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.     

The dynamics of protons,aluminium, and calcium in the rhizosphere of maize cultivated in tropical acid soils: experimental study and modelling

The goals of this work were to understand the dynamics of H⁺, Al and Ca in the rhizosphere of maize cultivated in tropical acid soils, and to evaluate the contribution of the dissolution kinetics of the Al-hydroxides to Al dynamics. The study of the dissolution kinetics was based on a comparison between experimental and simulated data, using a model of the chemical processes in the rhizosphere. Two Oxisols, pH 5.1 and 4.6, and one Ultisol, pH 5.2, were studied. An Al-tolerant maize variety (Zea mays L.) was grown for 14 days on a 3-mm thick soil layer. The composition of the soil and the soil solution, together with the concentration of Al in the roots, were determined throughout the experiment. The results showed that root growth (i) decreased the soil solution pH, up to one pH unit, (ii) increased Al concentration in the soil solution, (iii) increased exchangeable Al, and (iv) decreased exchangeable Ca. Soil solution pH, exchangeable Al, and exchangeable Ca were closely linked. Exchangeable Al increased 1.5 – 3.0 times, due to the dissolution of easily mobilised Al components. In addition, Al accumulation in roots depended mainly on Al in the soil solution. Modelling the interactions between H⁺, Al, and Ca proved that the main factor determining Al in the soil solution was the kinetic reactivity of the easily mobilised Al components. These components, probably poorly crystallised Al-hydroxides, are key players in the functioning of the rhizosphere in tropical acid soils.