Earthworm-assisted remediation of aged benzo[a]pyrene-contaminated soil and its impact on microbial community...

Benzo[a]pyrene (BaP) is a persistent organic pollutant (POP) that deposits in the soil through industrial and related anthropogenic inputs, and it resists to degrade naturally. Like most POPs, BaP binds strongly to organic matter thus affecting microbial removal owing to its limited bioavailability. This study evaluates BaP removal from soil inoculated with Pheretima guillelmi under freshly spiked and aged contaminated conditions while assessing its impact on soil chemistry, enzyme activities, and microbial community structure. Results revealed that BaP removal rates were 36.96 % and 39.47 % in fresh and aged contaminated soils, respectively, after 60 days of incubation in the presence of earthworms. The residual concentration of BaP as well as its bioaccumulation factor found in earthworm tissue drastically reduced under aged treatments thus revealing reduced or limited bioavailability. The electrical conductivity, ammonium nitrogen (NH₄⁺-N) and nitrate nitrogen (NO₃^--N), available potassium content; catalase and dehydrogenase activities increased due to earthworm burrowing, feeding, and casting activities whereas urease and protease were unaffected under aged conditions. 16S rRNA gene sequencing analysis indicated an increase in bacterial α-diversity accompanied by the enrichment of putative BaP-degrading genera Flavobacterium, Sphingomonas, Aeromonas, Mesobacillus, and Microvirga. Results of functional prediction and co-occurrence network analyses demonstrated that BaP perturbed microbial associations, however earthworm containing treatments maintained higher bacterial association as well as functional resilience. These results validate that P. guillelmi enhances remediation potential in aged BaP-contaminated soils through adjustment of nutrient balance in the soil, stimulation of pollutant-degrading taxa, and strengthening microbial interactions.