Rhizodegradation of diesel and PAH contaminated soils with Miscanthus × giganteus: Soil, plants, microbes and pollutants interactions after two seasons

Miscanthus × giganteus, a high-yielding perennial grass, has recently shown promise for phytomanagement of petroleum hydrocarbons (PHC) contaminated sites, however the mechanisms of plant-soil interactions are not clear. This study followed the second growing season of miscanthus cultivation on soil spiked with representatives of the most common PHC pollutants: diesel (dominantly aliphatic hydrocarbons), pyrene + phenanthrene (polycyclic aromatic hydrocarbons; PAH) and their mixture. Miscanthus demonstrated tolerance to PHC-contaminated soils, although high diesel concentrations significantly reduced biomass, limiting the overall benefits of cultivation. This was evident in reduced carbon sequestration, plant-induced soil respiration and root exudate content in diesel-treated soils. Despite comparable PHC levels in planted and unplanted soils after two seasons, several indicators of ongoing rhizodegradation were observed. These included increased root exudates production, a higher fungal-to-bacterial ratio and, at lower diesel levels, increased abundance of actinobacteria, i.e. shifts towards biodegraders capable of biodegradation of more recalcitrant components of petroleum. A comprehensive analysis revealed significant PHC impacts on soil microbial communities. While biomass and respiration increased, bacterial diversity decreased with increasing diesel concentrations. The microbial community shifted towards potentially PHC-degrading microorganisms, such as fungi and specific bacterial genera (e.g., AlkB gene abundance increased 100-fold). PAH contamination primarily affected the abundance of the pahGP marker, but its overall impact was limited due to low residual PAH levels in the second season. These findings show changing role of M. × giganteus in PHC bioremediation from the support of biodegradation in the first year to stabilization and enrichment of soil in the second.