Long-term distribution, mobility and plant availability of compost-derived heavy metals in a landfill covering soil

The application of municipal waste compost (MWC) and other organic materials may serve to enhance soil fertility of earthen materials and mine spoils used in land reclamation activities, particularly in the recovery of degraded areas left by exhausted quarries, mines and landfill sites among others. The long-term distribution, mobility and phytoavailability of heavy metals in such anthropogenic soils were studied by collecting soil samples at different depths over a 10 y chronosequence subsequent to amendment of the top layer of a landfill covering soil with a single dose of mechanically-separated MWC. Amendment resulted in a significant enhancement of the metal loadings in the amended topsoils particularly for Cu, Zn and Pb, which were also the predominant metals in the compost utilised. Although metals were predominantly retained in the compost amended soil horizon, with time their vertical distribution resulted in a moderate enrichment of the underlying mineral horizons, not directly influenced by compost amendment. This enrichment generally resulted from the leaching of soluble organo-metal complexes and subsequent adsorption to mineral horizons. However, in the course of the 10-y experimental period, metal concentrations in the underlying horizons generally returned to background concentrations suggesting a potential loss of metals from the soil system. Analysis of the tissues of plants growing spontaneously on the landfill site suggests that metal phytoavailability was limited and generally species-dependent.