| Greenhouse vegetable soil is a major source of N2O.Based on the flux measurements for decades in situ,the majority of N2O emissions have been attributed to N2O flushing emission peaks resulted from anthropogenic perturbations in greenhouse vegetable soil.These peaks usually occur 1-3 days after fertilization irrigation and natural precipitation.High N2O emission occurs in the early season of vegetable growth in greenhouses and more than 50%of the total annual N20 emissions for the season come from this period.It is usually accepted that denitrification is the most important processe for N2O production.However,in the recent years,importance of nitrification and nitrification coupled processes has been emphasized.To have a better understanding of processes responding to N20 emission from greenhouse vegetable soils,parameters for N2O emission related to processes,functional genes and soil microbial community structure are of great significance for N2O emission and reduction of gaseous N losses from the soil.In the present studies,soil samples were collected from two on-set experiments in a typical concentrated greenhouse vegetable production region,Shouguang,Shandong Province,in the North China plain for laboratory incubations.A robotized incubation system combined with molecular biology technology were used to monitoring gaseous products,mineral N changes,nitrification and denitrification related genes(amoA,Arch-amoA,narG,nirK,nirS,nosZ)and their transcription copies under different environmental factors,such as different O2 concentration,pH and carbon input.cDNA based on microbial community structure was analysis simultaneous.The overall research is to further explore the sources and mechanisms of N2O production in vegetable soils.The main results and conclusions are as follows:(1)N2O emissions declined exponentially with the increased soil oxygen partial pressure(R2=0.82,P<0.001)and nitrogen gases loss mainly occured under anaerobic and 1%of O2.3%of O2 was the inflection point for the functional genes transcription,N2O emission and nitrogen gases loss(N2O+N2),regardless of the adequacy substrate and carbon source.Available carbon would reduce the N2O emission,while increase the N2O production from heterotrophic denitrification at 1%of O2-(2)The transitional accumulation of NO2-in soil was closely related to N2O emission.Although excessive NO2-in soil would inhibit soil microbial activity,it would stimulate the transcription of mRNA by soil microorganisms.For VRS soils,exogenous NO2-stimulated the transcription of amoA,narG,and nirS genes at 21%of O2,and stimulated the transcription of nirK at 0%of O2.The NO2-accumulation in VRS and LJ also stimulated the transcription of nirS gene at 1%of 02.In addition,the addition of nitrification inhibitors not only reduced N2O emissions,but also reduced N2 emissions.All these indicated that heterotrophic denitrification and nitrification coupled denitrification might have important contributions to N2O emission.Soils with long-term crop straws addition had potential higher N2O emission,especially when the content of oxygen was 1%or less.(3)When the carbon source in vegetable soil was sufficient,heterotrophic denitrification process was the main route of N2O pulsed discharge,especially in low pH soil.Compared to nitrifiers,heterotrophic denitrifiers respond more quickly to the reduced soil oxygen content,and the increased amoA transcriptions at 30 h after the start of incubation might be the reason for the increased N2O in VRS soil at 10%of O2.However,it could be speculated that soil N loss from nitrification was seldom based on the N2 production and mass balance results.In addition,the high proportion of Firmicute(mainly Bacilllus)during the short-term incubation also confirmed the important role of heterotrophic denitrification process for N2O emission in vegetable soils.(4)Both the substrates and O2 levels would change the structure of the soil active bacteria,while the latter showed a more pronounced effect.At 10%and 21%of 02,no significant differences in genes transcript abundances and species diversity,but significantly difference in N2O production indicated that only a small portion of the active bacteria was involved in the formation of N2O,which leaded to no significant increase of the dominant activity bacteria in soil during the enrichment incubation.The active microbial community changed during the incubation time,and the dominant bacteria might be transforme from the Firmicute to indigenous soil bacteria,such as Proteobacteria.(5)Gene-induced(RNA)rather than cell growth(DNA)determined the N2O emissions from soil.Research based on transcription level could reflect the inorganic nitrogen changes and gas emissions to some extent.The gene and transcription abundance of nirK were generally higher than that of nirS,while no significant correlation was observed between nirK transcription and N2O emission.Furthetrmore,the transcriptional abundance of nosZ and the ratio of nosZ/(nirS+nirK)showed no significantly differences in both soils,while the N2O emission in LJ soil was significant higher than that in VRS soil.All of these indicated that not all the functional genes involved in transcription were involved in soil nitrogen transformation.It was the reason for the mismatched genotypes and phenotypes currently. |
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