Accumulation of policyclic aromatic hydrocarbons (PAHs) in surface litter and soils in four forests in the United States

Abstract:

Polycyclic aromatic hydrocarbons (PAHs) are toxic environmental pollutants originating from the incomplete combustion of organic material, both from natural or anthropogenic sources. Once emitted, they can be transported across thousands of kilometers impacting remote environments. Here, we characterize the distribution of 23 PAHs and 9 oxygenated PAHs (Σ32PAH) in litter and soils in four remote forests in the United States. Concentrations of Σ32PAH in fresh surface litter (Oi layers) showed very low levels in three of the four forests (mixed coniferous forest in Maine, deciduous blue oak forest in California, and a coniferous forest in Washington State), with PAHs levels much lower than those reported in previous studies from Europe. The analysis showed that PAHs represented a mix of regional background sources. Highest PAH levels were observed in a coniferous forest floor in Florida, attributable to frequent prescribed burning of understory vegetation at this site, and supported by high contributions of retene (>7%; compared to <1% at other sites). Σ32PAH increased in deeper, more decomposed organic litter layers, increasing from 57±19 ng g^-1 (in Oi layer) to 511± 285 ng g^-1 in decomposed, humidified litter layers (Oe and Oa horizons). In mineral soils, Σ32PAH were over an order of magnitude lower (average 37±8 ng g^-1), which was attributed to higher bulk densities of mineral soils. However, standardized per unit of organic carbon (OC), Σ32PAH:OC ratios in mineral soils also were below levels observed in overlying litter, indicating a strong sorption capacity of organic horizons for atmospheric deposition. Within mineral soils, Σ32PAH:OC ratios increased with depth (Ah horizons: 750±198 ng g^-1; B horizons: 1,202±97 ng g^-1), indicating that vertical transfer in mineral soils leads to significant accumulation of PAH in subsoils. ΣPAH:OC increases observed in deeper soil layers may be attributed to slower mineralization rates of PAHs compared to OC, plus vertical transport as indicated by preferential enrichment of PAHs with low K_ow (i.e., more water-soluble PAHs). Finally, percentage of potentially biologically produced PAH (Σ Naph+Phen+Pery) were low and consistent across the litter/soil horizons, suggesting that biological production is minor or absent at our sites.