Salinity and redox-driven niche differentiation of ammonia-oxidizing microbes in a saline-alkaline river system

The ammonia oxidation process constitutes a critical step in the nitrogen cycle within river ecosystems. Ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) are the major contributors to this process; however, their relative contributions differ substantially across different environments, particularly in saline-alkaline regions. To investigate the differences in ammonia oxidation processes between soil and sediment across various riparian zones in the Yinbei Irrigation District in Ningxia, samples were collected from five representative riparian types along the Third Drainage Ditch: a gravel-reed mixed zones, a reed zones, a high-salt Kochia scoparia zones, an Iris lactea embankment zones, and a bare soil zone. The potential nitrification rate (PNR) and associated environmental factors were quantified, and the community structure of microorganisms was analyzed by metagenomic sequencing, while the abundances of AOA and AOB functional genes (amoA) were quantified by quantitative PCR (qPCR). Through a multi-dimensional investigation of the ammonia oxidation process in riparian zones and sediments of saline-alkali wetlands, this study provided critical evidence for the mechanisms underlying habitat-specific nitrogen transformations. Pronounced physical and chemical differences were observed between sediments and riparian zones. Whereas the sediments exhibited strong reducing conditions, the riparian zones were characterized by pronounced saline-alkali stress. PCoA analysis revealed a clear separation between AOA and AOB communities, with saline-alkali-tolerant AOA predominating in riparian zones and AOB predominating in sediments. The strong reducing condition impedes the activity of AOB in sediments, thereby resulting in the accumulation of NH₄⁺ -N. AOA contributed 67-68.9 % of the total PNR in riparian zones, thereby driving substantial NO₃^--N production. Seasonal variations exhibited no significant influence on the partitioning of microbial functions, which were jointly regulated by the saline-alkali gradient and redox state. This study demonstrates that the ammonia oxidation process of saline-alkali wetlands exhibits a pronounced mechanism of habitat-specific functional differentiation: AOB in sediments are constrained by the anoxic conditions induced by elevated total organic carbon (TOC), whereas AOA in riparian zones overcome the inhibition of nitrification under high pH or electrical conductivity (EC) through evolved salt-tolerant adaptations This discovery provides a novel theoretical framework for elucidating the microbial mechanisms driving the nitrogen cycle in saline-alkaline environments.