Effects Of Rice Residue Incorporation On The Speciation And Exposure Risk Of Mercury In Mining Paddy Soils

To reduce air pollution,straw return instead of burning is being strongly encouraged in China,including some mercury polluted areas.Nevertheless,the possible influences of straw return on methylation,bioavailability and exposure risk of mercury were relatively unknown.In this study,different amounts of rice straw or root were added into a mercury contaminated soil from Wanshan mining area,Guizhou Province(referred to as WS soil).Mercury extraction by calcium chloride(CaCl2)or bovine serum albumin(BSA)was used to assess bioavailability of soil-bound mercury to crops or soil deposit-feeders.Bioavailability of inorganic mercury(IHg)or methylmercury(MMHg)decreased significantly in rice residue amended soils,possibly due to the strong binding of mercury with rice residue organic matter.Meanwhile,MMHg concentrations increased by 2-8 times in amended soils.Such increases were attributed to enhanced microbial activities and/or formation of Hg-S-DOM complexes after rice residue incorporation and decomposition.Consequently,exposure risk of IHg(quantified as concentration of bioavailable IHg in soil)decreased significantly while that of MMHg increased up to 4 times.To explore the possible influence of incorporation of decomposed crop straw on the speciation(i.e.,IHg or MMHg)and phytoavailability of mercury,WS soil was amended with different amounts(i.e.,low,medium or high)of straw organic fertilizer(SF,mainly consisting of decomposed rice straw)or humus(HU)and incubated for a month.Methylmercury levels in SF/HU amended soils were generally lower than that in the control,probably because complexation of IHg with SF/HU organics decreased IHg availability to methylation microorganisms.Phytoavailability of IHg,assessed by CaCl2 extraction,was significantly lower in soils amended with low/medium SF,possibly due to the immobilization effect of SF-organic matter on IHg.In contrast,phytoavailability of IHg was significantly higher in soils incorporated with high HU,possibly explained by the leaching effect of dissolved HU on soil-bound IHg.For MMHg,incorporation of medium/high HU significantly increased MMHg phytoavailability,while SF addition had little effect,suggesting that effects of organic matter on MMHg are less evident than that on IHg.To our knowledge,this is the first study demonstrating that rice residue incorporation could significantly affect mercury biogeochemistry in soil.However,it remains unknown whether these conclusions can be generalized to realistic paddy fields.To investigate potential effects of rice residue incorporation on mercury accumulation in rice,WS soil was added with rice root(RR),rice straw(RS)or composted rice straw(CS),and planted with rice.Incorporating RS or CS increased grain MMHg concentration by 14% or 11%.The observed increases could be attributed to elevated porewater MMHg levels and increased MMHg translocation to grains within plants,but was partly offset by increased grain biomass(i.e.biomass dilution).In contrast,IHg levels in grains decreased sharply(33-43%),probably explained by biomass dilution and reduced IHg translocation to grains.Our results indicated for the first time that rice residue incorporation could significantly affect mercury accumulation in rice grains,thus increase environmental and ecological risk of straw return activities in mercury polluted areas.Since that straw return activities may increase risk of MMHg in contaminated areas,it is important to find ways to mitigate the risk associated with straw return.Therefore,we carried out another pot experiment.Rice straw was added into a paddy soil sampled from Xunyang mercury mining area of Shannxi Province,and controls of environmental parameters(i.e.,water management and sulfate content in soils)on MMHg behaviors and bioaccumulation were investigated.We found that rice straw incorporation could largely increase grain MMHg concentration(94%).Drying soil before rice transplanting could significantly decrease risk of MMHg intensified by straw return,but would also result in evidently decreased rice yield.Sulfate addition into soils had no effect on grain MMHg levels,but co-application of sulfate would decrease risk of MMHg,and rice yield was not affected.Therefore,incorporation of crop straw and sulfate simultaneously could possibly be considered in mercury contaminated areas.