Influence of anaerobic digestate and wood ash on phenanthrene bioaccessibility and mineralisation in soil

The environmental persistence and ecotoxicity of polycyclic aromatic hydrocarbons (PAHs) have stimulated considerable interest in understanding their bioaccessibility and biodegradation in soils. Factors such as contact time, organic amendments, and microbial inoculation can influence these processes. This study examined how contact time, bacterial inoculation, and amendments affect the chemical extractability and biodegradation of 9-¹⁴C-phenanthrene in soil. Soils were amended with anaerobic digestate (AD), wood ash (WA), or their combination (AD + WA) and monitored over 90 days. Lower recoveries of 9-¹⁴C-phenanthrene activity were observed in amended soils compared with the control, particularly in AD + WA treatments, with recoveries declining over time. Both dichloromethane (DCM) and hydroxypropyl-β-cyclodextrin (HP-β-CD) extractability decreased with increasing soil - PAH contact time, with a greater reduction in HP-β-CD extractability observed in AD + WA. Inoculated systems exhibited shorter lag phases than uninoculated systems, although bacterial numbers, mineralisation rates, and extents were similar across both inoculation conditions. Lower mineralisation extents occurred in AD + WA under both inoculation conditions. This study provides new insights into how AD, WA, and AD + WA influence PAH bioaccessibility and mineralisation kinetics in soil. The findings indicate that AD and/or WA can stimulate 9-¹⁴C-phenanthrene biodegradation under nutrient-limited conditions, primarily through biostimulation of indigenous microbial communities; however, bioaccessibility rather than nutrient availability or microbial abundance, ultimately constrained degradation endpoints. These results highlight the need for (bio)remediation strategies that enhance contaminant availability, rather than focusing solely on nutrient inputs or microbial abundance.