Toxicity Evaluation Of Typical Low-ring Polycyclic Aromatic Hydrocarbons On Soil Microbial Activities

With the rapid development of industry and the continuous increase of population,the problem of environmental pollution is becoming more and more serious.Polycyclic aromatic hydrocarbons（PAHs）are typical persistent organic pollutants widely existing in the environment,which pose a serious threat to human health and ecological environment security.PAHs in the environment first enter the atmosphere and are adsorbed by atmospheric particles.Then,the soil becomes the main carrier of PAHs in the environment through dry and wet atmospheric deposition,irrigation by agricultural effluents,sorption and deposition,and subsequent accumulation.PAHs entering the soil will have an effect on soil microbial activity and lead to a disruption of the normal material cycle in the soil.Therefore,biochemical processes such as soil enzyme activity,microbial biomass and soil basal respiration are all affected by PAH contamination and are also the most useful biochemical indicators for evaluating PAH contamination in soils.At present,studies on the effects of PAHs with different ring numbers on soil microbial activity and soil quality evaluation are scarce,especially studies on the inhibition characteristics of PAHs on soil enzyme kinetics have not been reported.Therefore,the study of the stress effect of PAHs on soil enzymes,the mechanism of action and the influencing factors are of great importance to elucidate the environmental risk of PAHs on soil.In this study,the ecological-dose effect relationship of low ring polycyclic aromatic hydrocarbons（PAHs）on soil under acute contamination and their mechanism of action on soil enzymes were revealed by means of soil enzyme kinetics through indoor simulated PAH contaminated soil tests with naphthalene,phenanthrene and anthracene as typical representatives of low ring PAHs.It also revealed the changes in the anthracene ageing process on soil microbial biomass carbon,soil basal respiration,soil microbial metabolic entropy and soil enzyme activity,providing a rich theoretical basis for quality monitoring and risk assessment of PAH-contaminated soil.The main research findings are as follows（1）Soil FDA hydrolase activity was inhibited under the contamination of all three PAHs,and there was a significant negative correlation between the contamination concentration and FDA hydrolase activity,indicating that FDA hydrolase activity can be used as one of the sensitive indicators to evaluate the degree of naphthalene,phenanthrene and anthracene.At the highest pollutant concentrations,anthracene had the strongest inhibitory effect on FDA hydrolase activity than the other two pollutants,with an inhibition rate of74.15%.The complete inhibition model was used to fit enzyme activity and pollutant concentrations,and the results of the fits all achieved significant or highly significant relationships.This suggests that to some extent,FDA hydrolase activity can monitor the extent of PAH contamination of soil with ecological dose values ED₁₀ ranging from 43.92 to 203.55 mg kg^-1,55.28 to224.19 mg kg^-1 and 2.19 to 17.53 mg kg^-1,respectively.It reveals that the magnitude of the ecotoxicity of PAH contamination to soil varies depending on the type of PAH and the properties of the soil.（2）The kinetic parameters K_m and V_max of the FDA hydrolases of the test soils showed a decreasing trend with increasing concentrations after the addition of the three PAHs（except for the Typic Gleyi-Stagnic Anthrosols）,indicating that all three PAHs inhibited the FDA hydrolases of the test soils in an anti-competitive manner.Fitting the relationship between V_max and pollutant concentrations using the complete inhibition model yielded ED₁₀ranging from 20.02 to 139.14 mg kg^-1,30.36 to 71.68 mg kg^-1,1.30 to 20.81mg kg^-1,ED₅₀ ranging from 180.18 to 684.93 mg kg^-1,273.22 to 645.16 mg kg^-1,11.74 to 27.62 mg kg^-1.Anthracene was significantly more toxic to FDA hydrolase in the test soils than naphthalene and phenanthrene.In addition,the inhibitory effect of naphthalene and anthracene on the test soils was related to the soil properties,with soil organic matter being the main control factor affecting their toxicity to FDA hydrolase.（3）During anthracene ageing,the FDA hydrolase activity,soil basal respiration rate,microbial biomass carbon and microbial metabolic entropy of the test soils were significantly affected by the anthracene contamination concentration and incubation time,indicating that they can be used as ecological indicators to evaluate soil anthracene contamination.In both test soils,FDA hydrolase activity showed a highly significant positive correlation with microbial biomass carbon and a significant or highly significant negative correlation with microbial metabolic entropy,indicating that FDA hydrolase activity can respond to changes in soil microbes.（4）The K_m and V_max of the test soil FDA hydrolase decreased with increasing pollutant concentration during anthracene ageing,indicating that the type of inhibition of the test soil FDA hydrolase during anthracene ageing was all anti-competitive.A complete inhibition model was used to fit the relationship between V_max and contaminant concentrations in the test soils,and the results of the fits all achieved significant or highly significant relationships,indicating that V_max of FDA hydrolase can be used as a sensitive indicator to evaluate the degree of anthracene contamination.The ED values of FDA hydrolase V_max in the Ochri-Aquic Cambosols throughout the incubation period were smaller than those of the Typic Gleyi-Stagnic Anthrosols,indicating that anthracene had a stronger toxic effect on the tidal soils.In addition,the FDA hydrolase K_iu was greater than K_m in the test soils at all times except on day 45 in the Ochri-Aquic Cambosols of Shandong,suggesting that the aging process of anthracene influenced the FDA hydrolase by binding to the FDA hydrolase-substrate complex.In summary,naphthalene,phenanthrene and anthracene inhibit soil FDA hydrolase activity mainly through anti-competitive inhibition.Soil organic matter is the main control factor affecting its toxicity to FDA hydrolase.Anthracene is the most toxic of the three polycyclic aromatic hydrocarbons.This study provides a theoretical basis for evaluating the ecological and environmental risks of soil PAH contamination and pollution remediation.