| The nitrogen cycle is closely related to agriculture and environment. On the one hand, crop yields increase via man-made nitrogen fertilization, but meanwhile the loss of nitrogen has led to global environmental problems. Till now most known nitrogen transforming pathways are mediated by prokaryotes. The biologically nitrogen fixation which convert nitrogen gas into ammonium, could occur as free living bacteria in soil, as loose associations with root surfaces (associative nitrogen fixation) or within highly specialised, symbiotic associations with legumes plants (symbiotic nitrogen fixation).Nitrification can directly use ammonium as substrate, meanwhile as a link to denitrification and other processes. Both bacteria and archaea are involved in this biological process. Nitrification is mediated by two mutualistic microorganisms (ammonia oxidizers and nitrite oxidizers).Maize and rice are two majoy cereal crops,and be viewed as ecosystem engineer strongly responsible for shaping the agricultural environment for cohabitating species. The rhizosphere is a critical interface supporting the exchange of resources between plants and their associated soil environment, and considerable progress is being made in understanding its ecology and evolution is key to enhancing plant productivity and ecosystem functioning. Maize rhizosphere is a great habitat for nitrogen-fixing microorganisms. Associative diazotrophic Pseudomonas stutzeri A1501, firstly isolated from rice rhizosphere, belonged to kind of plant growth-promoting rhizobacteria (PGPR), but its root colony of maize remains unknown. Thus we investigated the root colony efficiency of P. stutzeri A1501 both in sterilized soil matrix and nutrient solution medium in terms of inoculation, coupling with visible technique of scanning electron microscope (SEM).15N isotope dilution method is employed to estimate the contribution of inoculated P. stutzeri A1501 to nitrogen fixed within maize. Although PGPR have been widely used as crop inoculant, the ecological impacts might have on the rhizosphere microbiota remained unknown.Therefore this design also allowed us to examine microbial community shift in maize rhizosphere by pyrosequencing, as well as population dynamic and activities of rhizosphere microbiome by quantifying the copy numbers of fuctional gene and transcripts. Due to its different physical and chemical properties, a series of redox processes including nitrification could occur in rice rhizosphere. Most of the earlier researches focused on community diversity of nitrifiers (mainly ammonia-oxidizing microorganisms) and effect of different environmental factors on soil nitrification activity, but seldom from the view of ecophysiology of nitrifiers, especially from changes in microbial population and activity. In this thesis, by using the typical Chinese paddy soil as objects, we investigated the effect of oxygen concentration on community and activity of nitrifiers in different compartments in paddy soil profile.Firstly, maize root colony of P. stutzeri A1501 is confirmed by plate counting method combined with SEM technique.15N dilution method revealed that the percentage of nitrogen from atmosphereoccupying the total nitrogen of maize shoot was 20.1% under water stress and 31.6% in well-watered condition, respectively.Correspondingly, the amount of N fixed of maize was 0.28 g/plant when water was deficient and 0.81 g/plant when well watered. According to the nitrogen fixation efficiency estimated in this study, the percent of N-fertilizers application on maize in the field could reduce approximately 14% and 40%, under poor and well water conditions respectively.Secondly, through establishing maize microcosms, we found marked differences in the distribution of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA), and of nitrogen-fixing bacteria in the different compartments of planted maize soil indicating niche differentiation.The nifH gene abundance was significantly higher in rhizospheric soil than in surface soil, whereas the abundance of amoA genes of AOB and AOA, and bacterial and archaeal 16S rRNA gene were significantly higher in surface soil than in rhizospheric soil. Beside, the different soil compartment had significant effect on nifH, bacterial and archaeal amoAtranscript copy numbers and accounted for most of the variance in the data. Moreover, community compostion of bacteria including diazotrophs shows obvious differentiation between rhizosphere soil and surface soil samples.It indicats that in maize fieldcommunity structure, population dynamics and activity of key N-cycling fuctional groups is mainly differentiated by the soil compartments. Water management sigficantly influences productivity of maize, but shows no relationship with community structure of microorganisms in rhizosphere. However, significant effect of water treatments on population of Bacteria and Archaea in rhizosphere, as well as nifH of diazotrophs, amoA of AOB and AOA are obvious.Finally, inoculation of P. stutzeri A1501 significantly affects nifH trnscripts, also the bacterial and diazotrophic community structre in rhizophere. However, population dynamic of Bacteria and Archaea are insensitive to the P. stutzeri A1501 inoculation.Finally, in soils and roots incubations, different oxygen concentration significantly affect nitration rate in paddy soil. In soils, greater populations and transcriptional activity of AOB were found under oxygen concentration than in low oxygen concentration. While populations and transcriptional activity of AOA gradually increased during incubation process in roots samples. Different oxygen concentrations also affected populations of Nitrospira and Nitrobacter genus, but did not change its relative abundance. AOB and Nitrobacter were more active in high oxygen concentration, performing as higher abundance and transcript activity. Correlation analysis showed significant correlation between nitrification rate and transcript activities of AOB and Nitrobacter under high oxygen concentration, but not under low oxygen concentration. Our results reveal that oxygen concentration influence nitrification process and community structure and activity of nitrifiers in paddy soil, as well as the relationship between them. |
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