Delayed recovery of nitrogen-related soil indicators during two-year phytomanagement of diesel-contaminated soil with Miscanthus × giganteus

Petroleum contamination disrupts soil nitrogen transformation pathways critical for ecosystem recovery during phytomanagement, yet their integrated response across microbial, chemical, and isotopic indicators remains poorly understood. This two-year pot experiment investigated how diesel contamination (5, 10, 25, 50 g/kg) and cultivation of the perennial grass Miscanthus × giganteus affect nitrogen availability, nitrogen-transforming microbial communities, free amino acid (FAA) profiles, and δ 15 N signatures in agricultural soil. Diesel increased soil C:N ratios up to 1.5-fold (22:1 vs. 14:1 in controls) and caused NH 4 + accumulation associated with up to 20-fold suppression of NH 3 -oxidizer gene abundance ( amoA ). Concurrently, while nitrogen fixation markers ( nifH ) increased up to 150-fold, soil δ 15 N was enriched (+3‰ at 50 g/kg), indicating that nitrogen losses associated with isotopic fractionation may have contributed more strongly to the observed isotopic signal than fixation inputs. Additionally, δ 15 N in plant biomass increased with diesel concentration, indicating uptake of isotopically enriched nitrogen. Total FAA content correlated with diesel concentration, with proline consistently elevated, reflecting a sustained stress response. M. × giganteus had a secondary, context-dependent effect, in that plant presence did not alter nitrogen-transforming microbial gene abundance or microbial activity but moderated mineral nitrogen distribution and isotopic signatures, particularly in the second season as hydrocarbon concentrations declined. These results indicate that substantial hydrocarbon removal during phytomanagement was not necessarily accompanied by recovery of multiple nitrogen-related indicators, suggesting that contaminant removal alone may not fully reflect restoration of soil functioning.