Soil seed banks in a heavy-metal polluted grassland at Prayon (Belgium)

Seed banks were investigated in a grassland dominated by Agrostis capillaris and Thlaspi caerulescens, established on soils polluted by atmospheric deposits of SO2 and heavy-metal rich dust (Zn, Pb, Cd) from an ore-smelter in Prayon (E Belgium). A total of 155 soil cores (0-7 cm depth) distributed among 10 plots were sampled in November and March. Each sampling plot was characterised for the botanical composition and standing biomass of the established vegetation as well as for four environmental parameters (soil pH and extractable zinc, volume percentage of stones, relative altitude within the site). Very high densities of germinable seeds were found (grand mean +/- SE, pooled over dates and plots: 28,950 +/- 6830 seeds m(-2)). Agrostis capillaris represented 95% of the seeds, followed by Thlaspi caerulescens (3%); the remaining 2% were shared among 24 other species. Two thirds of the seeds were concentrated in the upper 3 cm of the soil. The high seed density might be due to a low rate of microbial decay of seeds due to heavy metal toxicity. The seed bank is almost entirely derived from those few heavy-metal tolerant species that have re-colonised the polluted area in the last three decades. Only two species (Anagallis arvensis and Urtica dioica) not represented in the established vegetation and with seeds concentrated in the lower soil layer are probably derived from seed buried prior to the onset of heavy-metal pollution. This study provides the first data suggesting that the heavy-metal hyperaccumulator Thlaspi caerulescens possesses persistent seed banks. Seed densities of Agrostis capillaris, Thlaspi caerulescens and Rumex acetosa were positively correlated with the standing biomass of grasses and negatively correlated with the volume percentage occupied by stones in the soil. It is argued that small-scale variation in microclimatic factors that cue germination (fluctuations in soil temperature and moisture) can account for the observed pattern of spatial variation in seed density. From the standpoint of restoration management, the results indicate that little change in the floristic composition of the heavy-metal tolerant grasslands of Prayon can be expected from the recruitment of buried seeds.