Disentangling the roles of soil edaphic properties and long-term nitrogen addition in controlling polycyclic aromatic hydrocarbons retention in temperate and boreal forest soils of Eastern Canada

Polycyclic aromatic hydrocarbons (PAHs) are persistent organic pollutants that accumulate in forest soils via atmospheric deposition. However, the combined effects of long-term nitrogen (N) addition and forest stand type on PAH retention and vertical distribution remain poorly understood. This study examined how three levels of long-term (>20 years) N addition (Control, Low, High) influence PAH concentrations across soil horizons (F, H, Top B, and Low B) under three dominant forest stands in Eastern Canada: sugar maple, balsam fir, and black spruce. We quantified total, medium molecular weight (MMW), and high molecular weight (HMW) PAHs, and assessed their responses to N treatments. Results showed no consistent or statistically robust effect of long-term nitrogen addition on PAH concentration or vertical distribution across forest stands and soil horizons. PAH accumulation and distribution were primarily associated with soil chemistry and soil horizon across forest types. Organic horizons (F and H) retained higher PAH concentrations than mineral horizons with forest-dependent variation. Among soil variables, nitrogen-related metrics, organic carbon, and pH emerged as key forest-specific predictors of PAH retention. These findings emphasize the dominant role of natural edaphic factors over N amendments in controlling PAH dynamics, offering new insights for environmental monitoring and forest soil management.