Transport And Transformation Behavior And Physiological Response Characteristics Of Pahs And Their Oxidized Derivatives (Oxy-PAHs) In Soil-rice System

The migration and transformation behavior of PAHs and their derivatives in different environmental media is now a hot issue in environmental sciences.The redox environment created by rice cultivation practices is similar to that of a wetland system.There are still relatively few researches focused on PAHs derivatives in the soil-rice system,especially for the more toxic,migratory and persistent Oxy-PAHs.Supported by the National Natural Science Foundation of China（42207432）,PAHs and Oxy-PAHs in the soil-rice system were studied-through field sampling surveys and potted plant simulation experiments.On the basis of obtaining the distribution,sources and health risks of PAHs and Oxy-PAHs at the sampling sites,we systematically investigated the characteristics of PAHs and Oxy-PAHs migration and accumulation in the soil-rice system,and explored the rhizosphere effects of rice on the environmental behaviors of PAHs and Oxy-PAHs migration and accumulation,and elucidated the physiological responses of rice plants to the toxicity of two typical Oxy-PAHs.The results of this study further enriched the biogeochemical cycling theory of POPs in soils and provided a scientific basis for effectively reducing the integrated ecological risk of organic pollutants in soil-rice systems.The main results are as follows:（1）The distribution,influencing factors,main sources and health risks of PAHs and their derivatives（Oxy-PAHs）in paddy soils along Yangtze estuary were clarified.The concentrations ofΣ₁₆PAHs andΣ₁₁Oxy-PAHs were greater in long-term paddy soils than those in short-term paddy soils.Microbial degradation,vegetation absorption and transformation,and exogenous input influenced by human activies were important factors influencing the spatial distribution of PAHs and Oxy-PAHs in paddy soils.The concentrations ofΣ₁₁Oxy-PAHs in soils of long-term rice-growing areas decreased with depth.The highest concentration ofΣ₁₁Oxy-PAHs in short-term rice-growing soil was found in the middle layer.Correlation analysis showed that water content,TOC,and finer clay silt and redox potential were the main influencing factors of paddy soils.The source determination results showed that PAHs in paddy soils were mainly from combustion sources and traffic emissions,which were complex influenced by human activities.（2）The uptake and accumulation characteristics of PAHs and Oxy-PAHs in soil-rice system were systematically studied.The results of pot experiment showed that rice planting can significantly promote the degradation of PAHs in soil and the transformation and generation of Oxy-PAHs.The concentrations of PAHs in rice plants were lower than those in soils,and the concentrations of PAHs in different rice tissues were significantly different.Rice roots could easily aborb PAHs,but the transport of PAHs from roots to stems and leaves was difficult.The concentration of Oxy-PAHs in different tissues of rice were as follows:root>stem>rice husk>rice>leaf.The concentrations ofΣ₁₁Oxy-PAHs in rice roots and leaves increased significantly with rice growth,while the concentrations ofΣ₁₁Oxy-PAHs in rice stems decreased gradually with rice growth.The uptake and accumulation of Oxy-PAHs in rice root was obviously compared with the upper rice tissues.（3）To elucidate the effect of rice inter-root effects on the transport,transformation and uptake and accumulation of PAHs and Oxy-PAHs in the soil-rice system.In the rhizosphere bag pot simulation experiment,the mean residual concentrations ofΣ₁₆PAHs in rice rhizosphere soils were lower in the PAHs treatment group than those in the non-rhizosphere soils,and the mean residual concentrations ofΣ₁₁Oxy-PAHs were higher in rhizosphere soils than in non-rhizosphere soils.The mean residual concentrations ofΣ₁₁Oxy-PAHs in rhizosphere soils were higher in the Oxy-PAHs treatment group than those in the non-rhizosphere soils.In the Oxy-PAHs treatment group,the concentrations ofΣ₁₁Oxy-PAHs in rice plants were high in roots,followd by stem and leaves,and the composition structure of Oxy-PAHs in each rice tissue at different fertility stages had large differences.Root activities such as releasing root exudates,absorbing nutrients,and releasing oxygen from rice roots can affect microbial activities and pollutant effectiveness,resulting in significant variations in s-UE,s-POD,s-PPO,and s-DHA activities in different treatment groups,which were generally related to carbon sources,microorganisms,rice rhizosphere effects,and pollutant toxicity.Rice rhizosphere soil is usually an acidic environment,but aerobic oxidation of PAHs in rhizosphere soil was still promoted and generated related intermediates,such as Oxy-PAHs.Correlation analysis showed that high molecular weight Oxy-PAHs had a negative effect on s-UE activity in soils.（4）The physiological response of rice plants to two typical Oxy-PAHs was explored.In the short-term toxicity simulation experiment,rice plant height and tiller numbers were inhibited with the increase of 9-FLO concentrations.Low 1,4-NQ concentrations stimulated rice plant height,but high treatment concentrations inhibited rice plant height.Influenced by the antioxidant defense system of the organism,the stress of different concentrations of 9-FLO and 1,4-NQ produced more significant effects on the relevant indicators of reactive oxygen metabolism(O₂^’-content,H₂O₂content,SOD activity,POD activity,CAT activity)in rice leaves.