sulfur to further reduce MeHg phytoavailability.