Effect Of Selenium And Chitosan-modified Biochars On Formation Of Methylmercury In Paddy Soils And Its Accumulation In Rice

Mercury(Hg)is the only toxic heavy metal element in liquid state at room temperature.Because of its high vapor pressure,it is easy to volatilize and can stay in the atmospheric environment for a long time,so it can be transported in the atmosphere for a long distance,causing global pollution.There are many forms of mercury in the environment.Among them,methyl mercury(Me Hg)is the most toxic form and the most harmful to organisms.The methylmercury in the environment mainly comes from the microbial methylation of inorganic mercury,and the long-term flooding environment is the main living environment of mercury methylating microorganisms.Therefore,paddy field has become an important place for microbial mercury methylation.Rice is well-known to accumulate Me Hg,and the consumption of rice in Hg polluted area has been confirmed to be a primary way of Me Hg exposure.Therefore,how to inhibit the formation and accumulation of Me Hg in paddy field system need to be solved urgently.This study treated biochar with sodium selenate and chitosan to obtain two types of modified biochar.Potted experiments were used to study the effect of modified biochar on reducing rice soil methylmercury production and rice mercury enrichment.And analyzed the Me Hg adsorption characteristics of different biochars,the Me Hg content in the porewater,soil,and rice plant tissues at different stages,the microbial community structure of the rhizosphere,discussed the effect mechanism of modified biochar on the formation of methylmercury in the soil during rice growth and the enrichment of methylmercury in grains.The specific research results are as follow:(1)At the maturation stage of rice,the Me Hg content of rice roots treated with chitosan modified biochar(CMBC)and selenium-loaded modified biochar(SMBC)are 77.8% and 73.1%lower than that without biochar(CK1),and 68.6% and 62.0% lower than that with unmodified biochar(CK2).During the whole growth process,the total mercury(THg)and Me Hg content in CK1 and CK2 stems were significantly higher than SMBC and CMBC.The difference between SMBC and CMBC are not obvious,and the THg transfer coefficient(TF)of CK1 is significantly higher than SMBC and CMBC.There are no significant difference in the THg content of the old leaves of the 4 treatments.Except for the old leaves,the THg content of the other leaves of CK1 is significantly higher than that of SMBC and CMBC.The THg content in the leaves are the first leaf>the second leaf>the middle leaf(or old leaf);the Me Hg content of each leave of CK1 are significantly higher than that of SMBC and CMBC;the Me Hg content in the leaves are the first leaf>the second leaf(or middle leaf)>the old leaf.The THg content of rice in SMBC and CMBC are 78.7%and 71.0%lower than CK1,The Me Hg of rice in SMBC and CMBC are 86.4%and 75.8%lower than CK1.This indicates that the addition of modified biochar can inhibit the accumulation of mercury in the aerial parts of rice to a certain extent,and ultimately reduce the mercury content in rice.(2)Compared with CK1,the soil mercury methylation rate in SMBC and CMBC are reduced by 62.7% ～89.6% and 51.1% ～79.1%,respectively,and reduced by 82.1% ～93.1% and55.8% ～93.3% compared with CK2.The trends of THg and Me Hg in the porewater of the four treatments are basically the same.After the jointing period,the mercury content in the porewater decreased significantly and tended to be stable.On the whole,the Me Hg of SMBC and CMBC are significantly lower than that of CK1 and CK1.It shows that adding modified biochar can effectively inhibit the formation of Me Hg in porewater and reduce the mercury content in the soil.(3)Compared with unmodified biochar,the surface of selenium loaded and chitosanmodified biochar contain more chemical bonds and elements such as Cl,K,Ca,and the specific surface area and pore volume are relatively reduced.According to the pseudo second order kinetic model,the theoretical adsorption capacity and adsorption rate of modified biochar for Hg(Ⅱ)are greater;according to Langmuir model,the theoretical adsorption capacity of Hg(Ⅱ)per unit specific surface area of selenium and chitosan modified biochar are larger than that of unmodified biochar.The theoretical adsorption capacity of Me Hg on the three biochars fitted by the kinetic model are similar;the adsorption isotherm fitting of Me Hg on selenium and chitosan modified biochars are in the exponential rising stage,not reach the equilibrium,indicating that the modified biochar may also have a strong adsorption effect on methylmercury(4)In terms of bulk soil,both SMBC and CMBC significantly reduced the relative abundances of Firmicutes,the relative abundances of Geobacteraceae are lower in SMBC and CMBC than CK1 and CK2,and the relative abundances of Clostridiaceae are the lowest in CMBC.Meanwhile,CK2 and SMBC significantly reduced the relative abundances of Bacillaceae,SMBC and CMBC significantly reduced the relative abundances of Hungateiclostridiaceae,CK2 and CMBC significantly reduced the relative abundances of Gracilibacteraceae.The dominant Archaea of SMBC and CMBC in non rhizosphere and rhizosphere is not Euryarchaeota,but Crenarchaeota with mercury resistant gene mer A.The results showes that biochar modified by selenium and chitosan changed the relative abundances and community structure of the dominant microbiota in the rhizosphere,and form different dominant bacteria in rhizosphere and non-rhizosphere soil,inhibiting the activity of methanogens,while the activity of mercury resistant microorganisms increased.