Contrasting effects of species richness on soil pollutant removal in herbaceous plant communities: the importance of individual species

Many studies have shown that increasing species diversity generally increases ecosystem functioning, but few have evaluated the role of species diversity in soil pollutant removal, especially for organic pollutants. We test the hypothesis that increasing plant diversity can enhance soil pollutant removal. To test the hypothesis, we conducted two experiments: one for removal of cadmium and the other for removal of cypermethrin from the soil. In both experiments, plants were grown in monocultures, four- and eight-species mixtures in soils contaminated with the pollutants. Contaminated soils without plants were also included as controls. Our results showed that the efficiency of cadmium removal from the soil decreased with increasing plant species richness. The negative effect of diversity on cadmium removal was largely attributed to species underyielding in plant mixtures. In contrast, the efficiency of cypermethrin removal from soils increased with increasing plant species richness. The positive effect of diversity on cypermethrin removal was likely due to the presence of Taraxacum mongolicum which had the lowest biomass but the highest removal efficiency. Our results suggest that species diversity may regulate pollutant remediation, and this is pollutant-dependent. Moreover, the performance of individual species can strongly influence the direction and magnitude of the diversity effect on soil pollutant removal. These species should be treated with cautious in the remediation of contaminated soils.