Influence of chelating agent addition on copper distribution and microbial activity in soil and copper uptake by brown mustard (Brassica juncea)

A pot culture experiment was conducted using a Cu-spiked soil to compare the effects of the application of ethylenediamine-N, N'-tetraacetate (EDTA) and [S, S]-ethylenediamine-N, N'-disuccinate (EDDS), a biodegradable chelating agent, on the soil Cu distribution, Cu. uptake by brown mustard and on the metabolic activity of the soil microorganisms for substrate utilization. Seedlings of brown mustard were planted in the pots fitted with hollow fibers for soil solution suction, and cultivated for 12 weeks. Solutions of chelating agents with pH 6.0 at a concentration of 1.0 mmol kg(-1) soil were supplied on the 35, 43, and 51 d after transplanting (total 3.0 mmol kg(-1)). The addition of EDTA and EDDS enhanced plant Cu uptake by 2.9 and 1.8 times compared with the uptake of the control plants which were grown without chelating agent supply. However, EDTA addition caused a significant growth reduction compared with the control, while the application of EDDS did not reduce the dry matter yield of the plants. The concentrations of Cu. and chelating agent in the soil solution of the EDTA-amended soil were much higher than those in the EDDS treatment during the cultivation, especially at the late stage. The results of sequential extraction of Cu in soil after cultivation showed that the addition of both EDTA and EDDS decreased the amount of Cu in the Mn oxide-occluded fraction and increased the amount in the water-soluble and exchangeable fractions significantly, compared with the control soil. The amount of Cu. in the acid-soluble and organically bound fractions of the soil supplied with both chelating agents was also lower than that in the control soil, although the value was not significant for the EDDS treatment. Based on the values of the average well color development (AWCD) on the Biolog ECO MicroPlates, the activity of the soil microorganisms for substrate utilization in the EDTA treatment after cultivation was lower than that in the EDDS treatment. These results suggested that the environmental risks caused by the leaching of Cu. and Cu-chelate and the influence on soil microbial activity were less pronounced in the case of EDDS addition than EDTA. Therefore EDDS could replace EDTA for chelate-assisted phytoremediation of soils contaminated with Cu, although the amount of Cu taken up by the plants was lower than that required for efficient reduction of the soil Cu content.