Effect Of Biochar Amendment On The Uptake Of Cd And Pb By Rice And Wheat Grain In Pollutanted Soils

With the development of industry and the extensive use of pesticides and fertilizers in agriculture,the pollution of heavy metals in farmland soil is becoming more and more serious.As the main food of China,the rice and wheat that grow in heavy metal polluted soil will be a serious threat to human health by transfer heavy metals through food chain,which has attract much more attention for the protection of soil condition and the safety of agricultural products,the technologies of reducing the concentration of heavy metals in wheat and rice extremely important.Biochar is a kind of solid material characterized by rich carbon content,insoluble,stable and highly aromatic structure derived from pyrolysis crop straw and other biomass materials in the aerobic or anaerobic conditions with low temperature or low temperature(<700 ?).Generally,biochar is applied for sorption material with its dense pore structure and large specific surface area and high sorption potential.Meanwhile,the high pH value and strong ion exchange ability entitled biochar with high fixation performance on heavy metal ions by complexation and chelation effects.Therefore,biochar has broad application prospects in the control and treatment of heavy metal pollution in soil with its potential in remediation of heavy metal pollution in soil.This study uses rice as the object,according to the mass ratio of wheat straw biochar application 0,1%,2%respectively,Cd and Zn combined pollution,soil Cd,Zn bioavailability and rice on the difference of Cd,Zn absorption of biochar;at the same time,using wheat as the object,select the Cd and Pb pollution in three years of field experiments based on a one-time application of wheat straw biochar were 0,20,40t/hm2,persistence effect on biochar effect on soil heavy metal pollution effectively and crop absorption and biochar,in order to provide a theoretical basis for the biochar in the treatment of heavy metal pollution in agricultural soils.The main results are as follows:(1)The application of biochar obviously improve the physical and chemical properties of soil,including soil pH and soil organic carbon content,both in upland and paddy soil.Among which,the soil organic carbon content in pot experiment increased by 33.47%-114.63%with the increase in biochar application amount,indicated that biochar provided suitable conditions for the growth of wheat and rice and guarantee stable yields.(2)In polluted soil with mono-element of Cd,Zn,the application of biochar can significantly reduce the content of soil by 22.99%and 17.77%for CaCl2-Cd and 28.89%and 18.63%for CaCl2-Zn,respectively.However,there was no significant difference between different biocahr application amounts.For the multi-element pollution with Cd-Zn,obvious effect from biochar was got under high concentration of Cd,in which the largest reduced tend is up to 50.92%,while with the increase in concentration of Zn,the effect was weakened;similarly,the effect of Zn also increases with the increase of Cd concentration and weakened.(3)Under the condition of mono-element pollution with Cd,Zn in the soil,the inhibition effect of biochar on Cd uptake in roots and grains decreased with the increase of Cd concentration in soil,which ultimately induced to the inhibition effect on Cd uptake in straw.Compared with control,treatments with bicohar got the largest decline in Cd accumulation in grain,stem and root were 29.41%,42.85%and 43.89%,respectively.(4)In the combined pollution of Cd-Zn,the inhibiting effect from biochar on Cd uptake in grains became more obvious with the increase of Zn concentration in the soil.While the inhibition effect of biochar on Zn accumulation in rice root decline with the increase of soil Cd concentration.The differences existed in the Zn concentration in root between control and biochar treatments disappeared as the simulated concentration of Cd reached the maximum in soil(Cd3),and no correlation between Zn content and Zn content in root,meanwhile the concentrations of Cd,Zn in the overall distribution characteristics of rice plants:root>stem>grain.(5)Pollution levels of Cd and Pb and application amount of biochar could also inject some effects on biochar inhibition performance.The concentration of Cd in wheat grains was better inhibited by higher biochar application,while lower pollution level and biochar application amount got lower concentration of Pb in wheat grains.However,the Cd concentrations of wheat grains in both SN and JZ with every treatment was 0.19-0.29 mg ·kg-1 and 0.28-0.35 mg · kg-1,which were still higher than the national food safety standard limit(0.1 mg · kg-1).Effects of continuous three years of biochar absorption on grain Cd with third years of high rates of low carbon content pollution level and the effect of the best,by grain Cd content of wheat was 17.39%;effect on grain Pb absorption,the effect with low carbon content and low pollution level was the best,three consecutive years of seed Pb content is the control significantly decreased by 27.78%,30.77%,67.19%,showed an increasing trend.(6)The effect of biochar on the absorption of wheat Cd and Pb was mainly to reduce the bioavailability of soil Cd and Pb at the beginning by increasing soil pH and organic carbon content,so as to give full play to the lime effect of biochar.The content of CaCl2-Cd and CaCl2-Pb in the first year of B40 treatment was significantly lower by 17.64,49.72%and 26.68%and 47.18%than that of the control two,respectively;under the influence of natural conditions and time changes,the performances of biochar in the continuous three years fluctuated greatly,and the different characteristics of Cd and Pb also altered the effects of bicohar.The most significant changes happened with soil variability of Cd and Pb fractions,compared to year 1,both the concentrations of CaCl2-Cd and CaCl2-Pb increased by 3.44,0.25,5.01 and 6.89 times in year 2;and that were 13.01,3.98,0.88 and 1.43 times in year 3.Soil CaCl2-Cd content showed an increasing trend for three consecutive years,with an increase of 160%and 210%respectively.(7)Results showed that soil Cd and Pb fractions changed largely with ime under the influence of many factors including biochar.The fraction of soil Cd predominated by F1 in year 1,which occupied 60%of the total fractions(F1+F2+F3+F4).However,F1 decreased and F2 increased with time going.The portion of available soil Cd fraction(F1+F2+F3)changed from 90%to 61%in SN,while that kept within 84%in JZ.Conversely,the different soil Pb fractions changed little and mainly existed as F2,while F1 was the least and F3 changed the most in three years.When it comes to SN,the application of biochar increased F4 to some extent from year 2 and the treatment of B40 effectively reduced the portion of(F1+F2+F3)from 86%to 79%,while there was no significant changed in JZ,which there was nearly no change in F4 and only F1 and F3 changed with time.