Comparative performance of lignin-based slow-release biostimulants in remediating PAH-contaminated soils

PET-based lignin biostimulants removed up to 76.0% of soil PAHs. • SRB achieved 65.8% removal of recalcitrant HMW-PAHs. • Moderate nitrogen pulses enhanced PAH removal more than release stability. • PET treatments enriched Mycobacterium and increased bacterial diversity. • SRR increased positive bacteria-bacteria interactions to 39.2%. Polycyclic aromatic hydrocarbons (PAHs) pose a major challenge for soil remediation owing to their structural recalcitrance, and conventional bioremediation is often ineffective against high-molecular-weight PAHs (HMW-PAHs). In this study, two lignin-based slow-release biostimulant strategies were compared for the remediation of PAH-contaminated soil collected from a coking site over 120 days: physically encapsulated materials, including lignin-biochar (SRB) and lignin-resin (SRR) coated fertilizers, and a chemically synthesized aminated lignin material (CSK). All biostimulants were applied at an equivalent lignin input of 1% of soil dry weight. These biostimulants were designed to enhance PAH degradation by regulating nutrient availability and stimulating PAH-degrading microbial communities, while lignin may additionally contribute substrates associated with co-metabolic processes. The materials were characterized by SEM, FTIR, XPS, TGA/DSC, 1 H NMR, and elemental analysis, confirming their distinct structural features and successful modification. Nutrient-release assays showed that the physically encapsulated materials released 26.2% and 24.4% of their total nitrogen by day 5 for SRB and SRR, respectively, followed by continued release to 61.5% and 57.8% by day 25. In contrast, CSK exhibited the strongest slow-release capacity, with cumulative nitrogen release of 11.2% by day 5 and 46.1% by day 25, but maintained a consistently lower release amplitude. The physically encapsulated materials showed superior remediation performance, achieving ΣPAHs removal rates of 76.0% for SRB and 74.4% for SRR, compared with 66.0% for CSK. They were particularly effective against HMW-PAHs, with removal rates of 65.8% and 57.1%, respectively, compared with 47.7% for CSK. The physically encapsulated treatments maintained relatively high and sustained available nitrogen concentrations, with coefficients of variation of 0.16-0.17. Available nitrogen concentrations were strongly positively correlated with residual ΣPAH, MMW-PAH, and HMW-PAH concentrations ( r  = 0.8-0.9). Despite its greater material stability and slow-release capacity, CSK released only 11.2% of its total nitrogen by day 5 and 46.1% by day 25 and maintained relatively small temporal variation in soil available nitrogen (CV ≈ 0.04); this nutrient-supply pattern was associated with lower PAH degradation, particularly for HMW-PAHs. These results indicate that the magnitude and temporal pattern of nitrogen supply, rather than release stability alone, are important factors associated with biostimulation efficacy. Microbial analysis showed that the physically encapsulated materials increased bacterial α-diversity and enriched PAH-degrading genera such as Mycobacterium , which was significantly correlated with PAH removal. Co-occurrence network analysis further indicated that SRR promoted positive bacteria-bacteria interactions (39.2%) while maintaining low cross-domain antagonism (6.3%). Overall, under the tested conditions, the physically coated lignin-based slow-release formulations evaluated here provided an effective strategy for the targeted remediation of PAH-contaminated soils, particularly those containing substantial HMW-PAH fractions, with their performance supported by coordinated nutrient-release and microbial-community responses.