Is metal extraction by Arabidopsis halleri related to exchangeable metal rates in soils amended with different metal-bearing solids?

Metals are associated to various constituents in polluted soils, and their availability is closely related to their chemical speciation. Studies on relations between metal extraction efficiency by hyperaccumulators and location of metals with respect to soil constituents are scarce. In this study. we investigate the relationship between metal extraction by Arabidopsis halleri and the exchangeable metals from substrates amended with various metal-bearing solids collected in the vicinity of a Zn smelter complex. These consisted of fresh and decomposing organic matter, the soil clay fraction, and two types of waste slags. ZnSO4 was also used as metal-bearing solid. Each was mixed with an unpolluted soil to produce two types of substrate, one moderately polluted and the other highly polluted. Total Zinc, Cd, Cu, and Pb were measured in substrates and in roots and shoots of A. halleri. Analysis of 0.01 M CaCl2 exchangeable metals in each substrate was performed before and after plant growth. The results showed different concentrations of exchangeable metals after plant growth, depending on the nature of the metal-bearing solids. In the ZnSO4 soil substrate, the proportion of exchangeable Zn decreased after plant growth, whilst it increased significantly on substrates amended with the two waste slags. For the other substrates, exchangeable Zn was not significantly different before and after plant growth. The same trend was observed for Cd. In the case of Cu, exchangeable rates increased in all substrates. The results were discussed according to the characteristics of the metal-bearing solids and to the metal-uptake strategy of A. halleri.