Arsenic Phyto-Toxic Threshold and Influence of Free Iron in the Acid Soils of Southern China.

The disparity of soil arsenic (As) phyto-toxicity was studied in greenhouse with nine soils with different properties collected from Fujian province, China, using rice as an indicator plant. The EC₁₀ (NaH₂PO₄-As), defined as effective concentration of soil As estimated by NaH₂PO₄-extraction method that results in a 10% inhibition, of rice shoot (fresh weight) and shoot height ranged from 3.22 to 27.96 mg·kg^-1 and 3.72 to 29.11 mg·kg^-1, representing 8.68 and 7.83-fold variations among the different soils. The EC₂₀ (NaH₂PO₄-As) of rice shoot (fresh weight) and shoot height varied between 5.83 and 38.87 mg·kg^-1, 7.12 and 45.60 mg·kg^-1, representing 6.67 and 6.70-fold variations among the different soils. Stepwise regression analysis indicated that free iron was the major factor that affects plant As bioavailability in the soils. The modified prediction models of the EC_X (shoot height) of soil As phyto-toxicity as NaH₂PO₄-As were:

                                                                                            [EC₁₀] = -1.451 +1.160 [free iron]       ( r ² = 0.718, P = 0.004 , n=9 )    

                                                                                            [EC₂₀] = 1.659 +1.791 [free iron]        ( r ² = 0.764, P = 0.002 , n=9 )     

                                                                                            [EC₅₀] = 16.367 +2.715[free iron]       ( r ² = 0.827, P < 0.001, n=9 )

    These models have potential application in the prediction and risk assessment of As phyto-toxicity in the acid soil regions.

Key word: Arsenic; acidic soil; phyto-toxicity; threshold; soil properties