Diversity Survey In Rhizasphere Of Diazotroph In The Exotic Invasive Species Spartina Alterniflora And Two Native Species(Phragmites Australis And Scirpus Mariqueter) In The Wetlands At Chongming Dongtan In The Yangtze River Estuary

The invasive species of Spartina alterniflora, as an ‘ecosystemengineer’ was introduced into many places in China, now, had rapidlyand widely invaded into the Chongming Dongtan wetland since1990s.Microorganisms are important components of wetland ecosystems,performing a range of vital ecosystem functions, especially nitrogenfixation microorganisms. But, we have not known how nitrogen fixationmicroorganisms have an influence on the invasion of Spartinaalterniflora in Chongming Dongtan. In order to study the nitrogen-fixingbacterial diversity and abundance in rhizosphere of invasive plantsSpartina Alterniflora growing in different tidelands of ChongmingDongtan Wetlands for different seasons, we selected6samplescollection points, which distributed on the tidal zones with plant and theblare flat zone without plant in Chongming Dongtan wetlands. The tidalzones contain low-tide zone, mid-tide zone and high-tide zone. TotalDNA was extracted from soil samples, and nifH gene-based PCR wasperformed. A comparative study of diversity of microorganisms inrhizosphere soils of invasive exotic species (Spartina alterniflora) andtwo native species (Phragmites australis and Scirpus mariqueter) wasconducted by denaturing gradient gel electrophoresis(DGGE) of thesePCR products, representing nitrogen fixation microorganisms inrhizosphere soils. Results showed that diversity and abundance ofnitrogen fixation bacterial community were different in the rhizosphereof Spartina alterniflora, Phragmites australis and Scirpus mariqueter inthe low-tide zone, mid-tide zone and high-tide zone in ChongmingDongtan wetlands.. We found that, in summer, rhizosphere bacterialcommunities in mid-tide zone had higher abundance than those inlow-tide zone and high-tide zone. A higher diversity of bacteria in therhizosphere soils of these plants was observed in summer than in other seasons. The average diversity index of nitrogen fixation bacterialcommunity was calculated and was found to be the highest in mid-tidezone. The abundance and diversity of bacterial community in invasivespecies Spartina alterniflora rhizosphere were significantly higher than inthe local plants P. australis and S.mariqueter, and the abundance anddiversity of S.mariqueter rhizosphere bacteria in summer slightly higherthan in other seasons. Also the abundance and diversity of S. alterniflorarhizosphere bacteria were significantly higher in tidal soils than those inthe mudflat soils. Overall，these results indicated the tides had a stronginfluence on the abundance and diversity of the plants rhizospherenitrogen fixation bacteria. And the exotic species Spartina alterniflorahad a favorable influence on soil microbial community in wetland. Wereasoned that high diversity and abundance nitrogen fixation bacteriain rhizosphere maybe were an important for the invasion of Spartinaalterniflora. A higher diversity of nitrogen fixation bacteria in therhizosphere soils of these plants observed in summer revealed that thesummer is more beneficial for bacteria living in vicinity of plant roots. Thefact that the flourishing Spartina rhizosphere was associated with largebacteria assemblage suggested that the high diversity and abundanceof nitrogen fixation microorganisms might be identified as one of theimportant factors of Spartina invasion mechanisms.