Effects Of The Biofertilizers On Soil Mercury Release In Vegetable Greenhouses

Mercury（Hg）is one of the most toxic heavy metals in the environment,which is volatile,persistent and long-distance migration,etc.The soil Hg can be absorbed by crops easily,then enlarged and accumulated in organisms,which threatens the human’s life safety severely.In fact,leafy crops not only accumulate the Hg in soil by their roots,but also absorb the Hg in air replying on the breath of the leaf stomas,double absorption and accumulation could cause more severe food safety issues.The vegetable greenhouse is a semi-enclosed,high-heat and high-humidity environment,which could promote the Hg release from soil and increase the gaseous Hg contents in greenhouse.Moreover,the application of biofertilizers maybe affect the Hg release and gaseous Hg contents in greenhouse through immobilizing or activating the soil Hg,the high or low gaseous Hg contents would influence the leaf Hg contents directly.Therefore,we need to ascertain the traits of soil Hg release under the applying biofertilizers for clearing the effects of biofertilizers on leafy crops’leaf Hg contents.This research built some small vegetable greenhouses with Hg-contaminated soil（P）where were cultivated Chinese cabbages and leeks,and applied with microbial agent fertilizer（MF）and Hugenling biofertilizer（HF）,exploring the effect of biofertilizers on Hg contents in leafy crops,gaseous Hg contents,soil-air surface Hg release flux,total soil Hg,Hg fractions and soil physico-chemical properties.In addition,we carried out adsorption test using biofertilizers and free Hg-contaminated soil,and took characterization techniques to observe the biofertilizers and soil adsorbing for Hg before and after,stating the processes of biofertilizer adsorbing Hg and the effect of biofertilizers on soil adsorbing for Hg clearly.Finally,combining the results of vegetable greenhouses and adsorption test,we got the mechanism of inhibition of the soil release Hg by biofertilizers.The main conclusions are as follows:（1）MF and HF biofertilizers promoted the growth of Chinese cabbages and leeks as well as decreased the Hg contents in leaves of Chinese cabbages and leeks.The average biomass of Chinese cabbages in P-MF-2%and P-HF-3%treatments increased by 493.7%and 243.7%,respectively,compared to the control-P.The average biomass of Chinese cabbages in two treatments of all MF or HF biofertilizers treatments increased mostly,the growth effect got the best.The leaf Hg contents of Chinese cabbages in P-MF-1%,P-MF-2%,P-MF-3%and P-MF-2%（succession）decreased by 66.4%,19.3%,42.2%and 42.8%,respectively,compared with the control-P treatment.1%MF biofertilizer can decreased Hg contents mostly and obtained the best effect.Leaf Hg contents of Chinese cabbages in P-HF-1%,P-HF-2%,P-HF-3%and P-HF-2%（succession）decreased by 41.6%,33.2%,70.7%and 59.8%,respectively,compared with the control-P treatment,and 3%HF biofertilizer decreased the most.The average biomass of leeks in P-MF-3%and P-HF-1%treatments increased by 49.5%and 45%,respectively,compared to the control-P.The average biomass of leeks in two treatments of all MF or HF biofertilizers treatments increased mostly,the growth effect got the best.Compared with the control-P,the leaf Hg contents of leeks in P-MF-1%,P-MF-2%,P-MF-3%and P-MF-2%（succession）decreased by 60.3%,35.7%,64.7%and 61.6%,the leaf Hg contents of leeks in P-HF-1%,P-HF-2%,P-HF-3%and P-HF-2%（succession）decreased by 44.8%,80.6%,73.9%and 66.6%,respectively,compared with the control-P treatment.In conclusion,under the dosage of 3%MF and 2%HF biofertilizers,the leaf Hg contents of leeks decreased the most,and the performance effect was the best.（2）MF and HF biofertilizers can decreased the gaseous Hg contents and Hg release flux in greenhouses differently,which can decrease the Hg content absorbed by leaf stoma availably.From the beginning to the end of the experiment,the gaseous Hg contents of P-MF-1%,P-MF-2%,P-MF-3%and P-MF-2%（succession）decreased by 373,641,422 and477 times,respectively,and 2%MF biofertilizer decreased by the largest multiple,got the best effect.The order of Hg release flux reduction was P-MF-2%（succession）>P-MF-2%>P-MF-3%>P-MF-1%,and the inhibition effect of 2%MF（succession）on soil mercury release was the best.The gaseous Hg contents of P-HF-1%,P-HF-2%,P-HF-3%and P-HF-2%（succession）decreased by 447,247,621 and 700 times,respectively,and 2%HF（succession）biofertilizer decreased by the largest multiple,got the best effect.The order of Hg release flux reduction was P-HF-2%>P-HF-3%>P-HF-1%>P-HF-2%（succession）,and the inhibition effect of 2%HF on soil mercury release was the best.Besides,there was a positive correlation between the gaseous Hg contents and soil-air surface Hg release flux.MF treatment kept a stronger positive correlation between the gaseous Hg contents and soil-air surface Hg release flux.This indicated that the lower the Hg release flux is,the lower the gaseous Hg contents in the greenhouses is.（3）Both MF and HF could immobilize the soil Hg and transform the potentially available Hg into unavailable Hg,decreasing the activity,migration and bio-availability of Hg.The order of difference value of soil total Hg contents between beginning and end in each treatment were:the control-P>P-MF-3%>P-MF-2%（succession）>P-MF-1%>P-MF-2%,and the control-P>P-HF-3%>P-HF-1%>P-HF-2%（succession）>P-HF-2%,indicating that applying biofertilizers can keep Hg in soil,and 2%MF and 2%HF biofertilizers could get the best effects.MF and HF can increase the ratio of unavailable Hg,2%MF（succession）and 2%HF increased the most.p H,SOM,Eh and CEC were influenced by biofertilizers.As the biofertilizers dosages increases,p H value decreased gradually.Two biofertilizers can improve the SOM and CEC.Eh in adding biofertilizers continuously were inferior to 400 m V on 90 day,these soils were not strong oxidation state no longer.Pearson correlation indicated that SOM and CEC were staple factors influencing the Hg release flux.（4）Langmuir and Freundlich models could describe the isothermal adsorption processes of Hg by HF primely,only Freundlich model could describe the isothermal adsorption processes of Hg by MF.The adsorption of Hg by MF and HF was also suitable to quasi-second-order kinetic model.Microscopic characterization manifested that MF and HF have a lot of chemical functional groups（-OH,-O-NO₂,C-OH and-C=O,etc.）,their particles surfaces have vast micropores and abundant microorganism,those are benefit for adsorbing Hg by MF or HF.The energy spectrum directly illustrated that MF and HF can adsorb Hg,the relative content of Hg in MF and HF biofertilizers after adsorbing Hg was2.9 and 8.7 times of that before adsorption,respectively.And the adsorption ability of HF was stronger than MF.When the biofertilizers mixed with natural soil,the relative Hg contents in Hg+MF+natural soil and Hg+HF+natural soil treatments increased by 0.06%and 0.75%,respectively,compared with Hg+natural soil treatment,which suggested that MF and HF biofertilizers could enhance the effect of soil adsorbing Hg.（5）Biofertilizers can immobilize the soil Hg through reducing the activity and migration of Hg in soil,and suppress the release Hg from soil,then resulted in decreasing gaseous Hg contents in greenhouses,finally,the leaf Hg contents was reduced.Those had a chain effect on vegetable greenhouse production.The primary mechanism maybe include physical adsorption,chemisorption,biological adsorption,and forming“membrane”to enclose soil particles to immobilize Hg during the inhibition of the soil release Hg by biofertilizers.Firstly,the abundant micropores on the surface of biofertilizers particles provide a mass of adsorption sites for Hg adsorption.Secondly,the different chemical functional groups contained in biofertilizers can chelate and complex with Hg.Moreover,abundant microorganisms could adsorb mercury through intracellular,extracellular and secretions,and could also bond with soil minerals to form a“membrane”to fix mercury.To sum up,microbial agent fertilizer and Hugenling biofertilizer can passivate and immobilize Hg in the soil by decreasing the activity,migration and bio-availability of Hg,and inhibit the Hg release from soil.Then the gaseous Hg contents was decreased.Ultimately,above those decreased the leaf Hg contents of Chinese cabbages and leeks.Therefore,in this study,the application of two kinds of biofertilizers has a positive effect on the planting of Hg-contaminated soil agricultural vegetable greenhouse,which has practical significance for the green and sustainable development of agriculture.