Interception Effect Of Biochar On Heavy Metals In A Paddy System

Screening a kind of biochar from corn straw biochar, rice straw biochar and rice husk biochar which has a better curing effect on soil heavy metals through indoor experiment, firstly, and then in the conditions of rice pot experiment, the biochar was applied to the pot soil. On the one hand, the effect of different biochar amount (biochar and soil mass ratio:0%,0.05%,0.1%,0.5%,1%,2%) on biological effectiveness of Cd,Pb,Zn in the soil was studied, on the other hand, and two different cadmium concentrations of irrigation water (C1, meet standards for irrigation water quality (GB5084-2005); C2, meet emission standard of pollutants for lead and zinc industry (GB25466-2010)) were irrigated in pot soils which biochar amount was 0%,0.1% and 2% in 5 rice growth periods(tillering stage, jointing stage, booting stage, flowering period, filling stage), and tap water (CK) as contrast, which researching on the effect of cadmium-containing water on cadmium accumulation in brown rice and intercept effects of biochar on cadmium. The main results were as follows:(1) The indoor experiment showed that the three kinds of biochar can improve varying degrees of soil pH, reduce the effective state contents and toxicity leaching contents of soil heavy metals of Cd、Pb、Zn. Comprehensive comparison, the curing effect of corn straw biochar was better than rice straw biochar and rice husk biochar.(2) Biochar amount had a significant or highly significant effect on the soil physical and chemical properties of seeding stage and maturity in the rice pot experiment.Especially, CEC of seeding stage was lower than maturity, on the contrary, The soil pH, organic matter, EB, BS of seeding stage were higher than maturity. The biochar amount didn’t have a significant effect on the effective state contents of soil Cd, Pb, Zn(p>0.05),but had an effect on their toxicity leaching contents.Especially,the effective state contents of soil Cd, Pb, Zn in seeding stage were higher than maturity. Pb, Zn toxicity leaching contents of seeding stage were lower than maturity. Soil Cd toxicity leaching contents of seeding stage were higher than maturity when the amount of the biochar in the soil was 0% and 0.05%, however, the result was on the contrary when the amount was 0.1%-2%.When the amount of the biochar was 2% in the soil, the toxicity leaching contents of soil Cd, Pb, Zn in seeding stage were reduced 31.3%,73.8%,22.5%, and had reached a significant level(p<0.05), and the contents in maturity were reduced 7.6%,25.9%,10.2%.(3)With the increase of the application amount of the biochar to the tested soil, the height of rice plants were slightly higher than the control plants, and which reached significant level(p<0.05) when the biochar amount of 2% was added to the soil. Biochar had an impact on dry weight of rice stems, leaves, husks, brown rices and biomass, but it was not significant(p>0.05). Under the biochar amounts of 2% conditions, rice stems, husks, brown rices and biomass reached maximum.The biochar had a non-significant impact on the contents of Cd, Pb and Zn in rice stems, leaves and husks.With the increase of the application amount of the biochar, the Cd contents in iron plaques on the surface of rice roots reduced gradually, and comparing with the control, the contents reduced 27.9% significantly (p<0.05) when the biochar amount was 2%.The contents of Pb and Zn in rice roots had a tendency to reduce with the increase of the application amount of the biochar, which reduced 33.9% and 31.2% when the biochar amount was 2%.The biochar had a non-significant impact on the contents of Cd, Pb and Zn in rice stems, leaves and husks, but But there was no significant impact on Cd contents of rice roots. The contents of Cd,Pb and Zn in brown rices reduced with the increase of the application amount of the biochar when the application amount of the biochar was 0%～1%, the reduction was 7.1%～24.2%,21.7%～76.2% and 8.3-27.3%.Obviously, the contents of Cd in brown rices met national food safety standard(≤0.2mg/kg) within the scope of 0%～1%biochar amount, and Pb content in brown rices was by overweight to standard(0.2mg/kg), when the biochar amount applied to the soil was 1%. But the contents of Cd and Pb in Brown rices had rebounded and beyond the standard limits when the biochar amount reached 2%.(4) The experiment of irrigating cadmium-containing water showed that total soil cadmium, effective cadmium, leaching of cadmium toxicity basically increased with the increase of the cadmium concentrations of irrigation water when none biochar was applied to the soil. The biochar amount of 0.1% could effectively intercept irrigation water of C1 and C2, which preventing cadmium accumulation and decreasing cadmium bioavailability of soil. The cadmium concentrations of different components of rice also increased with the increase of the cadmium concentrations in irrigation water whatever the biochar were applied to the soil or not. The content of Cd(0.15mg/kg) in brown rice met the standard with non-biochar when irrigating the tap water(CK).Cd content in brown rice was more than the standard when the water of C1 and C2 was irrigated to the rice potted with non-biochar, which by standard to exceed bid, and which showed that when growing rice in Cd polluted soil in a certain extent, even if the Cd concentration in the irrigation water met the demand of agricultural irrigation, it would happen that the content of Cd in brown rice exceeds the standard.The Cd content in brown rice reduced from 0.15mg/kg> 0.24mg/kg、0.28mg/kg to0.13mg/kg、0.13mg/kg、0.14mg/kg, respectively, after both fall by excessive compliance when the 0.1% biochar amount was applied to the soil in each irrigation, and the reduction was 45.6% and 50.7%. However,the Cd content in brown rice was excessive when biochar amount in soil was 2%, which was more than the content that non-biochar in soil.(5)Analysis of the absorption coefficient and accumulations of Cd showed that the absorption coefficient of Cd was the highest in rice roots and stems.the accumulations of Cd was the most in rice roots,stems and brown rices.The biochar amount of 0.1% could effectively reduce Cd absorption of rice roots and stems, which promoted rice husks absorbing Cd, thereby reducing Cd migration to brown rices.The absorption coefficient and accumulations of Cd in brown rice had no obvious change with the increase of the cadmium concentrations in irrigation water when the biochar amount was 0.1%.However, the absorption coefficient and accumulations of Cd in rice roots,stems,leaves and husks increased with the increase of the Cd concentrations in irrigation water when the biochar amount was 2%, which made Cd content in brown rice exceed the standard.