Study On The Control Of Cd Accumulation In Rice By Exogenous Organic Matter And Biochar Under Water Management

Rice is the most important food crop in China and other Asian countries.In order to reduce the Cd accumulation in rice,the Chinese government has invested a lot of money in contaminated soil remediation,but the efficiency is not significant.So it is imperative to explore the control technology for Cd accumulation of rice in Cd-contaminated soil.In recent years,water management has been attracting much attention in the study of Cd accumulation in rice.Exogenous organic matter could increase soil organic matter content and fertility.Biochar,as a new type of environmental material,has shown its tremendous potential at the remediation of Cd-contaminated soil,but in which there is little information on the effects of those on Cd accumulation in rice.Moreover,it is still unclear that the mechanisms of water management and organic matter on the migration and transformation of Cd in soils.Therefore in this paper,we explored the control mechanism of different additive amounts of exogenous organic matter and application time of biochar on Cd accumulation in rice and the migration of existing form of Cd in soil.The main results of all works are presented as following:(1)Water management had an important influence on Cd accumulation in rice.During the entire growth period,the contents of Cd in grain rice were the lower with increase of soil water contents.Moreover,with increase of the soil moisture,the soil redox potential(Eh)decreased significantly(p<0.05),and Fe2+ contents could increase significantly(p<0.05),the pH value,biomass and yield of rice had no significant difference(p>0.05).The dissolve organic matters(DOM)and DTPA-Cd concentrations reached the peak during the flowing stage and reduced in maturing stage.(2)Under the two different water managements(the drying-wetting alternate irrigation and the conventional irrigation),there were consistent results for the treatments with exogenous organic matter.Addition of the exogenous organic matter could reduce the concentrations of Cd in grain rice.The concentrations of DOM in soil solution could increase significantly(p<0.05)and there were no significant effect(p>0.05)on the Eh value in soil and the pH value in the soil solution.However,in the treatments with 0.5%exogenous organic matter,the aboveground biomass of rice incresed,but the concentrations of available Cd had no significant effect(p>0.05).Whereas in the other treatments with 4%organic matter the concentrations of available Cd significantly reduced(p<0.05)and the aboveground biomass had no significant effect(p>0.05).(3)Addition of biochar could have an important effect on accumulation of Cd in rice under the two water managements.Under the conventional irrigation,biochar of 5%added in the stage before transplanting(or after tillering)could reduce the Cd content in grain rice significantly(p<0.05),but the highly could reduce contents of Cd for the treatments with biochar added after tillering.Comparatively,under drying-wetting alternate irrigation,biochar application could increase the content of Cd mainly in grain rice and roots of rice and reduce higher contents of Cd when biochar were added after tillering.The addition of biochar could have different effects on the physical and chemical properties of soil and soil solution under the two water managements.Under the conventional irrigation,the soil Eh values could decrease before the heading stage and increased after the maturity period.The biochar treatment had no significant effect(p>0.05)on the pH values in the soil solution.Under the drying-wetting alternate irrigation,the Eh values increased and the pH value in soil solution decreased with the biochar treatment.In addition,under the two water managements,biochar treatment could reduce the available Cd content in soil significantly(p<0.05),and the DOM contents had no significant effect(p>0.05).(4)The combined effect of exogenous organic matter and biochar could have different effects on Cd accumulation in rice under different water managements.Under the conventional irrigation,the Cd contents in grain rice treated in each group were generally lower than those under drying-wetting alternate irrigation treatment groups.Compared with the control group(non-exogenous organic and non-biochar treatment),the combined treatments could reduce the Cd content in grain rice,and the Cd contents in grain rice were lower after tillering application of biochar treatment groups than that before transplanting stage treatment groups under the conventional irrigation.The combined treatment could increase the Cd contents in grain rice,and the content of Cd after tillering stage were higher than the stage before transplanting under drying-wetting alternate irrigation.Under the two kinds of water management,the order of Cd contents in rice plants was:root>grain>stems and leaves>rice hulls,and the contents of Cd in roots were much higher than other parts of rice plants.(5)The effects of exogenous organic matter and biochar on the soil available Cd contents were more complicated under water managements.Under the conventional irrigation,the soil available Cd contents in all treatment groups were higher at the tillering stage and heading stage than those in the drying-wetting alternate irrigation treatment groups.In the filling and maturation periods,those were lower than that in drying-wetting alternate irrigation groups.Compared with the control group,under the conventional irrigation,the combined treatment could increase the pH values and the contents of DOM in soil solution.Moreover,the Eh values in soil had no significant effect(p>0.05).The pH values increased before the tillering and decreased after tillering stage under the drying-wetting alternate irrigation.Eh values in soil had no significant difference(p>0.05).