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

Seed banks were investigated in a grassland dominated byAgrostis capillaris and Thlaspicaerulescens, established on soils polluted by atmospheric depositsof SO₂ and heavy-metal rich dust (Zn, Pb, Cd) from an ore-smelter inPrayon (E Belgium). A total of 155 soil cores (0–7 depth)distributed among 10 plots were sampled in November and March. Each samplingplot was characterised for the botanical composition and standing biomass of theestablished vegetation as well as for four environmental parameters (soil pH andextractable zinc, volume percentage of stones, relative altitude within thesite). Very high densities of germinable seeds were found (grand mean ±SE, pooled over dates and plots: 28,950 ± 6830 ). Agrostis capillaris represented 95% of theseeds, followed by Thlaspi caerulescens (3%); the remaining2% were shared among 24 other species. Two thirds of the seeds were concentratedin the upper 3 of the soil. The high seed density might be dueto a low rate of microbial decay of seeds due to heavy metal toxicity. The seedbank is almost entirely derived from those few heavy-metal tolerant species thathave 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 thelower soil layer are probably derived from seed buried prior to the onset ofheavy-metal pollution. This study provides the first data suggesting that theheavy-metal hyperaccumulator Thlaspi caerulescens possessespersistent seed banks. Seed densities of Agrostiscapillaris, Thlaspi caerulescens andRumex acetosa were positively correlated with the standingbiomass of grasses and negatively correlated with the volume percentage occupiedby stones in the soil. It is argued that small-scale variation in microclimaticfactors that cue germination (fluctuations in soil temperature and moisture) canaccount for the observed pattern of spatial variation in seed density. From thestandpoint of restoration management, the results indicate that little change inthe floristic composition of the heavy-metal tolerant grasslands of Prayon canbe expected from the recruitment of buried seeds.