Effects Of Nitrogen Regulation On Remediation Of Diesel-contaminated Soil And Its Microbial Mechanism

Diesel oil is one of the most widely used fossil fuels in all crude oil refining products.It is easy to cause hydrocarbon organic pollutants to enter the environment during its production and processing.Organic pollutants have a long degradation cycle in the environment and will continue to be input and accumulated with the production process.Therefore,the remediation of organic hydrocarbon-contaminated soil is of great significance for maintaining the sustainable development of the ecosystem.Nitrogen is a key element necessary for microbial growth.Adjusting the soil carbon-nitrogen ratio by nitrogen addition can significantly improve the degradation efficiency of organic pollutants by microorganisms,which is a very practical technology for organically contaminated soil remediation.However,there are still many scientific problems to be solved in the study of the effect of nitrogen regulation on the remediation of diesel-contaminated soil and the microbial mechanism.In this study,indoor simulation was used to design experiments from two aspects:nitrogen source type and nitrogen addition concentration.Diesel oil was the most representative petroleum pollutant.The effects of nitrogen regulation on the physical and chemical properties,microbial community structure and nitrogen cycle process of contaminated soil were analyzed.The key microbial groups involved in the nitrogen cycle of diesel contaminated soil,nitrogen utilization preference and its coupling relationship with petroleum hydrocarbon degradation were clarified.On this basis,the effects of nitrogen regulation on the degradation of petroleum hydrocarbons under anaerobic conditions were analyzed by limiting the oxygen conditions of the bioremediation process,and the differences in soil physical and chemical properties and microorganisms involved in degradation between anaerobic and aerobic environments were compared.The conclusions were reached as follows:（1）After 30 days of incubation,the addition of ammonium significantly promoted the degradation rate of total petroleum hydrocarbons（TPH）in soil（79%～81%）compared with the control treatment group（38%）and other non-ammonium nitrogen amendments（43%～57%）.Metagenomic analysis showed that nitrogen addition significantly changed the microbial community structure and the abundance of functional enzymes in the soil.Proteobacteria and Actinobacteria were the dominant bacteria in the remediation of diesel-contaminated soil.The main metabolic pathway for nitrogen is the conversion of ammonium nitrogen to glutamine and then to glutamate.The key enzymes involved in this conversion process are glutamine synthetase（GS）and glutamate synthase（GOGAT）.（2）After 60 days of indoor culture,compared with CKoil（32.03%）,the soil TPH degradation rate of T7 treatment（KNO3;C:N=10:1;44.74%）was the highest,but T1 treatment（Peptone;C:N=20:1;44.39%）was listed as the optimal treatment because of the least amount of nitrogen added.Alphaproteobacteria and Gammaproteobacteria were the dominant bacteria in the process of biological repair,belonging to the Proteobacteria.In addition to Proteobacteria,Actinobacteriota is also a functional bacterium with high TPH degradation ability.Among them,Rubrobacter,Solirubrobacter and Gaiella of Actinobacteria are the three bacteria genera most related to the degradation of petroleum hydrocarbon pollutants in soil after nitrogen regulation.The addition of peptone and KNO₃ had different mechanisms in bioremediation of diesel-contaminated soil.（3）After 30 days of anaerobic remediation,the TPH degradation rate of CKoil was 19.16%,the TPH degradation rate of the D4 treatment（Peptone;C:N=5:1）was the highest,followed by the D3 treatment（Peptone;C:N=10:1）,and the degradation rates were 38.29%and 27.98%,respectively.Under anaerobic culture conditions,Firmicutes were the dominant bacteria for diesel degradation.The addition of ammonium nitrogen could promote the increase of Firmicutes abundance in soil,thereby increasing the degradation rate of petroleum hydrocarbons in soil.When nitrate nitrogen is added as the only electron acceptor,it will stimulate the enhancement of soil denitrification.At the same time,a large amount of nitrite will accumulate in the denitrification process,which will inhibit the nitrate reducing bacteria and affect the degradation of soil petroleum hydrocarbons.