Enhanced uptake of soil Pb and Zn by Indian mustard and winter wheat following combined soil application of elemental sulphur and EDTA

Enhancement of Pb and Zn uptake by Indian mustard ( Brassica juncea ( L.) Czern.) and winter wheat ( Triticum aestivum L.) grown for 50 days in pots of contaminated soil was studied with application of elemental sulphur ( S) and EDTA. Sulphur was added to the soil at 5 rates ( 0 - 160 mmol kg(-1)) before planting, and EDTA was added in solution at 4 rates ( 0 - 8 mmol kg(-1)) after 40 days of plant growth. Additional pots were established with the same rates of S and EDTA but without plants to monitor soil pH and CaCl2-extractable heavy metals. The highest application rate of S acidified the soil from pH 7.1 to 6.0. Soil extractable Pb and Zn and shoot uptake of Pb and Zn increased as soil pH decreased. Both S and EDTA increased soil extractable Pb and Zn and shoot Pb and Zn uptake. EDTA was more effective than S in increasing soil extractable Pb and Zn, and the two amendments combined had a synergistic effect, raising extractable Pb to > 1000 and Zn to > 6 times their concentrations in unamended control soil. Wheat had higher shoot yields than Indian mustard and increasing application rates of both S and EDTA reduced the shoot dry matter yields of both plant species to as low as about half those of unamended controls. However, Indian mustard hyperaccumulated Pb in all EDTA treatments tested except the treatment with no S applied, and the maximum shoot Pb concentration was 7100 mg kg(-1) under the highest application rates of S and EDTA combined. Wheat showed similar trends, but hyperaccumulation (1095 mg kg(-1)) occurred only at the highest rates of S and EDTA combined. Similar trends in shoot Zn were found, but with lower concentrations than Pb and far below hyperaccumulation, with maxima of 777 and 480 mg kg(-1) in Indian mustard and wheat. Despite their lower yields, Indian mustard shoots extracted more Pb and Zn from the soil ( up to 4.1 and 0.45 mg pot(-1)) than did winter wheat ( up to 0.72 and 0.28 mg pot(-1)), indicating that the effects of S and EDTA on shoot metal concentration were more important than yield effects in determining rates of metal removal over the growth period of 50 days. Phytoextraction of Pb from this highly contaminated soil would require the growth of Indian mustard for nearly 100 years and is therefore impractical.