Mechanisms underlying immobilization of short-chain perfluoroalkyl substances in paddy soils

<bold>Objective</bold> Perfluoroalkyl substances（PFAS）enter farmland fields such as paddy fields through surface water irrigation and biosolid applications，causing PFAS pollution in paddy soils.Among various of PFAS，short-chain PFAS has higher environmental mobility compared to PFAS with longer carbon chains，causing them easier to enter into the rice plant and pose a threat to human food security.The present study aims to investigate the immobilization remediation mechanism of short-chain PFAS pollution in paddy soil through laboratory soil cultivation experiments. <bold>Method</bold> By setting up seven soil improvement measures，including ammonium fertilizer（ammonium chloride），nitrate fertilizer（potassium nitrate），urea fertilizer（urea），goat manure，chicken manure，earthworm manure，and quicklime（calcium oxide），the immobilization enhancement of short-chain PFAS under different improvement measures was explored.Furthermore，by combining fourier transform infrared spectroscopy，X-ray photoelectron spectroscopy，and microbial amplicon sequencing，the geochemical and microbiological mechanisms of soil amendments on short-chain PFAS immobilization were analyzed. <bold>Result</bold> The results found that ammonium chloride had the highest enhancement effects on short-chain PFAS immobilization（63.21%，comprehensive contribution of physical chemistry and microbiology，<italic>P</italic>&lt;0.05，n=3）.The structural equation modeling revealed that soil improvement promoted immobilization by reducing the occurrence of PFAS in soil solution（<italic>P</italic>&lt;0.05）.The dose-effect experiment showed that the 0.1% mass ratios of ammonium chloride significantly promoted the consumption of dissolved organic matter in soil，thus weakening their physical and chemical bonding with PFAS.Meanwhile，0.1% ammonium chloride enhanced the adsorption and fixation of PFAS on soil particle surfaces through the transformation of soil particle organic matter.Furthermore，based on PICRUSt2 metabolism analyses of microbial community，the expression of ABC transporter pathway in paddy soil microorganisms was elucidated as the core driving factor for the regulation of soil organic matter conversion and following PFAS immobilization（with a contribution rate of 49.84%）. <bold>Conclusion</bold> A 0.1% mass ratio of ammonium chloride is the optimal amendment measure for improving the immobilization efficiency of PFAS in paddy soil.Its main mechanism of action lies in significantly altering the expression of the ABC transporter pathway in soil microorganisms and the microbial consumption of organic matter in the soil solution，enhancing the hydrophobic interaction between organic groups on the surface of soil particles and PFAS，thereby comprehensively strengthening the degree of PFAS immobilization.