Degradation of low molecular weight organic acids complexed with heavy metals in soil

Soils contaminated with heavy metals also contain a number of organic ligands, particularly in the rhizosphere and thus, a fraction of the bioavailable metals in soils likely exists in a complexed form. The presence of soluble, metal-complexing organic ligands can influence the fate and transport of metals as well as mineralisation of organic compounds. Oxalate and citrate complexed with Cd, Cu, Mg, Pb and Zn were used as model metal–organic complexes possibly occurring in heavy metal contaminated soils. Soil respiration was used to determine the biodegradation of these compounds in a clay alkaline soil kept under different management regimes. Overall, the results indicated that metal complexes with citrate generally were more degradable than oxalate–metal complexes. For each organic acid, biodegradability of the metal–organic complexes varied for different metals, following the ranking order: Mg>Zn>Cu≅Pb>Cd. Addition of Cd complexes decreased the soil respiration. These results indicate that the formation of the complexes between heavy metals and low molecular weight organic ligands might affect the soil functionality, especially in the rhizosphere of contaminated soils, as determined by soil respiration.