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 used 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 metallophyte grassland highly polluted by industrial dust fallout. Both litter and soil horizons were sampled. Particle-size and density fractionations were carried out to separate particulate organic matter (POM), i.e. the light fraction >50 μm. 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 grasslands, SOM displayed similar quantities but differed significantly as to its quality and dynamics. The main differences were a lack of incorporation of plant returns in the soil profile and an imbalance of SOM composition towards more POM and less fine humified material. Zn, Pb, and, to a lesser extent, Cd were located mainly in the organic-rich, superficial soil layer. Heavy metal concentrations of POM of different sizes were similar within each horizon. Heavy metal concentrations of total POM increased strongly according to a depth–time scale. Our results suggest a selective decomposition of portions of metallophyte-derived debris with initially low heavy metal concentrations and resistance to biodegradation of those portions with initially high heavy metal concentrations. Such a mechanism may constitute a process of mutual protection, in this soil–plant system, of plant debris towards biodegradation and of heavy metals towards mobility.