Effects And Mechanisms Of Soil Fumigation On Nitrogen Cycling Microorganisms And N₂O Production

Despite soil fumigants were widely used to prevent and control devastating soil-borne pathogens including bacteria,fungi and nematodes before planting crops,the influence of soil fumigation on microorganisms involved in nitrogen-transforming process remains little understood.We investigate the emission characteristics of N₂O following chloropicrin?CP?,1,3-dichloropropylene?1,3-D?,dazomet?DZ?,dimethyl disulfide?DMDS?,allyl isocyanate?AITC?fumigation through headspace-gas chromatography.High-throughput sequence and quantitative PCR were used to determine the responses of 11 functional microorganisms associated with nitrogen transfer.Metatranscriptomes and dual-label¹⁵N-¹⁸O isotope analysis were applied to study the responses of key nitrogen cycle processes such as ammonia oxidation or denitrification,as well as the change in N₂O emission pathways.Results of our study would provide important scientific evidence for understanding the response characteristics of nitrogen cycling functional microorganisms under the fumigation stress,and clarify the formation mechanism of N₂O in fumigated soil.1.Effects of 1,3-D fumigation on nitrogen cycling microorganismsIn two soil types,1,3-D temporarily reduced the abundance of 11 functional genes involved in nitrogen cycling,and a stronger and longer inhibition effect on these functional genes was observed in acidic Jiangxi red soil than in the almost pH neutral Beijing fluvo-aquic soil.Among the observed 11nitrogen-cycling groups,microorganisms containing nifH,nxrB,napA,and qnorB genes were most vulnerable to 1,3-D fumigation.At the same time,the abundance of nifH,AOBamoA,nirS,qnorB and cnorB genes was significantly increased at a later stage following 1,3-D fumigation.Fumigation with 1,3-D significantly reduced the relative abundance of nitrogen-fixing bacteria Azospirillum and Paenibacillus;the nitrifiers Nitrosomonas and Nitrospira;and the denitrifiers Pseudomonas,Paracoccus and Sphingomonas.Fumigation with 1,3-D significantly increased the relative abundance of BradyRhizobium and Rhizobium that are involved in N-fixing;Nitrosospira and Nitrolancea that are involved in nitrification;and Sphingobium,Alcanivorax,Bacillus,Streptomyces and Aeromonas that are involved in denitrification.Overall,our results substantiate that 1,3-D fumigation significantly influenced the microbial nitrogen cycle,but different nitrogen cycling groups recovered to the unfumigated levels in various incubation phases.2.Effects of DZ fumigation on nitrogen cycling microorganisms and N₂O emissionFumigation with DZ produced a significant decrease in the abundance of 16S rRNA and 11 functional genes present in microbes involved in N-cycling in a lateritic red soil?Jiangxi soil?.This inhibition effect was also present in DZ fumigated fluvo-aquic soil?Beijing soil?but weaker.DZ also temporarily stimulated bacterial diversity as well as caused a significant change in bacterial community composition.The N₂O emissions in fumigated soil were significantly correlated with key soil environmental factors?NH₄⁺,DAA,MBN?and not functional gene abundance.However,when the concentrations of the fumigant declined and the inhibitory effects of DZ fumigation disappeared,the soil microbial community recovered to population levels observed in unfumigated soils.The microbial recovery rate,however,depended on the physicochemical properties of the soil.3.Effects of CP fumigation on N₂O emission and productionCP fumigation significantly increased N₂O emission but decreased the abundance of 16S rRNA and N cycling functional genes.Meanwhile,CP decreased soil bacterial diversity and caused a shift in the community composition.The N₂O emissions in fumigated soil were significantly correlated with soil environmental factors?such as NH₄⁺,DAA,MBN,and NO₃^-?and were not correlated with functional gene abundance.The significant increase in the availability of NH₄⁺-N and DAA?acting as N pools?in fumigated soil was a significant driver of N₂O metabolism.CP inhibited the expression of gene families involved in both N₂O production and sink processes and caused a shift from the nitrification N₂O emissions pathway to the denitrification pathway,which became the main pathway of N₂O production following CP fumigation.When the effects of the fumigants in the soil were absent,the inhibitory effects of fumigants disappeared,and the soil microbial community recovered to normal levels.However,the recovery rate depended on the soil characteristics.4.Mechanism of N₂O production following DMDS or AITC fumigationBoth DMDS and AITC produced a decrease in the abundance of 16S rRNA and N cycling functional genes in the short term.When the effects of the fumigants in the soil were no longer present the effects of the stimulation or inhibitory on microorganism were no longer present.The recovery rate,however,appeared to depend on the soil characteristics.The N₂O emissions in fumigated soil were significantly correlated with soil environmental factors?such as NH₄⁺,DAA,MBN,NO₃^-?and not correlated with the abundance of functional genes.The significant increase in the availability of NH₄⁺and DAA?which acted as nitrogen pools?in fumigated soil was a significant driver of N₂O production.DMDS stimulated the expression of gene families involved in both N₂O production and sink processes,while AITC only increased the process of NO₂^-to NO and organic decomposition.Fumigation caused a switch from the nitrification N₂O emissions pathway to the denitrification or nitrifier denitrification pathway,which became the main pathway of N₂O production following DMDS and AITC fumigation.