Effects Of Biochar On Soil Cd Fractions And Cd Accumulation In Rice

Cadmium(Cd),as a heavy metal,has strong toxicity to human body.China's recent official soil pollution investigation report showed that,nearly 20%of the cultivated land had been contaminated,and most of which were inorganic pollution,the excess rate of Cd was the highest among above inorganic pollutants,and the rate was much higher than other elements.Soil Cd pollution is most harmful to rice in bulk crops,in recent years,the food safety issue of Cd exceeded rice have been a strong social concern.Biochar has a variety of environmental functional effects,such as improve soil quality,increase crop yields,and reduce greenhouse gas emissions,and biochar is being widely concerned in studies of repairing contaminated soil,because of its high porosity,specific surface area,pH and other physical and chemical properties.The practical problem,how to reduce the soil Cd bioavailability and Cd uptake by rice,is the starting point of this study,and the main study purpose is to clarify the effects of biochar on Cd behaviors in "soil-rice" system.Biochar application was tested as a soil conditioner to immobilize heavy metal,and three test methods were used,including culture experiment,field experiment and rhizosphere simulation experiment,effects of biochar on the bioavailability and fractions of Cd in soil and the Cd content and accumulation in different organs of rice were studied emphatically,finally to discuss the potential,mechanism and method of biochar application to Cd contaminated soil remediation.In the culture experiment,6 different types of soil were used to simulate Cd contaminated soil,the effects of biochar on the soil Cd bioavailability and Cd content of rice seedlings were studied under different biochar application rates(0,1,5%)and different culture conditions(culture times,with rice seedling or not),5 extraction methods(HCl,Mehlich 3,DTPA,CaCl2,TCLP)were used to test the soil Cd bioavailability.In field experiment,a paddy Cd contaminated field due to sewage irrigation in the suburbs of Shenyang was selected as the test site,and treated with different biochar applications(0,0.5,1.0,2.0 t/mu,and 0.2 t/mu·year).The Cd fractions and main physical and chemical properties of soil and Cd contents and accumulations of various rice organs were studied for 3 years.In rhizosphere simulation experiment,a rhizobox was used to cultivate rice and collect rhizosphere soil samples,this rhizobox could divide the closest 1-5mm rice rhizosphere soil into five 1mm thick layers,and soil layers could be removed and analyzed separately,the effects of biochar on available Cd content in different rhizosphere soil layers and Cd uptake by rice were studied in this part.The main results are as follows:under conditions of cultivation,biochar significantly reduced soil availability Cd contents and Cd contents of rice seedlings in 6 types of soil;the extraction rates of different extractions were not the same,TCLP was the lowest,CaCl2 was the second lowest,Mehlich 3 and DTPA were relatively high,HCl was generally the highest;the extraction results of Mehlich 3 and DTPA showed better correlations with Cd contents of rice seedlings,and DTPA was considered as a recommended method to analyze the changes of soil available Cd content under the biochar addition.Under field conditions,there was no significant effect oi different biochar application treatments on the dry matter accumulations of various organs of rice for three years;the high level treatments of biochar application significantly reduced the Cd contents of brown rice,root,stem and sheath,but had no significant effect on Cd contents in leaf and husk;biochar significantly reduced Cd accumulation in several organs of rice in the second year of this experiment;biochar treatments had no significant effect on Cd transfer coefficient and Cd distribution coefficient,but could significantly reduce the Cd enrichment coefficient of rice from soil;the medium and high level biochar application treatments could significantly reduce the Cd content of exchangeable fraction and increase the Cd content of Fe-Mn oxide bound fraction,the content of organic bound fraction was increased slightly in the last year of the experiment,there was no significant effect on the carbonate-bound and residual fractions;biochar could increase soil pH and SOC significantly.In the rhizosphere experiment,biochar had no significant effect on the dry matter accumulation of various rice organs,but the Cd contents in the stem,sheath and brown rice were significantly decreased,and biochar significantly reduced the Cd content and dry matter accumulation of total above-ground rice.Biochar reduced the available Cd content in the rhizosphere soil,and biochar application addition reduced the available range of Cd absorbed by rice root in the soil.