Effects Of Mixed Addition Of Shrub And Grass Litters On Their Remediation Effect On Crude Oil Contaminated Soil

In this study,11 of well grown shrub and grass species in crude oil contaminated regions in northern Shaanxi Province that have favorable potential necrophytoremediation effects were selected for this study.The litters of them were collected and combined into 30 litter mixtures according to their common community composition in the nature.At room temperature(20-25℃)and constant humidity,the crude oil contaminated soil of 15g/kg(slightly contaminated)and 45g/kg(seriously contaminated)were treated with monospecific and mixed litters for 150 days.The remediation effect of each treatment(including contaminants removal and restoration of the damaged biological and chemical properties of soil)was detected.Then,the actual remediation effect of mixed litters and their corresponding theoretical predicted values,which were calculated based on the remediation effect of monospecific litters and their mass proportion in the mixtures,were compared,to analyze whether mixed addition of litters can strengthen or weaken their remediation effects on contaminated soil and the underlying mechanisms of these phenomena.Aiming to analyze the potential effects of mixed addition of litters on their remediation effects on crude oil-contaminated soil,and to provide a scientific basis for selecting appropriate litter combinations to strengthen the effects of necrophytoremediation.The main findings are as follows:(1)All litter treatments significantly increased the degradation rate of crude oil and its components in slightly and serously contaminated soil(increased by 22.56%-78.60%,relative to natural attenuation,P<0.05),and most of the litter treatments could increase the contents of ammonium nitrate nitrogen,available phosphorus and available potassium,as well as the activities of urease,alkaline phosphatase,catalase and dehydrogenase in contaminated soil,while they generally reduced the activity of polyphenol oxidase.The p H of slightly contaminated soil was generally decreased,while that of seriously contaminated soil was increased.The sucrase activity of slightly contaminated soil was generally increased,while that of seriously contaminated soil was decreased.(2)For slightly contaminated soil,a total of eight forms of litter mixing synergistically enhanced their overall remediation effects including contaminants removal and restoration of the damaged biological and chemical properties of soil.Especially,the remediation effects of mixing forms Lespedeza davurica + Artemisia gmelinii + A.scoparia,Heteropappus altaicus + Hippophae rhamnoides and A.gmelinii + A.scoparia were generally stronger than the corresponding predicted value(increased by 30.12%-197.82%,P<0.05),as well as the measured value of any corresponding monospecific litter treatments,exhibiting the best potential for the practice of necrophytoremediation.In addition,mixed addition of litters in forms of Bothriochloa ischaemum + Periploca sepium + Sophora davidii,L.davurica + A.gmelinii and L.davurica + A.gmelinii + H.altaicus + A.scoparia tended to synergistically enhance their contaminants removal effects,but generally lead to antagonistic inhibition on their restoration effects on the damaged soil biological and chemical properties.For seriously contaminated soil,a total of five forms of litter mixing synergistically enhanced their overall remediation effects including contaminants removal and restoration of the damaged biological and chemical properties of soil.Especially,the remediation effects of mixing forms B.ischaemum + S.davidii,P.sepium + S.davidii,and A.cristatum + Bromus inermis exhibiting the best potential for the practice of necrophytoremediation(increased by 1.68%-712.47%,P<0.05).In addition,mixed addition of litters in forms of B.ischaemum + P.sepium tended to synergistically enhance their contaminants removal effects,but generally lead to antagonistic inhibition on their restoration effects on the damaged soil biological and chemical properties.(3)In general,the stimulating effects of mixed litters treatments on contaminants degradation was positively correlated with their nutrient content(especially nitrogen and phosphorus)and chemical richness,but negatively correlated with their carbon content and stoichiometric ratio(C/N,C/P and N/P)in slightly contaminated soil.Their restoration effects on the damaged biological and chemical properties of soil was generally positively correlated with their content of nutrients(nitrogen,phosphorus,potassium,and labile carbon and nitrogen sources such as soluble sugars,amino acids),but negatively correlated with the contents of co-metabolic substrates and surfactants(terpenoids,phenols,total organic acids and other organic substances)and carbon.The stimulating effects of mixed litters treatments on contaminants degradation was positively correlated with their nutrient content,while partially negatively correlated with its chemical richness in slightly contaminated soil.Their restoration effects on damaged biological and chemical properties of soil were generally positively correlated with their nutrient content and negatively correlated with their stoichiometric ratio.However,their supplementary effects on soil ammonium nitrogen were positively correlated with their degradation-auxiliary substance content,and their supplementary effects on nitrate nitrogen and the stimulating effects on soil enzyme activity were negatively correlated with them.(4)In slightly contaminated soil,the increase in chemical richness of mixed litters was conducive to synergistically enhance contaminants degradation and stimulate soil catalase activity.In seriously contaminated soil,the increase in chemical diversity of mixed litters tended to weaken the synergistic strengthening of the effects of stimulating contaminants degradation,supplementing soil available potassium and stimulating polyphenol oxidase activity,and even led to the antagonistic inhibitory effect.However,it might conducive to synergistically enhance the stimulating effects on catalase activity.(5)Both slight and serious contamination and natural attenuation treatments resulted in significant changes in soil bacterial or fungal community structure,while all litter treatments did not restore them to uncontaminated status.In slightly contaminated soil,the bacterial community structure under mixed litters treatments was more distinguished from to that of uncontaminated soil relative to that under monospecific litter treatments,while the fungal community structure under mixed litters treatments was more similar to that of uncontaminated soil relative to A.gmelinii litter treatment.In seriously contaminated soil,the bacterial community structure under mixed litters treatments was more similar to that of uncontaminated soil relative to that under monospecific litter treatments,while there were no significant differences of the fungal community structures between mixed or monospecific litters treatments.In slightly contaminated soil,the relative abundance of Nocardioides,Coprinellus,Nocardia and Stachybotrys,which were significantly related to contaminants degradation and soil biological and chemical indices recovery,significantly increased after mixed litter treatment,while in seriously contaminated soil,the relative abundances of Chaetomium,Brevundimonas and Nocardia,which were significantly related to contaminants degradation and soil biological and chemical indices recovery,were significantly increased.