Characteristics Of N₂O Emission Pathways And Relative Dynamics Of Microbial Community In Vegetable Soils

Nitrous oxide is one of the most important greenhouse gases in the atmosphere, mainly derived from the biological emissions. Vegetable soil may contribute great share to the national N2 O emissions due to the higher fertilization rate, multiple application and irrigation. Both field experiment and laboratory incubation were used to do the research, and we also combined with the technology of next generation sequencing to explain the contribution of different pathways of N2 O emissions after fertilized with ammonium N,including mainly ammonium N, ammonium N was equal to nitrate nitrogen, mainly nitrate nitrogen. Through the above, we will analysis the variation in abundance, diversity and composition of two important nitrifiers（AOB and AOA） and three denitrifiers（nirS, nirK and nosZ）. This studies were aimed to investigate the N2 O emission fluxes、different N2 O production processes and the mechanism of microbial in the vegetable soils; provide valuable and scientific evidence to controllingthe emission of N2 O in the vegetable soils.Three different land use of fields, including a maize field（maize）、 a newly established open-ground vegetable field converted from a maize field four years earlier（OV4）、an established sunlight heated greenhouse vegetable field converted from a maize field more than 20 years ago（GV20） were set in the experiment. Among them, the sample of open ground vegetable field were use the treatment of organic fertilizer only. Laboratory incubation were used acalcareous vegetable soil and a vegetable soil of brown soil with low pH inShandong province to explore the relationship between relevant contribution sourced from different pathway to N2O（Nitrifier Nitrification NN、 Nitrifier Denitrification NiD、Denitrifier Denitrification DD、Heterotrophicnitrification HN et al） and the variety of nitrification（AOB、AOA） and denitrification（nirS、nirK、nosZ） on copy number、diversity and composition after ammonium N was applied in different vegetable soils, which would provide the evidence of reducing the emissions of N2 O,adjustment of nitrogen transformation processes and enhancement of nitrogen use efficiency.The order of annual cumulative N2 O emissionss in different land use of soils is: open-ground vegetables field>greenhouse vegetable field>maize. This may be due to thedifferent soil temperature and moisture content, which has different impact on. microbial activity in variety soils. â‘¡The copy number and diversity of AOB were highest in the greenhouse vegetable and Nitrosospira_sp._Wyke8 was the dominant species in the three types of vegetable soils.â‘¢While the open-ground vegetable soil has the highest copy number of AOA and it has the common dominant species with maize field--Nitrososphaera_sp._JG1. These bacteria may play a major role in the process of nitrification.â‘£The copy number of nirS, nirK and nosZ were highest in greenhouse field. The composition ofthis three denitrifying microbialwere similarity but the proportion of them were differently. The dominant species of nirS in the open-ground vegetable field were Rhodanobacter_sp._D206a and Pseudomonas_sp._I-Bh4-8（Pseudomonassp.） In the microbial groups which can be identified, Pseudomonas_fluorescens_group was the predominant denityification bacteria of nir S in greenhouse field. In the three types ofsoils Achromobacter_cycloclastes（colorless spp.）, Bosea_sp._MF18,and Bradyrhizobiaceae_bacterium_D214 were based, but each of them has different proportion in solis.In type of nosZ,Achromobacter_cycloclastes and Sinorhizobium_fredii_group were main components.Nitrifier Denitrification and Nitrifier Nitrification contributing the majority of N2 O emissions within the short term of applications after fertilization with ammonium N in the calcareous vegetables field, each proportion of them was 55-61% and 26-32% of N2 O emission flux。In this period the copy number and diversity of nitrifier were higher but the copy number of Nitrosospira_sp and Nitrosospira_sp._EnI299 in AOB were decreased. They may play a leading role in the nitrification. But as the incubation continued, the contribution of Nitrifier Nitrification to N2 O emissions was lower than before（23-26% of N2 O emission flux）. From the middle of incubation, the contribution of Nitrifier Denitrification to the N2 O emissions was droped sharply（only 9% of N2 O emission flux）, while the contribution of denitrification increased significantly（from 9% at beginning to 67% in the end）. Accordingly, the copy number of nir K and nosZ was increased and the diversity of nirS was added too with the incubation continues. The species which can be identified in different periods were still Achromobacter_cycloclastes, Bradyrhizobiaceae_bacterium_D214, Bradyrhizobium_sp._GSM-471 and Bradyrhizobium_sp._ORS₂78,they may occupied the main advantage in the denitrification and t had little change among incubation process.The N2 O emissions from the acidified vegetable soil after ammonium N was applied at the primary period of incubation was mainly from denitrification（50-51% of N2 O emission flux）. But with the prolonging of incubation, the relative contribution of denitrification was decreased（in the middle was 34%,in the end was 17%）. In the incubation, the copy number of nirS and nirK were increased and the diversity of nosZ was added too. Nir K may play an important role in denitrification and in the dominant species which could be identified was Nitrosomonas_sp._TA-921i-NH4 and Nitrosospira_tenuis, they perhaps had an important impact on N2 O emissions. After the primary period of incubation, the relative contribution of NiD was increased gradually., which took the place of denitrification and became the major contribution of N2 O emissions. In this process, maybe both of AOB and AOA were participated in, the dominant species of them were Nitrosospira_sp, Nitrosospira_sp._PJA1, Nitrosospira_sp._Wyke2 and Nitrososphaera_unclassified. However, because of the low soil pH, the activity of Nitrososphaera_unclassified may be higher than others, and do more to the contribution of N2 O emissions. The N2 O emissions from the acidified vegetable soil after ammonium N was applied at 18 days of incubation was mainly from denitrification（50-51% of N2 O emission flux）.The N2 O emissions of other processes including Heterotrophic nitrification in acidic soil after ammonium N was applied during 18 days were more higher than the contribution of calcareous soils（5.5%）, which may related with the increase of the copy number of（AOA / 16 S rRNA）at last and the diversity of AOA decreased but a proportion of the dominant species were increased.The dominant species in acidic vegetable AOB were Nitrosospirahe and Nitrosomonadacea, but in calcareous soils the dominant species in the AOB were Nitrosospira and Nitrosomonas, which may be related to the lower nitrification rate in acid soils. The copy number and diversity of denitrifier in acid soils were lower than them in the calcareous soils and the composition of some species at genus level were higher in acid soils. But in calcareous soils the dominant species were iform relatively, some belong to a particularly high proportion of the situation did not occur.