Effects Of Straw Amendment On Methylmercury Accumulation In Soil And Crop Plants Under Wheat-rice Rotation

Mercury(Hg),especially for methylmercury(MeHg),massive production and bioaccumulation of neurotoxic methylmercury in contaminated soil-rice systems raise concerns about human exposure to MeHg.Some studies reveal that consumption of rice rather than fish could be an important MeHg exposure route for residents in some mercury polluted areas,and crop straw return is a most common way of organic input into paddy soils.Nevertheless,the potential impacts of straw amendment on methylmercury accumulation in soil and crop plants under wheat-rice rotation were largely unknown.Therefore,it is necessary to explored by a series of experiments,including pot and batch experiments,including pot and batch experiments.To investigate the possible influences of straw amendment on soil methylmercury levels and its accumulation in crop plants,mercury-contaminated paddy soil was collected from Wanshan mercury mine area and amended with 1%(w/w)crop straw(wheat or rice)in a wheat-rice rotation system.At different stages(e.g.,seedling,tillering,jointing,flowering and harvesting)of plant growth,the soil and plant samples were collected and analyzed for MeHg.The results indicated that rice straw amendment could evidently enhance MeHg levels in soils and MeHg accumulation in wheat,while wheat straw amendment had minor effects on net production and bioaccumulation of MeHg during rice cultivation.To further explore the underlying mechanisms of changes in mercury dynamics under straw amendment,laboratory experiments in batch reactors(quantifying methylating bacteria,as well as dynamics of Eh,sulfate,iron and dissolved organic matter)were also carried out,to provide mechanistic explanation to the potential differences between rice and wheat.The results indicated that:1).Rice straw amendment resulted in significantly higher soil MeHg levels during the wheat cultivation period(p<0.05).Differently,wheat straw amendment had minor and insignificant effects of soil MeHg levels during rice cultivation(p> 0.05,on and after day 30),except at the start of rice cultivation(i.e.,on day 0,10 days following wheat straw amendment).Similar like those in batch experiments,Soil MeHg concentrations have increased by 3 times in straw amended soils under rice season than wheat season.This was possibly due to the different compose of the rice and wheat straw under under wheat-rice rotation.Of course,flooded rice can more increase the risk of a mercury exposure.We suggest that incorporating wheat instead of rice straw,e.g.,in rice-wheat rotation systems,may result in less/minor increase in MeHg bioaccumulation.2).Concentrations of MeHg in plant tissues have increased(especially for grain)under straw return.Wheat grain MeHg increased by 169% under wheat-straw covered return,rice grain Me Hg increased by 25.1% under rice-straw amendment covered return.When we take the mixture of straw return,the grain MeHg increased by 225.4% and increased by 19.5% under rice-straw amendment.Our results indicated that crop straw incorporation could significantly affect mercury accumulation in grains,thus increase environmental and ecological risk of straw return activities in mercury polluted areas.3).Neither rice straw nor wheat straw amendment,the DOC levels in all treatments have sharp increases at the start of incubation,then have decreased with increasing incubation time.There is a positive correlation between DOC levels and soil MeHg(r2=0.67,p<0.05)under oxic incubation.There is significant correlation between sulfate levels and the dissolved iron levels and soil MeHg concentration under the anoxic conditions.Those factors(sulfate,DOC,Fe)could be attributed to mercury methylation.4).Amendment of either rice or wheat straw evidently increased SRB abundances in soil(p<0.05),clearly demonstrating positive effects of straw amendment on SRB growth.The total number of the SRB on day 25 have 997.8×103 MPN/g dry soil under wheat-straw amendment and 1407.2×103 MPN/g dry soil under rice-straw amendment,respectively.We may believe that the sulfate reducing bacteria play a key role of the mercury methylation in soil under straw return.These results suggest that it should be cautious when promoting straw amendment in mercury-contaminated areas.