Zinc mobilisation from a contaminated soil by three genotypes of tobacco as affected by soil and rhizosphere pH

The aim of this research was to evaluate the effect of soil and rhizosphere pH on the mobilisation of Zn by various tobacco genotypes. One-month-old tobacco plants were grown for 8 days on top of a thin layer of an arable soil that had been sampled near a Zn smelter. A range of rhizosphere pH values was obtained either by growing nitrate-fed tobacco on top of the soil amended with various amounts of acid or lime, or by growing tobacco on top of the unamended soil with nitrate or ammonium supply. In the latter case, we used three genotypes that were assumed to differ in their ability to accumulate Zn or acidify the rhizosphere and, hence, mobilise soil Zn. In spite of the moderate level of contamination of the soil, tobacco took up substantial amounts of soil Zn. No difference was found between the three genotypes. Exchangeable Zn steeply increased with decreasing soil pH, which could be adequately modelled with a simple model. Whatever the source of nitrogen supplied, a significant acidification occurred in the rhizosphere. This explains why the observed Zn mobilisation was larger than expected on the basis of bulk soil pH values. Taking account of the change of pH induced by tobacco roots is thus of prime importance for better predicting the actual amount of exchangeable Zn in the rhizosphere and, thereafter the bioavailability of soil Zn.