Mutual effects of soil organic matter dynamics and heavy metals fate in a metallophyte grassland

Metallophytes, plant species that grow only on soils rich in metals, are us ed for bioremediation of polluted soils. However, little is known about the functioning of such soil-plant systems. We have investigated soil organic matter (SOM) dynamics and its effects on the fate of heavy metals under a m etallophyte grassland highly polluted by industrial dust fallout. Both litt er and soil horizons were sampled. Particle-size and density fractionations were carried out to separate particulate organic matter (POM), i.e. the li ght fraction > 50 mu m. All samples were analysed for C, N, Zn, Pb, and Cd. Bulk densities were determined for all horizons and stocks of elements per unit area were calculated. Compared to broad uncontaminated temperate gras slands, SOM displayed similar quantities but differed significantly as to i ts quality and dynamics. The main differences were a lack of incorporation of plant returns in the soil profile and an imbalance of SOM composition to wards more POM and less fine humified material. Zn, Pb, and, to a lesser ex tent, Cd were located mainly in the organic-rich, superficial soil layer. H eavy metal concentrations of POM of different sizes were similar within eac h horizon. Heavy metal concentrations of total POM increased strongly accor ding to a depth-time scale. Our results suggest a selective decomposition o f portions of metallophyte-derived debris with initially low heavy metal co ncentrations and resistance to biodegradation of those portions with initia lly high heavy metal concentrations. Such a mechanism may constitute a proc ess of mutual protection, in this soil-plant system, of plant debris toward s biodegradation and of heavy metals towards mobility. (C) 1999 Elsevier Sc ience Ltd. All rights reserved.