Effects of humic and fulvic acids on the rhizotoxicity of lanthanum and aluminium to corn

Abstract

Effects of varying lanthanum (La) or aluminium (Al) concentrations (0-30 µM) on corn (Zea mays L.) root elongation were examined in the presence and absence of (i) humic acid (HA) at 35 mg carbon (C)/L, or (ii) fulvic acid (FA) at 15 mg C/L, using dilute nutrient solutions. The organic acids were extracted from a mixture of decomposed grass (Sorghum halepense) and lucerne (Medicago sativa) hay. In the absence of added HA or FA, the addition of La at ≥5 µM and Al at 30 µM was toxic to the root growth of corn. The rhizotoxic effects of La at 5 and 10 µM were negated by HA. The ability of FA to overcome La rhizotoxicity was much less, significantly ameliorating the toxic effects of 5 µM La but not those of 10 or 30 µM La. HA and FA did not precipitate La from solution. Both organic acids ameliorated Al toxicity by complexing Al and reducing monomeric Al in solution. It is concluded that concentrations of HA and FA, commonly present in soil solutions, are capable of forming non-rhizotoxic complexes with La, hence plant tolerance to La in the soil solution may be appreciably higher than would be indicated by results of solution culture experiments in which these ligands are not present.