Influence Of Submergence And Organic Matter Addition On The Variation Of Cadmium Activity In Soils

Cadmium activity in soil may increase or decrease upon submergence and organic matter addition.Based on the literature review we posed questions to CdS precipitation as the mechanism of controlling Cd activity in flooded rice paddy soils.In this thesis we propose a hypothesis that the redistribution of Cd fractions was controlled by the transformation of iron oxides during the reductive process after submergence.This mechanism can be characterised with the parameter of pe+pH.To prove this,a red soil (pH4.46) and a fluvo-aquic soil(pH7.10) were chosen to be incubated under nitrogen coverage and constant temperature(25±0.5℃).Three treatments,including control(no addition of organic materials),clover blossom and straw addition,were set to determine soil Eh,pH,and Fe/Cd fractionation.We studied the relations between Cd activity and Eh,pH, DOM,iron oxides according to the principles of chemical and electrochemical equilibrium to investigate the relationship between transformation of Fe/Cd components and variation of pe+pH in soils after submergence and organic matter addition.The experimental results, which were summarized below,revealed the relationship between Cd behaviors and the redox state in soil under submerged condition.1.The pH of the red soil rose to 5.92 after 60 days of submergence.For the ftuvo-aquic soil,it dropped to 6.62 after 3 days and then increased to 6.89.The pe+pH of two soils descended from 14.16 to 5.38.The variation of pH and pe+pH caused increase of the redistribution ratio of Exe-Fe,Carb-Fe,Oxide-Fe,Org-Fe in soil solid phase,and transformation of Exc-Cd to Carb-Cd,Oxide-Cd and Org-Cd.The negative correlations were significant or highly significant between Exc-Cd and Exc-Fe,between Exc-Cd and (Carb-Cd+Oxide-Cd+Org-Cd) or(Carb-Fe+Oxide-Fe+Org-Fe).This demonstrates that Fe redistribution controlled Cd redistribution.We also put forward a preliminary range of pe+pH and pH for the redistribution of Cd fractions.2.The variation ofpe+pH and pH affected the concentration of Fe/Cd in solution phase. During the 1-7 days after submergence in the red soil,Exc-Cd was the dominated forms in soild phase,and the concentration of Cd in solution phase increased with the decrease of pe+pH.In the later periods for the red soil and the whole incubation for the fluvo-aquic soil, Carb-Cd and Oxide-Cd were the dominated forms in soild phase,and the concentration of Cd in solution phase decreased with the decrease of pe+pH.3.The change of dissolved organic matter(DOM) concentration following submergence did not significantly affect soluble Cd concentration.4.Compared to the control group,the average pH of the red soil with clover blossom and straw addition increased respectively 0.39 and 0.34,the average pe+pH decreased 2.12 and 2.52,and the average DOM concentration was 19.1 and 3.4 times higher.For the fluvo-aquic soil,the average pH with clover blossom and straw addition decreased 0.27 and 0.11,the average pe+pH decreased 0.85 and 0.65,and the average DOM concentration was 3.6 and 1.1 times higher respectively.The changes of pH and pe+pH were the result of multiple chemical and electrochemical reactions between different Fe minerals in soil and organic matter.Iron redistribution controlled Cd redistribution.These reactions caused pe+pH declined and pH,Exc-Fe,Oxide-Fe increased for the red soils added with organic matter,and the average Exc-Cd with the clover and straw treatments respectively decline 0.101mg·1^-1 and 0.098mg·1^-1.The trend for the fluvo-aquic soil was opposite.Compare to the control group,the pe+pH,pH,Exc-Fe decreased and Carb-Fe increased when adding organic matter,and the average Exc-Cd with the clover and straw treatments respectively increased 0.441mg·1^-1 and 0.10mg·1^-1.5.Compare to the control group,the concentration of Cd²⁺ in soil solution of the organic matter-treated soils was more clearly affected by complexation of DOM and diffusion of Exc-Cd.6.The difference in composition of organic materials directly affected the content of Exc-Cd in solid phase and the variation of Cd²⁺ concentration.