Flood-Induced Shifts in Nitrogen Speciation Alter Nitrogen-Transforming Microbial Communities and Functional Potentials in a Shallow Lake Ecosystem

Nitrogen cycling in freshwater lakes is mediated by diverse microbial communities, yet how nitrogen-transforming communities respond to flood perturbations across multiple habitats remains poorly characterized. In this study, we investigated nitrogen forms and associated microbial communities in Baiyangdian Lake before (Phase 1, May) and after (Phase 2, August) a major summer flood, using 16S rRNA gene sequencing and Phase 1 metagenomics. Flooding was associated with increased water temperature and dissolved oxygen, and moderate declines in nitrogen and phosphorus concentrations, consistent with a dilution-driven effect. Key nitrogen-cycling taxa showed apparent abundance shifts between phases. Baseline metagenomics revealed comprehensive nitrogen cycling functional potential across water, sediment, and soil habitats, with denitrification genes most abundant in sediments, nitrification genes in the water column, and nitrogen fixation genes in soils. Temperature, dissolved oxygen, and nitrogen forms were identified as primary environmental drivers of community variation. Co-occurrence network analysis identified modular organization of nitrogen-cycling taxa. These findings demonstrate that flood events can alter lake nitrogen regimes through dilution, with implications for shallow lake nutrient management under changing hydrological regimes.