The Effect Of Water Management On Methylmercury Accumulation In Agricultural Soils

Mercury（Hg）is a global pollutant that enters the soils may be transformed to highly neurotoxic methylmercury（MeHg）by specific anaerobic microorganisms,while changes in soil environmental conditions may affect the diversity and activity of these functional microorganism.The fluctuation of water conditions will affect the soils oxygen condition and redox process,change microbial community composition,and finally may affect the formation of MeHg.Upland soils are typical aerobic environment,which are not conducive to the survival of anaerobic microorganisms.However,agricultural water management and rainfall will change the instantaneous water conditions of upland soils,the effect on Hg methylation still remains unclear.In this study,two typical upland soils（Tongren yellow soil and Xianning red soil）and long-term flooded farmland soils were sampled to analyze the effect of water management on soil MeHg accumulation.The relationship between mercury methylators or bacterial community composition and MeHg accumulation was investigated by high-throughput sequencing techniques,and the microbial ecological mechanism was also revealed.The main results obtained are as follows:（1）Water management influenced the accumulation of MeHg in upland soils and the abundance of mercury methylation gene（hgc A）.The 25%moisture content did not alter the MeHg concentration in upland soils.Increasing the moisture content to60%,the MeHg content in upland yellow soils increased,while it was no significant change in red soils.Flooded increased the MeHg content in two upland soils,and the warming had a positive effect on the formation of MeHg.The increase of moisture and temperature increased the abundance of hgc A in red soils,but no hgc A genes could be detected under the condition of 25%moisture content.Flooded and warming did not affect the abundance of hgc A in yellow soils.（2）The main mercury methylators in tested soils were Geobacter,methanogens and SRB,and their relative abundance was influenced by the change of moisture and temperature.With the increase of water content,the relative abundance of Geobacter increased first and then decreased in yellow soils,the relative abundance of methanogens decreased,and the relative abundance of SRB keep constant.Warming had no effect on the relative abundance of Geobacter in yellow soils,but increased the relative abundance of methanogens and SRB.The relative abundance of Geobacter and SRB in red soils increased,while the relative abundance of methanogens decreased with the increase of moisture content.With the increase of temperature,the relative abundance of Geobacter and methanogens increased in red soils,while the relative abundance of SRB increased first and then decreased.（3）The concentration of MeHg in the long-term flooded soils was 0.85μg·kg^-1,accounting for 1.44%in total mercury（%MeHg）.The concentration of MeHg in the control soils（intermittent flooded）was 0.78μg·kg^-1and the%MeHg was 1.33%.44.3%of the samples（n=131）in the long-term flooded treatment had a significant difference in%MeHg in comparison with the control.The difference may be related to the disparate composition of soil bacterial communities.24 taxa of potential mercury methylators were found in this study,of which Geobacteraceae and Desulfobulbaceae were positively correlated with%MeHg.In addition,the study found that non-Hg methylators such as Christensellaceae,Desulfocapsaceae and Prolixibacteraceae had a strong linear positive correlation with%MeHg.