Accumulation of polycyclic aromatic hydrocarbons (PAHs) and oxygenated PAHs (OPAHs) in organic and mineral soil horizons from four US remote forests

We characterized distributions of 23 polycyclic aromatic hydrocarbons (Sigma 23PAH) and nine oxygenated PAHs (Sigma 9OPAH) in four remote forests. We observed highest Sigma 23PAH and Sigma 9OPAH concentrations in a coniferous forest in Florida, particularly in organic layers which we attributed to frequent prescribed burning. Across sites, Sigma 23PAH and Sigma 9OPAH concentrations strongly increased from surface to humidified organic layers (+1626%) where thncentrations reached up to 584 ng g(-1). Concentrations in mineral soils were lower (average 37 +/- 8 ng g(-1)); but when standardized per unit organic carbon (OC), PAH/OC and OPAH/OC ratios were at or above levels observed in organic layers. Accumulation in litter and soils (i.e., enrichment factors with depth) negatively correlated with octanol-water partition coefficients (K-ow) and therefore was linked to water solubility of compounds. Concentrations of Sigma 9OPAHs ranged from 6 +/- 6 ng g(-1) to 39 +/- 25 ng g(-1) in organic layers, and from 3 +/- 1 ng g(-1) to 11 +/- 3 ng g(-1) in mineral soils, and were significantly and positively correlated to Sigma 23PAHs concentrations (r(2) of 0.90) across sites and horizons. While OPAH concentrations generally decreased from organic layers to mineral soil horizons, OPAH/OC ratios increased more strongly with depth compared to PAHs, in particular for anthrone, anthraquinone, fluorenone, and acenaphthenequinone. The strong vertical accumulation of OPAH relative to OC was exponentially and negatively correlated to C/N ratios (r(2) = 0.67), a measure that often is used for tissue age. In fact, C/N ratios alone explained two-thirds of the variability in OPAH/OC ratios suggesting particularly high retention, sorption, and persistency of OPAHs in old, decomposed carbon fractions. (C) 2015 Elsevier Ltd. All rights reserved.