Effects Of Different Ph Values Of Irrigation Water On Migration And Accumulation Of Cd In Rice Plants

In this study,the effects of different pH values of irrigation water(pH = 3.5±0.05,4.5±0.05,5.5±0.05,6.5±0.05)on two Cd pollution levels of soil basic physical and chemical properties(soil pH values,organic matter,cation exchange capacity),Cd bioavailability in soil(determined by toxicity characteristic leaching procedure(TCLP)and 1.0 mol/L MgCl2 Cd-extracts),and total Cd contents in different rice tissues(roots,stems,husks,brown rice),the migration and accumulation of Cd in rice plants were simultneously investigated.The main results showed that:a)At three rice growth stage(tillering stage,filling stage,and maturity stage),the effects of different pH values of irrigation water on soil pH and soil cation exchange capacity(CEC)were significant,although there were no significant effects on soil organic matter(OM).With the increase of pH values of irrigation water,the soil pH values increased gradually.With the growth of rice,the soil pH values of each treatment showed a decreasing trend.There were no significant differences between the two rice varieties and the two levels of Cd pollution.Compared with no-planted soil,the soil OM of each treatment gradually decreased with the growth of rice.There was no significant difference between the two rice varieties and the two levels of Cd pollution.The soil CEC decreased gradually with the increase of pH values of irrigation water at two levels of Cd pollution for the two rice varieties.Additionally,the soil CEC of each treatment showed a trend of increasing with the growth of rice.The soil Cd bioavailability(exchangeable and TCLP-extricable Cd contents)decreased gradually with the increase of pH values of irrigation water for each treatment.b)The effects of pH values of irrigation water on the total Cd contents in rice root,stem,husk,and brown rice were significant.With increasing of pH values of irrigation water,the total Cd contents in the brown rice were significantly reduced for Xiangwanxian 12,and increased for Weiyou 46.In Cd low-contamination soil,compared with the treatment of pH 3.5,the total Cd contents in brown rice decreased by 4.9%?48.1%(from 0.45 mg/kg to 0.04-0.12 mg/kg)for Xiangwanxian 12,and increased by 65.2%-140.3%(from 0.05 mg/kg to 0.09?0.13 mg/kg)for Weiyou 46 for the treatments of irrigation water with pH 4.5 to pH 6.5.The same case were found in Cd high-contamintaion soil,compared with the treatment of pH 3.5,the total Cd contents in brown rice decreased by 45.6%?73.9%(from 0.45 mg/kg to 0.12?0.25 mg/kg)for Xiangwanxian 12,and increased by 71.0%?177.2%(from 0.16 mg/kg to 0.28?0.45 mg/kg)for Weiyou 46 for the treatments of irrigation water with pH 4.5 to pH 6.5.It was obvious that increasing pH values of irrigation water was benefit to reducing Cd contents in brown rice of Xiangwanxian 12(conventional rice),but was unfavorable for reducing Cd contents in brown rice of Weiyou 46(hybrid rice).c)At the rice mature stage,the Cd accumulated amount,Cd translocation factor(TF),and Cd bioaccumulation factors(BCF)were all reduced gradually with increasing of pH values of irrigation water for Xiangwanxian 12,and increased gradually for Weiyou 46.Therefore,at range of 4.8?5.2 soil pH value,irrigating the water with higher pH would be beneficial in reducing the risk of Cd pollution for Xiangwanxian 12,but would not be effective for Weiyou 46.