| Differently-treated crops straw is being widely used to fertilize soil,while the potential impacts of straw amendment on the biogeochemistry and phytoavailability of mercury in contaminated soils are largely unknown.In the present study,differently-treated rice straw(dry straw,composted straw,straw biochar,and straw ash)was incorporated into mercury-contaminated soil at an environment relevant level(1/100,w/w),and methylmercury(MeHg)concentration,MeHgphytoavailability(using ammonium thiosulfate extraction method,validated elsewhere)and bioaccumulation(in Indian mustard Brassica junceas)were quantified.Our results indicated that incorporating straw biochar or composted straw into soil would decrease phytoavailable MeHg levels,possibly due to the strong binding of MeHg with particulate organic matter in amended straw('MeHg immobilization effect').Consequently,MeHg accumulation in aboveground tissue of Indian mustard harvested from straw biochar-amended soil decreased by 20%compared to the control.Differently,incorporation of dry straw resulted in elevated MeHg levels in soil('Mercury methylation effect').Decomposition of amended dry straw in soil would evidently increase DOC levels(averagely 40%-195%higher than the control),which may subsequently mobilize MeHg in the soil('MeHg mobilization effect').Accordingly,incorporation of dry straw led to increased phytoavailable MeHg levels in the soil and doubled MeHg accumulation in Indian mustard.Our results provided the first evidence that incorporating differently-treated rice straw into soil could have diverse effects on mercury biogeochemistry and phytoavailability,which should be taken into account in risk assessment or soil remediation.There is growing concern about methylmercury(MeHg)accumulation in rice grains and thus enhanced dietary exposure to MeHg in Asian countries.To further explored the possibility of reducing grain MeHg levels by biochar amendment,and the underlying mechanisms,pot(i.e.,rice cultivation in biochar amended soils)and batch experiments(i.e.,incubation of amended soils under laboratory conditions)were carried out,investigating MeHg dynamics(i.e.,MeHg production,partitioning and phytoavailability in paddy soils,and MeHg uptake by rice)under biochar amendment(1-4%of soil mass).We demonstrate for the first time that biochar amendment could evidently reduce grain MeHg levels(49-92%).The declines could be attributed to the combined effects of:(1)increased soil MeHg concentrations('Mercury methylation effect'),probably explained by the release of sulfate from biochar and thus enhanced microbial production of MeHg(e.g.,by sulfate-reducing bacteria),(2)MeHg immobilization in soils('MeHg immobilization effect'),facilitated by the large surface areas and high organosulfur content of biochar,and(3)biodilution of MeHg in rice grains('Biodilution effect'),due to the increased grain biomass under biochar amendment(35-79%).These observations together with mechanistic explanations improve understanding of MeHg dynamics in soil-rice systems,and support the possibility of reducing MeHg phytoaccumulation under biochar amendment.To further explore the long-term effects of biochar on MeHg accumulation in crops.Here,pot experiments(wheat-rice rotation)continued to be carried out using previous experiment soils.Our study suggested that biochar amendment may have different effects on bioaccumulation of MeHg in rice and wheat plants.Specifically,straw biochar incorporation could increase the MeHg accumulation in wheat grain(48-84%)but still decrease the rice grain MeHg concentration(63-88%).These different results may be explained by the different conditions required for the plants growth.Our study provided further evidence that biochar amendment could remarkablly reduce MeHg accumulation in rice grain in the long-term.Therefore,incorporation of straw biochar could possibly be considered as a practical mean to reduce the MeHg exposure risk in mercury-contaminated areas. |
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