Derivation of reliable empirical models describing lead transfer from metal-polluted soils to radish (Raphanus sativa L.): Determining factors and soil criteria

Abstract

Reliable models describing Pb transfer from soils to food crops are useful in the improvement of soil protection guidelines. This study provides mechanistic insights from in-situ soil solution measurement on the Pb uptake in the root tissues (RF) of radish, grown in 25 representative Pb-contaminated agricultural soils. Lead speciation and regression analysis indicate that >88.6% of the variation in RF Pb is attributable to free Pb²⁺ activity (aPb²⁺) in the soil solution, which is predominantly controlled by pH and DOC. Higher DOC would increase the total dissolved Pb (C_Sol-Pb) in the soil solution but reduce the bioavailability of Pb to radish. C_Sol-Pb performs poorly in predicting RF Pb unless pH and DOC are included. However, 0.01M CaCl₂ extractable Pb (C_CC-Pb) alone can satisfactorily predict RF Pb, attributable to the fact that C_CC-Pb is consistent with aPb²⁺. C_CC-Pb can be predicted using C_Sol-Pb and pH. Total soil Pb (C_T-Pb), or 0.43M HNO₃ extractable Pb (C_NA-Pb) has a strong, non-linear correlation with C_Sol-Pb or C_CC-Pb and it is therefore not surprising that C_T-Pb or C_NA-Pb, together with pH and CEC, can also satisfactorily predict RF Pb. Derived models are effective in identification of soils where RF Pb exceeds the food quality standard (FQS). Soil Pb criteria based on C_T-Pb, C_NA-Pb and C_CC-Pb are derived by inverse use of empirical models. The derived Pb criterion (target value) based on C_CC-Pb is 0.02mgkg^-1 and the stricter criterion (safe value) is 0.01mgkg^-1, which allows a 5% probability for RF Pb to exceed FQS. Safe values based on C_T-Pb and C_NA-Pb ranged from 26 to 1036mgkg^-1 and 9 to 745mgkg^-1, respectively.

Keywords: Bioavailability; Dissolved organic matter; Peeled radish; Smelting soils; Soil solution.