nosZI-Type N2O Reducer Communities in Three Macrophyte Rhizospheres and Denitrification Potential in Phragmites australis-Associated Sediments of a Saline Eutrophic Lake

Shallow eutrophic wetlands are a significant source of nitrous oxide (N2O). Mitigation of N2O emission largely depends on nitrous oxide reductase (NosZ), which is carried by both canonical denitrifiers and non-denitrifying N2O reducers. We characterized nosZI-type N2O reducers in rhizosphere and bulk sediment of a shallow saline eutrophic lake in northern China, and assessed potential denitrification and N2O production in Phragmites australis-associated and bulk sediments, with the aim of identifying taxa potentially involved in N2O reduction. The abundance of nosZI genes and the diversity of nosZI-type N2O reducers were assessed in sediments associated with three emergent macrophytes and bulk sediments using qPCR and clone library analysis. Potential activities were measured using acetylene inhibition and slurry incubations for the P. australis–sediment system only. Bulk sediment showed a higher potential denitrification rate but a lower potential N2O production rate than the rhizosphere of P. australis (p < 0.05), corresponding to a higher ratio of N2O/(N2O + N2) in the rhizosphere. In this saline and eutrophic sediment, nosZII-type N2O reducers were not amplified with the primers used, whereas nosZI-type N2O reducers were detected in all samples, with higher copy numbers of the nosZI gene in the bulk sediment than in rhizospheres of the three macrophytes (p < 0.05). Within the resolution of the clone library, bulk sediment nosZI-type communities were dominated by a Halomonas-related lineage (relative abundance, 23.9%) within Gammaproteobacteria, along with Hydrogenophaga (18.2%), Rhodocyclaceae (12.5%), and Thiobacillus (8.0%) of Betaproteobacteria. In contrast, the same Halomonas-related lineage (37.0–66.7%) dominated all three macrophyte rhizospheres. An isolated salt-tolerant strain, Halomonas sp. W13, reduced N2O under the tested conditions. These findings indicate that rhizosphere and bulk sediments harbor distinct nosZI-type communities, with denitrification potential specific to the P. australis–sediment system.