Influence of increased salt contents in compost-peat substrates on the phytoavailability of Cd, Cu, Zn, and Mn

Heavy metal phytoavailability might be increased by high salt contents in the substrate. The relevance of this factor was investigated by growing Petunia X hybrida in compost-pear substrates with increased Cd (3, 10 mg kg(-1)), Cu (200, 1000 mg kg(-1)), Zn (800, 1600 mg kg(-1)), and Mn (1500, 3000 mg kg(-1)) contents tall in aqua regia) and addition of KCl or MgCl2 (300 mg Cl- l(-1) substrate). Plant dry matter yield was significantly reduced by Zn, Mn, and Cu supply, whereas no dry matter reduction was observed for the Cd treatment or as a result of KCl or MgCl2 addition. Petunias showed Cd, Cu, Zn, and Mn induced Fe deficiency chlorosis, which was increased due to KCl or MgCl2 addition in the case of Zn and Mn treatments, with an indication of a higher increase under MgCl2 addition than under KCl addition. Higher heavy metal phytoavailability was reflected by increased plant Cd, Cu, Zn, and Mn contents under KCl or MgCl2 treatment. We hypothesized that Cd, Cu, Zn, and Mn phytoavailability was higher due to increased exchange of adsorbed heavy metals by the cations of the salts, For Cd in addition the formation of soluble chloro-Cd-complexes might have increased Cd phytoavailability. The increase in phytoavailability was not reflected by the amount of extractable (phytoavailable) Zn (0.1 M CaCl2), Cu (0.01 M CaCl2 + 0.002 M DTPA), and Mn (1 M NH4OAc) contents of the compost-pear substrates. This might be attributed to the high salt concentrations in the extraction solutions that simulate heavy metal phytoavailability at high salt concentrations.