Soil enzyme dynamics in arid and Mediterranean soils exposed to environmentally relevant PFAS concentrations

This study investigated the short- and long-term effects of perfluorooctanoic acid (PFOA) and 6:2 fluorotelomer sulfonic acid (6:2 FTSA) on the biochemical functioning of agricultural soils with contrasting organic matter content, specifically Mediterranean and arid soils. Soil samples were spiked with environmentally relevant concentrations of both PFAS (10, 50, and 250 ng g⁻¹ dry mass) and incubated for 210 days to monitor changes in microbial and enzymatic activities. The results revealed that both PFAS significantly impaired soil biochemical functioning in the arid soil, with Treated-Soil Quality Index (T-SQI) values declining to less than 70% of control levels. Specifically, 6:2 FTSA caused significant inhibition of carbon- and phosphorus-acquiring enzymes such as β-glucosidase and alkaline phosphomonoesterase, with inhibition levels reaching up to 81.7%. In contrast, the Mediterranean soil exhibited minimal enzymatic response, which was attributed to its higher organic matter content that likely decreased PFAS bioavailability. Eco-enzymatic stoichiometric modeling further indicated that PFAS exposure increased microbial carbon limitation and altered nutrient acquisition strategies in arid soils over the long term. While PFOA concentrations remained relatively stable throughout the study, 6:2 FTSA underwent microbially mediated transformation to perfluoroheptanoic acid (PFHpA), with more pronounced dissipation occurring in the Mediterranean soil. Overall, these findings highlight the greater vulnerability of low-organic matter arid soils to both legacy and emerging PFAS, emphasize the risk of impaired nutrient cycling associated with PFAS-contaminated organic amendments, and underscore the importance of careful management of biosolids and municipal composts in dryland agricultural systems.