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.
Keywords: Cd speciation; Cd translocation; Halophytes; N form; Rhizosphere pH; XANES.
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