Distribution Of Low-molecular-weight Organic Acids And Their Effects On The Mercury Methylation In A Water-level-fluctuating Zone Of The Three Gorges Reservoir Area

Mercury?Hg?,especially its methylation product?Methylmercury,MeHg?,have extremely high bioaccumulation factors and biotoxicity.Aquatic ecosystem is a unit that facilitates Hg methylation,and its biogeochemical properties will affect the safety and health of wildlife and humans.Reservoirs are the important composition of aquatic ecosystems.Since the finding of the extremely high levels of biological Hg in newly-built reservoirs,the"activation effect"of Hg in reservoirs has been widely concerned,and it has been confirmed that reservoir is a"Hg-sensitive ecosystem".The inundated soil and plants are responsible for the elevated Hg levels in aquatic organisms.The Three Gorges Reservoir?TGR?,one of the greatest reservoirs worldwide with the area up to 1080 km²,is formed in the upper reach of the Three Gorges Dam following its fixed operation strategy.The water-level-fluctuating zone?WLFZ?,a region of approximately 350 km² with its water level periodically fluctuating between 145 m above sea level?a.s.l.?and 175 m a.s.l.,was also generated around the reservoir.Plants grow abundantly during the drying period of the WLFZ,the season with the warm-moist climate.After flooding,these plants will be fully flooded and further release a large quantity of dissolved organic matter?DOM?to water and sediment.Then,the WLFZ has typical characteristics of newly-constructed reservoirs.Plant growth and soaking can release large amounts of DOM into soil and sediment.Among which,low-molecular-weight organic acids?LMWOAs?,as important parts of DOM,is a kind of the small organic compounds with higher bioavailability.It not only participates in the activation of soil nutrients,but also can influence the migration and transformation,bioaccumulation,and biotoxicity of trace metals through the coordinated complexation and/or stimulating the microbial activity.Different from other trace elements,the interaction between LMWOAs and Hg may affect the fate of ambient MeHg.However,experiencing several years of periodically flooding stress,the flood-tolerant plant species have gradually become the dominant species in the WLFZ.Among these adaptable species,bermudagrass?cynodon dactylon?L.?Pers.,?,a common species with the extremely high biomass,coverage area,flooding survival,and low decomposition rate,is probably changing the features of Hg biogeochemistry,however,this is unclear.Although many researchers have explored the environmental chemical process influenced by plants in the WLFZ,they mainly focused on the nutrients release under flooding conditions.However,till now,whether those plants will affect the Hg migration,transformation and methylation plus the corresponding ways has not been well understood.As such,following the interaction between Hg and LMWOAs influenced by plants,the field studies and laboratory simulation experiments were conducted.The objectives of this study are 1)to investigate the distributions of LMWOAs and Hg in dominant plants and their rhizosphere soils;2)to explore the effects of growth and flooding of bermudagrass on the production of MeHg and its migration in soil and/or sediment;3)to confirm the roles of LMWOAs in these processes.The main results are as follows,In the detected LMWOAs,malic acid and acetic acid were dominant components in plants,while the citric acid accounted for a lower proportion.The total LMWOAs(T_LMWOAs)content in leaf was higher than that in stem and root,indicating that leaf was the main storage site.Specifically,T_LMWOAsMWOAs content in leaf in the WLFZ was higher than that in the permanently non-flooded zone,which was contrary to that in stem and root,indicating that water-level fluctuation had altered plant metabolism.In rhizosphere soil,T_LMWOAs content showed a significantly positive relationship with root secretion rate of LMWOAs?p<0.01?,and their distributions in different rhizosphere environments were varied largely?p<001?.It should be noted here that the occurrence of such acids were under the joint influence of root metabolism and rhizosphere environment.In the bulk soils,the tartaric acid content was the highest component of all detected LMWOAs with 107.67±116.64 mg kg^-1?d.w.?,and the citric acid accounted for the lowest level with content of 4.50±4.46 mg kg^-1?d.w.?.The T_LMWOAsMWOAs contents in the bulk soil was elevated in WLFZ,and this illustrated that WLFZ was a unit favorable to accumulate LMWOAs.Additionally,soil T_LMWOAs pool reached the lowest value in the initial period after drying(0.06±0.05 mg cm^-3),followed by the maximum of 0.28±0.16 mg cm^-3 in July,and reduced to 0.13±0.08 mg cm^-3 in September.It was seen that soil T_LMWOAs increased approximately 2.31 times in the whole dry-period,which also positively correlated with the soil temperature?p<0.05?.Plants,as the main parts of the WLFZ ecosystem,are important Hg pool.The total Hg?THg?contents in root,stem,and leaf of the dominant plants were 9.62±8.20-28.45±13.33,4.74±1.57-14.23±13.87 and 10.37±6.82-34.24±12.60 ng g^-1,which were firstly increased,and then decreased over time.Meanwhile,the methylmercury?MeHg?contents in root,stem,and leaf of plants were varied with sampling time from 0.55±0.32 to 0.78±0.39,0.27±0.12 to 0.56±0.27,and0.45±0.28 to 0.90±0.24 ng g^-1,respectively.Different from the THg distribution,MeHg concentrations in different parts of plant in the initial dry-period were higher than other periods due to the increased net accumulation of MeHg in the flooding period.With a few exceptions,MeHg in root,stem and leaf in most of the plants had no significant variation?p>0.05?,which indicated that MeHg accumulation in plant might be more dependent on the environmental factors.Plant root and leaf were the mainly storage sites for THg and MeHg,while stem served as a transport function in the body.Bermudagrass,the predominant plant in the WLFZ,was found the stronger ability to accumulate Hg in its root and stem than other plants,whereas the Hg contents in leaf was at the low level.The THg contents varied significantly in rhizosphere soil between Shibao zhai?SBZ?and Qukou zhen?QKZ?sites,ranging from 22.79 to 117.03 ng g^-1 plus small variations over time.Specifically,higher MeHg concentration was observed in the initial(April,0.72±0.20 ng g^-1)and medium(July,0.75±0.24 ng g^-1)stages of the dry-period,which was concluded to be responsible by both flooding process and root metabolism.Similar with the distribution of LMWOAs,soil MeHg contents were elevated in WLFZ compared to permanently non-inundated area,indicating that WLFZ was also favorable to Hg methylation.In rhizosphere soils,T_LMWOAsMWOAs showed the positively relationship with corresponding MeHg contents?p<0.01?,indicating that root-derived LMWOAs could promote soil Hg methylation.In addition,MeHg contents in rhizosphere soils were significantly correlated with those in roots?p<0.05?,but were not obviously correlated with those in stem and leaf?p>0.05?.Then,the LMWOAs accumulated in the rhizosphere may be responsible for the accumulation of MeHg in plants.Dominant plants,such as bermudagrass,Echinochloa crusgalli?L.?Beauv.,Bidens tripartita L.,Xanthium sibiricum Patrin ex Widder.,and Setaria viridis?L.?Beauv.,were found the higher bioaccumulation factor of soil MeHg(BAF_MeHg)ranging from0.56±0.24 to 1.73±0.92 with the average value being 1.24±0.88,which were affected by MeHg level in rhizosphere soils and plant species.The effects of bermudagrass on Hg methylation in soils and MeHg release from sediments were studied by artificially removing it as control.Results showed that,different from the MeHg contents and the temporal distribution in rhizosphere soils,the net accumulation of soil MeHg declined considering the effect of the eliminated plants with non-obvious temporal variations.This implied that rhizosphere environment was a hotspot of Hg methylation.THg contents were 116.87±17.29and 82.95±13.83 ng g^-1 in vegetated soils in SBZ and QKZ sites,respectively,higher that those in the corresponding non-vegetated soils(106.32±15.71 and 70.66±10.55 ng g^-1).Due to the retention of plants,THg contents in vegetated sediment were increased by flooding which exerted small effects on non-vegetated areas.Contents of MeHg in vegetated and non-vegetated soil/sediment in SBZ were 0.81±0.24 and 0.64±0.20 ng g^-1,respectively.While MeHg contents in vegetated and non-vegetated soil/sediment in QKZ site were 1.08±0.45 and 0.58±0.16 ng g^-1,respectively.It was thus concluded that vegetated soil had the significantly higher MeHg contents than non-vegetated areas did?p<0.05?.The MeHg contents in vegetated sediments were considerably increased by flooding?p<0.01?;however,non-obvious increase was found in non-vegetated areas?p>0.05?,indicating that plant flooding could elevate the MeHg production in sediment.Hg enriched stable isotope addition experiments showed that Hg methylation rate constant in non-vegetated soil was0.0003±0.0001d^-1 of which was significantly lower than that in vegetated soil(0.0035±0.0012d^-1).Nevertheless,such variation was not dependent on the bioavailable Hg level.Meanwhile,T_LMWOAsMWOAs contents in vegetated soil were 51.74±30.96 mg kg^-1 that was significantly higher than that in non-vegetated soil with values of 28.46±10.61 mg kg^-1?p<0.01?.Moreover,soil T_LMWOAs was also found to be positively correlated with the corresponding MeHg contents,supporting that the higher LMWOAs contents in vegetated soils were responsible for the elevated Hg methylation potential.Hg enriched stable isotope addition experiments confirmed that Hg methylation rate was significantly increased in non-vegetated soil with the addition of citric acid and plant extracts,and their rate constants increased by more than 10 times compared to the control group.This indicated that the removal of plants resulted in the LMWOAs deficiency,which was responsible for the decrease of Hg methylation rate in non-vegetated soils.However,solid-liquid partitioning coefficient of MeHg were2.97±0.24 L kg^-1?log-transformation?in vegetated sediment,respectively,which was significantly higher than that in non-vegetated sediment(2.48±0.26 L kg^-1)?p<0.05?.Additionally,it was found that the diffusion flux of MeHg from sediment to overlying water was lower in vegetated region(1.49±0.61ng m^-2 d^-1)than that in non-vegetated sediment(2.50±1.51ng m^-2 d^-1)as well.Apart from that,addition of citric acid and plant extracts increased the solid-liquid partitioning coefficient of MeHg possibly due to the increased quantity of organic ligands in soil.Overall,bermudagrass,as the predominant plant in the WLFZ,was featured with its obviously environmental effects for promoting Hg methylation in soil and sediment,but not in favor of Hg release towards overlying water.This may be attributed to that the enhanced organic environment resulted from plant growth had a strong retention effect for MeHg in sediment.To study the effects of bermudagrass flooding on the production and migration of MeHg in sediment,a series of incubation experiments including"plant+water","plant+soil+water"and"soil+water"treatment groups were performed.The UV-vis and fluorescence spectral demonstrated that both composition and structure of DOM in overlying water was apparently altered by plant flooding that increased the fraction of chromophore and fluorescence DOM.In a system including both plant and soil,the spectral slope ratio was gradually reduced,and DOM humification index?HIX?was increased in overlying water over time,indicating that the fraction of low-molecular-weight components was gradually reduced and the fraction of high-molecular-weight humus increased.The presence of soil was responsible for the accelerated humification of plant-derived DOM.Additionally,HIX value in overlying water showed a remarkably negative correlation with the T_LMWOAs concentration?p<0.01?,illustrated that enhanced DOM humification was served as one of the reasons responsible for the decline of LMWOAs concentrations over time.Moreover,the fluorescence intensity of tyrosine peaks in?plant+soil+water‘and?plant+water‘treatments were 2.70 and 7.95 times higher than those in?soil+water‘treatment,respectively,with the corresponding multiples of 2.21 and 2.79 for tryptophan peak.It was indicated that plants could release a large amount of protein-like substances into water in the incubation process.Concentrations of dissolved THg and MeHg in overlying water increased significantly in?soil+water‘treatment over time;however,their concentrations in the presence of plants were firstly increased in the initial period,whereas they were decreased in the later period of incubation due to the adsorption and/or complexation of the plant residues,protein and humus.MeHg concentrations in solid phase gradually increased over time in the?soil+water‘treatment;however,it was firstly increased and then decreased in the presence of plants.Moreover,dissolved MeHg concentrations in porewater were firstly increased and then decreased over time.In the later period of incubation,plant-based humic substances entered into the sediment,they not only competed for MeHg with solid phase,but also promoted the formation of macromolecular organic-MeHg in the porewater,which further decreased the proportion of the dissolved MeHg.In the presence of plants,the partitioning coefficient of MeHg showed a significant negative relationship with LMWOAs concentrations in overlying water?p<0.05?,indicating that elevated LMWOAs levels could enhance the production of MeHg in porewater,and this partially increased the relative fractions of MeHg in porewater compared to that in solid phase.