Effects Of Organic Materials On Iron Morphology In Paddy Soil And Cadmium Absorption In Rice

Rice is the largest grain crop in China.It has a strong ability to enrich cadmium.The contamination of paddy soil and rice cadmium has attracted much attention.The uptake of cadmium by rice is affected by soil redox potential（Eh）,pH,organic matter and iron oxides.Iron is an essential element for rice growth.It can affect the effectiveness of Cd by its competitive relationship with cadmium and its morphological changes,or by forming iron film on the root surface to affect Cd uptake and transport in rice.Iron oxides are the most active minerals in soil.They have large specific surface area and surface activity,and have strong adsorptive effect on cadmium.However,different iron oxides have different adsorptive capacity for cadmium due to different crystallinity and surface active adsorption sites.Therefore,the redistribution of Fe forms will determine the redistribution of cadmium forms.The form and content of iron in soil are susceptible to environmental conditions.When organic materials are applied to soil,the physical and chemical properties of soil change,which will affect the bioavailability of Cd and also affect Fe.It is not clear whether the effect of organic materials on Cd availability is related to the change of redox potential of paddy soils and the interaction of iron with cadmium.In this study,different amounts of straw（S）,humus soil（H）,farm manure（FM）and particulate organic matter（POM）were added to Acidic Purple Paddy soil.The changes of pH,Eh,dissolved organic matter（DOC）,free iron（Fed）,amorphous iron（Feo）and complex iron（Fep）in paddy soils were determined.The results showed that paddy soil with different amounts of organic materials had different pH,Eh,dissolved organic matter（DOC）,iron forms.Coupling law of Cd change with Fe morphology and DOC change.A pot experiment was conducted to determine Cd form,rice biomass and Cd content,Fe form and available Fe content in soils at maturity stage of rice by adding4 kinds of organic materials:straw（S）,humus soil（HS）,pig manure compost（PM）and particulate organic matter（POM）to acidic,calcareous purple paddy soils（APS,CPS）and yellow loam paddy soils（YS）at 5 gC/kg.The effects of organic materials on soil Cd availability and rice Cd uptake were studied.The conclusions are as follows:（1）The addition of organic materials increased the pH and pe+pH of paddy soil,intensified the reduction potential and increased the DOC content;the more the organic materials were added,the more significant the effect was.Organic materials reduced Fed content,increased Feo,Fep content and Feo/Fed content;Fed and Feo content increased overall with the increase of the amount added,while Feo/Fed and Fep content decreased with the increase of the amount added.The content of available Cd in paddy soils decreased significantly at the later stage of incubation,which was negatively correlated with DOC and pH,and positively correlated with Eh and pe+pH.The content of available Cd was negatively correlated with amorphous iron oxide at 35,77 and 120days,negatively correlated with activation at 7,35 and 120 days,and negatively correlated with complex iron at 14,35,49,98 and 120 days.（2）Organic materials increased soil pH and decreased available Fe and Fe²⁺contents during rice maturity,which may be related to the adsorption and absorption of active iron by plants.Except for Fed in YS,organic materials increase the contents of Fed,Feo and Fep,and iron oxides are negatively correlated with pH value.The effects of different organic materials on effective Fe,Fe²⁺and iron oxides are similar in APS and YS,but opposite in CPS.Organic materials increased Fe content in APS rice plants,decreased CPS,decreased root and stem in YS,and increased grain.（3）Three iron oxides are the main factors affecting the occurrence of Cd in paddy soils.Fep in APS and YS is negatively correlated with available Cd content,but in CPS is positively correlated with exchangeable Cd content and negatively correlated with organic bound Cd content.Organic materials reduced the effective Cd content in paddy soil,increased the Cd content in iron film,and decreased the Cd content in rice plants.Although the Cd content in iron film was positively correlated with the Cd content in roots,it was negatively correlated with the Cd content in stems,leaves and grains.Organic materials inhibited the transport of roots to stems and leaves.More Cd was still concentrated in iron film on the surface of roots,although it promoted the Cd absorption in roots.But it was not transported to the above ground part.