Plant diversity enhances the natural attenuation of polycyclic aromatic compounds (PAHs and oxygenated PAHs OPAHs) in grassland soils

Increasing plant species richness stimulates microbial activity in soil, which might favor biodegradation of polycyclic aromatic compounds (PACs). To explore the relationship between plant community composition and PACs in grassland soils (Fluvisols exposed to an urban atmosphere), we determined the concentrations of 29 polycyclic aromatic hydrocarbons (PAHs) and 15 oxygenated PAHs (OPAHs) in topsoils of 80 plots of a grassland biodiversity experiment. The plots included different levels of plant species richness (1, 2, 4, 8, 16, 60 species) and 1–4 plant functional groups (grasses, small herbs, tall herbs, and legumes) in a randomized block design. The concentrations (ng g−1) of ∑29PAHs and ∑15OPAHs in the soils were 271–2407 and 57–329, respectively. Concentrations of 16 (out of 44) PACs and ∑29PAHs decreased significantly with increasing plant species richness, after accounting for the effects of block and initial soil organic C concentration (ANCOVA, p < 0.05). Microbial turnover as the mechanism underlying this relationship was supported by the findings that (i) the regression of the concentrations of PAH with >4 aromatic rings on plant species richness yielded slopes that were negatively correlated with their octanol-water partitioning coefficients, (ii) two OPAHs accumulated in soils with higher plant species richness, and (iii) higher plant species richness increased four OPAH/parent-PAH ratios. Accordingly, structural equation modeling indicated that the higher concentration of 1,2-acenaphthenequinone (a metabolite of acenaphthene) and the higher 1,2-acenaphthenequinone/acenaphthene and 1-indanone/fluorene ratios in plots with higher plant species richness were partly explained by higher soil microbial biomass on plots with higher plant species richness. We conclude that higher plant species richness can be used to enhance biodegradation of aged PACs in soil. We however caution that OPAHs (some of which are more toxic than their related PAHs) might accumulate in soils during such a plant-assisted remediation process.