Reactivity of added humic substances towards plant available heavy metals in soils

Abstract

The objective of this work was to study the capacity of humic substances added to soils to influence the availability of heavy metals to plants. A solution at pH=7 of a purified and characterized Leonardite extracted humic acid was added to an organic and a mineral soil in order to enhance the soils organic carbon content by 1 and 2%. The treated samples were further added with solutions of heavy metals (Cu, Pb, Cd, Zn, Ni) to reach a concentration for each metal of 0, 20, and 50 ug/g. Soils were allowed two periods of incubation of 1 and 2 months and then extracted with, 1: 2.5% acetic acid for readily soluble and easily exchangeable forms of metals; 2: DTPA (Diethylenetriaminepentaacetic acid) solution at pH=7.3 for the organic matter (OM) complexed metals; 3: 1 N HNO 3 for metals present in carbonates, phosphates, and oxides. Results showed that addition of humic substances efficiently immobilize the soluble and exchangeable forms of all metals and to a larger extent in the mineral soil. Retention capacity was directly related to the amount of added humic substances and increased with incubation time. Metal forms extracted with DTPA showed that the amount of soil-borne OM determined metals availability. When the latter was present, its interaction with added humic materials concurred in reducing metals extractability. Conversely, in the soil low in OM, freshly added humic substances bound metals in readily accessible complexes which were then displaced by DTPA, thereby enhancing their availability. Extraction with 1 N HNO 3 followed the same mechanism. Effectiveness in decreasing metals extractibility was of the same order found for stability constants of humic substances: Pb Cu Cd Ni Zn. These results may have practical implications in soil management for the reduction of metal toxicity to plants and of watertable contamination risks.