The complete ammonia-oxidizing bacteria (comammox) oxidize NH₃ to NO₃^– and affect key processes such as water quality safety and greenhouse gas generation and emissions. The reservoir riparian zone, characterized by periodic water level fluctuations, forms an anoxic/aerobic interface where the majority of ammonia oxidation takes place. Comammox can independently perform nitrification and their ecological distribution and diversity influence the ammonia oxidation process. The Daning River riparian zone in the Three Gorges Reservoir was designated as the study area, where we examined the spatiotemporal variations in comammox bacterial abundance and diversity within both rhizosphere and non-rhizosphere soils during the low water level period. The findings revealed the widespread presence of comammox's two branches, clades A and B, in soil derived from the Cynodon dactylon plant belt. Clade A exhibited a higher number of operational taxonomic units (OTUs) compared to clade B; however, no significant distinction was observed between rhizosphere and non-rhizosphere soils. The relative abundance of comammox OTUs increased with decreasing altitude. In August, the abundance of comammox clade B (1.17 × 10⁷ copies/g dry sediment) was significantly lower than in July (1.46 × 10⁸ copies/g dry sediment). Furthermore, it exhibited a significant decrease as exposure time increased in the riparian zone and showed a significant positive correlation with NH₄⁺-N, NO₃^–-N, and total nitrogen levels. The abundance of amoA gene in clade A demonstrated a positive correlation with ammonia-oxidizing archaea (AOA) and bacteria (AOB), while an opposite trend was observed for amoA gene abundance in clade B. Clade A adapted better to environmental disturbance, while clade B quickly responded to changes in the riparian zone. This study improves our understanding of comammox bacteria distribution and factors influencing them, providing valuable insights into nitrogen cycling in reservoir riparian zones and their resilience to climate change.
 

Comammox,Riparian zone,Soil,Nitrogen,water level

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