Water level fluctuations (WLFs) are typical characteristic of floodplain lakes and dominant forces regulating the structure and function of lacustrine ecosystems. The sediment diazotrophs play important roles in contributing bioavailable nitrogen to the aquatic environment. However, the relationship between the diazotrophic community and WLFs in floodplain lakes is unknown. In this paper, we carried out a comprehensive investigation on the alpha diversity, abundance, composition and co-occurrence network of the sediment diazotrophs during different water level phases in Poyang Lake. There were no regular variation patterns in the alpha diversity and abundance of the sediment diazotrophs with the water level phase transitions. The relative abundance of some diazotrophic phyla (including Alphaproteobacteria, Deltaproteobacteri, Euryarchaeota, and Firmicutes) and genera (including Geobacter, Deferrisoma, Desulfuromonas, Rivicola, Paraburkholderia, Methylophilus, Methanothrix, Methanobacterium, and Clostridium) was found to change with the water level phase transitions. The results of ANOSIM, PerMANOVA, and DCA at the OTU level showed that the diazotrophic community structure in the low water level phase was significantly different from that in the two high water level phases, while there was no significant difference between the two high water level phases. These results indicated that the diazotrophic community was affected by the declining water level in terms of the composition, while the rising water level contributed to the recoveries of the diazotrophic community. The diazotrophs co-occurrence network was disrupted by the declining water level, but it was strengthened by the rising water level. Moreover, redundancy analysis showed that the variation of the diazotrophic community composition was mostly related to sediment total nitrogen (TN) and total phosphorous (TP). Interestingly, the levels of sediment TN and TP were also found to vary with the water level phase transitions. Therefore, it might be speculated that the WLFs may influence the sediment TN and TP, and in turn influence the diazotrophic community composition. These data can contribute to broadening our understanding of the ecological impacts of WLFs and the nitrogen fixation process in floodplain lakes.

The sub-lakes of Poyang Lake have significant ecological value. To date, the diazotrophs in sub-lakes are unknown. Moreover, no study has simultaneously focused on diazotrophic communities in the water and sediment in natural freshwater ecosystems. In this study, we investigated the diazotrophic alpha diversity, structure, abundance, molecular ecological network, and community assembly mechanism in the water and sediment of sub-lakes using Illumina MiSeq sequencing and a quantitative polymerase chain reaction assay. The results showed that the sediment had a greater diversity of diazotrophs than the water. Proteobacteria and Spirochaetes were the dominant diazotrophic phyla in the water, whereas Proteobacteria was the dominant diazotrophic phylum in the sediment. There were significant differences in the composition of diazotrophic communities between the water and sediment. The sediment had a more complex co-occurrence network of diazotrophs than the water. Deterministic processes dominate the community assembly of diazotrophs in both the water and sediment of the sub-lakes, and the relative role of deterministic processes was stronger for sediment than water. Our study is the first to reveal the differences in the diazotrophic communities between the water and sediment in natural freshwater ecosystems and provides the fundamental scientific datasets for understanding the nitrogen fixation process in sub-lakes.