Immobilization Of Cd In A Paddy Soil Under Different Agronomy

As the development of industrialization and urbanization, heavy metal Cd pollution of paddy soilshave become increasingly prominent, which has posed potential hazard to ecosystem and human health.In situ chemical stabilization process for contaminated soils has been gaining prominence for itscost-effectiveness and rapid implementation as well as its appearance as a alternative technique for awide range of polluted sites. Furthermore, different agronomy such as moisture management, varietyscreening and foliar microelement fertilizer spraying, had important influences on soil Cd uptake by riceplant. In this study, immobilization of Cd in paddy soils using different agronomy and mechanism werediscussed. The main results as follows:Firstly, under no amendments(sepiolite combined with phosphate fertilizer, biochar with organicfertilizer, and bentonite), Cd in brown rice in the continuous flooding reduced by 37.9% and 52.6%compared to conventional and wetting irrigation treatments; and paddy rice biomass in the conventionalirrigation was 1.1 and 1.3 times of continuous flooding and wetting irrigation treatments. In contrast toconventional irrigation and wetting irrigation, under no amendments, gradual increase of Fe2+ etc.reduced form bivalent ions, and p H value in continuous flooding soil promoted the transition of soil Cdchemical fraction from exchangeable to Fe/Mn oxides bound, and the competition for adsorption sites inroot coating of rice plant between Cd2+and Fe2+ etc. bivalent ions, which were reasons for lower Cdconcentration in plant in the continuous flooding soil treatment, brown rice Cd(all lower than 0.2mg?kg-1) significantly decreased after applying amendments to soil, the lower availability of Cd in soilwas main reason for lower Cd concentration in plant in amended soils.Secondly, available Cd in soil and brown rice Cd significantly decreased, and paddy rice biomassincreased after applying different additive amounts of palygorskite and humus acid to paddy soil,presenting positive correlations with additive amount of amendments. The remediation efficiency ofcontaminated soil treated by palygorskite was superior to that of contaminated soil treated by humusacid under the same additive amount. In the control group, brown rice Cd of the variety TY 272 was1.45 and 1.90 times of the variety FY 9 and JD 78. As for FY 9 and JD 78, brown rice Cd(FW) in 1.5%palygorskite soil treatment were lower than 0.2 mg?kg-1(Maximum permit concentration for rice Cd inChina, GB 2175-2005), and brown rice Cd of TY 272 was lower than 0.4 mg?kg-1(Codex AlimentariusCommission, Codex Stan 1993-1995).Thirdly, the growth status of rice plant improved markedly after spraying of micronutrient fertilizer(Si, Se). In contrast to control group, the biomass of paddy rice and rice straw significantly increased,and amount of increase in paddy rice production was better in the heading stage than in the tilleringstage. The brown rice Cd decreased significantly after application of trace-element fertilizer to foliage,showing negative correlation with spraying concentration of micronutrient fertilizer. The amount ofdecrease in brown rice Cd was better in tillering stage than in heading stage, and brown rice Cd was thelowest in tillering plus heading stage treatment.Finally, successive two-year pot experiment showed that, in compared to first-year experiment, thebrown rice Cd increased as a whole in second-year experiment with a 10% increasing amplitude onaverage. The brown rice Cd were higher than 0.2 mg?kg-1 in amended soils with wetting irrigationmanagement in second-year experiment, and paddy rice Cd were still lower than 0.2 mg?kg-1 inamended soils with continuous flooding and conventional irrigation management.In conclusion, immobilization remediation efficiency for Cd-contaminated paddy soils weredistinct remarkably among different agronomic measures. Chemical immobilization process combinedwith rational agronomy could produce the paddy rice meeting National Standard of Food Hygiene forCd, alleviating the adverse influences of long-term application of chemical amendments on soilecosystem.