Effects Of Different Nitrogen Fertilizer Treatment On Mercury Accumulation In Rice And Their Mechanisms

Mercury(Hg),as a toxic and harmful heavy metal element,has attracted wide attention since the 20 th century.In 2014,the Soil Heavy Metal Pollution Bulletin showed that the soil mercury pollution in China was very serious,and the Wanshan Mercury Mine in Tongren City,Guizhou Province,produced a lot of wastewater,waste residue and waste gas during its mining and smelting activities,which had caused serious damage to the farmland ecosystem.As the main farming method of residents in mercury mining area,contamination of edible rice has become the main way of methylmercury exposure in mercury mining area.Nitrogen,as a very important factor in rice growth process,is not only related to the yield of rice,but also has a certain impact on the uptake and accumulation of heavy metal ions by rice.It is also the most commonly used agronomic measures to improve soil properties by farmers.Therefore,it is of great practical significance to actively explore the control methods of mercury pollution in paddy soil and reduce the accumulation of mercury in rice.Based on the above research background,this paper selected the mercury-contaminated soil in Wanshan Wukeng(WK)mercury mining area,and conducted an open-air pot experiment in Wanshan Dixi soil method mercury mining area.Ten treatment groups were set up: blank control(CK),total urea fertilizer(CNL,CNM,CNH),total urea as base fertilizer,tillering fertilizer and spike-grain fertilizer(CN1,CN2,CN)with low,normal and high application rates.3)The effects of urea+calcium nitrate+ammonium sulfate on mercury uptake and accumulation in rice were studied by adding urea+calcium nitrate+ammonium sulfate three times in growth period,tillering fertilizer and one-time spike-grain fertilizer(CNN3,NNN2,NNNN3).The analysis and determination of mercury content in the preliminary test were the most suitable treatment.Four treatment groups were set up in the follow-up simulation experiment: total urea,urea + calcium nitrate,urea + ammonium sulfate,urea + calcium nitrate + ammonium sulfate as one-time addition of spike grain fertilizer(NS,NO,NH,NOH).The dynamic changes of physical and chemical properties and mercury speciation in soil under different nitrogen fertilizer treatments were explored,and the dynamic changes of mercury speciation at different nitrogen fertilizer sites were analyzed.The mechanism of mercury uptake and accumulation in rice was studied.The following conclusions were drawn:(1)The results of different nitrogen fertilizer treatments on rice biomass showed that compared with CK treatment,adding nitrogen fertilizer could promote the growth of rice biomass.The seed setting rate of the treatments with different kinds of nitrogen fertilizers was higher.The seed setting rate of the treatments with NN2,N3 and CNN3 increased by 2%,3% and 9% respectively.The results showed that the effect of three nitrogen fertilizers on rice biomass was better than that of single fertilizer.(2)The results of different nitrogen fertilizer treatments on total mercury content in rice showed that the highest total mercury content was found in high nitrogen fertilizer treatments.In high biomass group,compared with conventional fertilization(CNM),CN1,NN2,N3 and CNN3 treatments could reduce the mercury uptake and accumulation in rice,but their effects on different parts of rice were different.The total mercury content in Rice Leaves under CNN3 treatment was 39% lower than that under CNM,and the total mercury content in Rice under NN3 treatment was 49% lower than that under CNM.(3)The results of different nitrogen fertilizer treatments on methylmercury in rice showed that the concentration of methylmercury under CN2 treatment was the highest in low biomass treatments compared with CK treatment,and rice was 105% higher than CK treatment.In high biomass group,compared with conventional fertilization(CNM),the content of methylmercury increased significantly in all parts of rice treated with nitrogen fertilizer at tillering stage,but decreased 32% in rice treated with nitrogen fertilizer at panicle and grain stage.At the same time,due to many factors affecting mercury methylation,the results show that methylmercury and total mercury show different trends.(4)The results of simulation test showed that soil EC and pH value increased after applying nitrogen fertilizer.The trend of EC value increased was NOH > NH > NO > NS,and the trend of pH value increased was NS > NO > NH > NOH.Meanwhile,the content of fulvic mercury in soil speciation mercury was NOH < NH < NO < NS;the content of humic mercury was NOH < NO < NS < NH;the content of strong complex mercury was NO < NS < NOH < NH;and the content of sulfide mercury was NS < NH < NO < NOH.In summary,the application of three nitrogen fertilizers(urea + calcium nitrate + ammonium sulfate)at panicle-grain stage can not only maximize the growth of rice biomass and yield,but also change the physical and chemical properties of the soil,so that the EC value of the soil rises the highest and the pH value rises the least,thus reducing the content of mercury in fulvic acid and humic acid,which are easy to be transferred and absorbed in the soil.Fixed strong complex and sulfide mercury content increased,which made the inhibition of mercury absorption and accumulation in soil by rice the strongest.It can be seen that the treatment of mixed application of three nitrogen fertilizers at spike and grain stage is the best agronomic measure to reduce farmland pollution in mercury mining area,which is of great significance to effectively control the absorption and enriCNMent of mercury in mercury polluted area and to realize agricultural production in mercury polluted area.