Effect of rice variety on zinc, cadmium, iron, and manganese content in rhizosphere and non‐rhizosphere soil fractions

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Abstract
In research on selecting rice (Oryza sativa L.) cultivars that are resistant to metal toxicity, an experiment was carried out with the objective of determining the effects of rice cultivars with differing oxidizing powers on the forms of metals in the rhizosphere soil. Four rice varieties were grown for 40 d in 3 soils which had differing amounts of iron (Fe) and manganese (Mn) oxides. The rhizosphere soil was kept separate from the non‐rhizosphere using nylon mesh “rhizobags”;. The soils were treated with either 5 mg/kg cadmium (Cd) or 20 mg/kg zinc (Zn). At the end of the experiment the soils were analyzed using a sequential fractionation scheme for five forms of Cd, Zn, Fe, and Mn. The rhizosphere soil was lower in pH (p=0.14) and P (p=0.05) than the non‐rhizosphere soil. Iron in the Mn oxide fraction was ten times higher in the rhizosphere as opposed to the non‐rhizosphere soil while the rhizosphere soil Mn was lower in this fraction. Cadmium in the rhizosphere soil was increased in both the exchangeable and organic matter fractions. Rice with higher oxidizing power decreased the Mn in the Mn oxide fraction, since the Mn reduced by submergence was oxidized by the oxygen supplied by the roots. Rhizosphere soil Zn increased in the Mn oxide fraction indicating that it may have been adsorbed or occluded by the oxides freshly precipitated by the roots. This increase coincided with a decrease of Zn in the exchangeable fraction. The increase in Zn in the Mn oxide fraction coupled with a decrease in more plant‐available fractions indicated that rice cultivars with high oxidizing power can alter the rhizosphere forms of metals so as to decrease their plant availability.