Ammonium-based fertilizers enhance Cd accumulation in Carpobrotus rossii grown in two soils differing in pH

Nitrogen fertilization has been shown to improve Cd uptake by plants but there is little information on the effect of N form. This study examined the effects of N form on Cd bioavailability and phytoextraction in two soils differing in pH. Plants of halophytic species Carpobrotus rossii were grown in an acidic Sodosol [pH (CaCl₂) 4.9] and a neutral Vertosol (pH 7.2) spiked with 20 mg kg^-1 Cd as CdCl₂. Three N forms, KNO₃, (NH₄)₂SO₄ and (NH₂)₂CO at a rate of 24 mg N kg^-1 were applied at weekly intervals, together with nitrification inhibitor dicyanodiamide. Cadmium availability was measured, and Cd speciation in the rhizosphere analysed using synchrotron-based X-ray absorption spectroscopy. The uptake, translocation and accumulation of Cd in plants were also assessed. The reduced N forms (NH₄⁺ and urea), compared to NO₃^--N, decreased rhizosphere pH by 0.25 units in Sodosol and 0.72 units in Vertosol, but decreased Cd-phosphate (by 23%) only in the Vertosol. Moreover, the reduced N forms increased the extractable Cd concentration in the rhizosphere of the Vertosol by 92% and of the Sodosol by 14%. They increased root Cd concentration by 70% and Cd uptake per unit root length by 40% in the Vertosol, and increased the translocation of Cd from the roots to the shoots by 76% in the Sodosol. The results suggest that the supply of NH₄⁺-based N favors Cd phytoextraction in C. rossii.