Effect of nitrogen form on the rhizosphere dynamics and uptake of cadmium and zinc by the hyperaccumulator Thlaspi caerulescens

Abstract

The effect of N form (NO3− versus NH4+) on growth and uptake of Cd and Zn by the hyperaccumulator Thlaspi caerulescens (Ganges ecotype) was investigated in hydroponic and rhizobox experiments. In the hydroponic experiments, NO3− or NH4+ was supplied to plants with the pH of the nutrient solution being unbuffered or buffered at around 6.0. A moderately contaminated soil was used in the rhizobox experiment with or without additions of NO3−, NH4+ or NH4+ + DCD (dicyanodiamide, a nitrification inhibitor). A higher biomass was obtained when N was supplied as NO3− in both experiments, indicating that T. caerulescens prefers NO3− over NH4+. In the hydroponic experiments, supplying NO3− resulted in a doubling of Cd concentration in the shoots compared with the NH4+ treatment, regardless whether solution pH was buffered or not. The form of N also had a noticeable effect on root Zn concentrations. In the rhizosphere box experiment, rhizosphere pH was markedly influenced by the N treatment. The acidification in the NH4+ and NH4+ + DCD treatments increased the concentrations of extractable Cd and Zn, both of which showed a considerable depletion in the rhizosphere. However, total uptake of Cd and Zn were highest in the NO3− treatment, despite the fact that concentrations of extractable Cd and Zn in the rhizosphere were the lowest in this treatment. The results showed that supplying N as NO3− promoted growth and phytoextraction of Cd and Zn by T. caerulescens compared with NH4+.