Per- and polyfluoroalkyl substances (PFASs) in humus-mineral layers of forest soils: Distribution, key drivers, and...

The toxicity and persistence of per- and polyfluoroalkyl substances (PFASs) have made their widespread environmental contamination a significant global issue. Forests can uptake and trap PFASs from the atmosphere, but their environmental behavior and fate in forest humus layer and mineral soil remain unclear. This study investigated PFAS distribution, drivers, and leaching rates in Mt. Shennongjia, China. The concentrations of total PFASs (Σ24PFAS) in the humus layer (average of 5.67 ng/g) were higher than in mineral soil (2.79 ng/g), with long-chain perfluoroalkyl carboxylic acids (PFCAs) (46.8%-59.3%) being the predominant PFASs. Spatial distribution and correlation analysis indicates that, altitude and forest vegetation types influence the accumulation of PFASs in forest humus and soil. The concentration of PFASs generally increased with increasing altitude, and the sub-alpine conifer forest exhibited higher PFASs level than other vegetation types. The concentration of PFASs was positively correlated with the TOC content (r = 0.356-0.745, p < 0.05). Enrichment factors (EF) of PFAS varied from 0.14 to 2.22, and EFs decreased with increasing carbon chain length, suggesting long-chain PFASs have a greater propensity to partition into humus. The leaching rates of PFAS from the humus layer in freely dissolved forms were significantly higher than those in the dissolved organic matter forms (p < 0.05). Short-chain and carboxylate-headed PFASs showed greater downward leaching potential, more attention should be paid to their transport to groundwater in ecologically sensitive areas.