Influence Mechanism Of Organic Amendments On Mercury Speciation And Its Biological Effects In Soil-Rice System

Mercury（Hg）pollution in the soil-rice system poses a potential threat to the global ecology and human health,and thus has become a very serious environmental issue.Humic substances and biochar are two commonly used organic amendments to control soil Hg pollution.However,the differential influence and its underlying mechanism of dissolved humic substances and particulate humic substances on Hg speciation and microbial transformation are not completely clear.In addition,in order to improve the immobilization efficiency of sulphur-modified biochar on soil Hg,for the first time this study tried to use sulphur-rich biomass to prepare natural sulphur-rich biochar,and evaluated its efficiency on reducing methylmercury（Me Hg）accumulation in rice seed.Therefore,this research selected organic waste compost and its humic extracts,humic acids（HA）and fulvic acids（FA）,and natural sulfur-rich biochar pyrolyzed from sulphur-rich oilseed rape straw as two kinds of organic amendments to investigate the mechanism of their effects on the Hg speciation as well as its biological effects in the soil-rice system,and reached the following conclusions:（1）The application of dissolved humic substances,that are HA and FA,caused the content of water-soluble Hg to increase by 2.3-5.6 times,which in turn led to a 28%decrease in the number of operational taxonomic units（OTUs）and a decrease of15.7%-19.9%in alpha diversity（calculated based on Shannon index）of bacterial community in paddy soil.In addition,under the application of HA and FA,the abundances of Hg-resistant genes mer A and mer B increased by 2.3-5.6 times and 5.6times,respectively,which was the adaptation mechanism for soil microbes to enhanced Hg stress.In contrast,the application of compost basically does not change the Hg stress degree on the bacterial community.（2）The natural sulphur-rich biochar promoted the reduction and volatilization of Hg in the rhizosphere paddy soil,resulting in a decrease of the total Hg（T-Hg）content by 9.2%,thereby reducing the degree of Hg stress on the soil bacterial community,thus elevating the number of OTUs.Mean increase of 149 OTUs was observed with a 1mg·kg^-1 decrease in T-Hg content.（3）Natural sulphur-rich biochar promoted the production of Me Hg in rhizosphere soil,which increased by about 1.4 times.However,it significantly reduced the Me Hg accumulation in rice grains by 75%,and meanwhile significantly increased rice biomass by about 24%.The findings indicate that natural sulphur-rich biochar can be used to control Hg pollution in rice fields and effectively improve the growth of rice.