Ammonia oxidizers are key players in the global nitrogen cycle, yet little is known about their ecological performances and adaptation strategies for growth in saline terrestrial ecosystems. This study combined ¹³C-DNA stable-isotope probing (SIP) microcosms with amplicon and shotgun sequencing to reveal the composition and genomic adaptations of active ammonia oxidizers in a saline-sodic (solonetz) soil with high salinity and pH (20.9 cmol_c exchangeable Na⁺ kg⁻¹ soil and pH 9.64). Both ammonia-oxidizing archaea (AOA) and bacteria (AOB) exhibited strong nitrification activities, although AOB performed most of the ammonia oxidation observed in the solonetz soil and in the farmland soil converted from solonetz soil. Members of the Nitrosococcus, which are more often associated with aquatic habitats, were identified as the dominant ammonia oxidizers in the solonetz soil with the first direct labeling evidence, while members of the Nitrosospira were the dominant ammonia oxidizers in the farmland soil, which had much lower salinity and pH. Metagenomic analysis of “Candidatus Nitrosococcus sp. Sol14”, a new species within the Nitrosococcus lineage, revealed multiple genomic adaptations predicted to facilitate osmotic and pH homeostasis in this extreme habitat, including direct Na⁺ extrusion/H⁺ import and the ability to increase intracellular osmotic pressure by accumulating compatible solutes. Comparative genomic analysis revealed that variation in salt-tolerance mechanisms was the primary driver for the niche differentiation of ammonia oxidizers in saline-sodic soils. These results demonstrate how ammonia oxidizers can adapt to saline-sodic soil with excessive Na⁺ content and provide new insights on the nitrogen cycle in extreme terrestrial ecosystems.

氨氧化剂是全球氮循环的关键参与者，但对其在盐碱地陆地生态系统中生长的生态表现和适应策略知之甚少。本研究将^{13 个C-DNA 稳定同位素探测 (SIP) 微观世界与扩增子和鸟枪测序相结合，揭示了高盐度和 pH 值（20.9 cmol} _c可交换）盐碱 (solonetz) 土壤中活性氨氧化剂的组成和基因组适应性。Na ⁺ kg ^-1土壤和 pH 值 9.64）。氨氧化古细菌 (AOA) 和细菌 (AOB) 都表现出强烈的硝化活性，尽管 AOB 执行了在 solonetz 土壤和从 solonetz 土壤转化而来的农田土壤中观察到的大部分氨氧化。成员Nitrosococcus更常与水生生境相关联，被首次直接标记证据确定为 solonetz 土壤中的主要氨氧化剂，而Nitrosococcus成员是农田土壤中的主要氨氧化剂，其盐度和盐度要低得多，酸碱度。“ Candidatus Nitrosococcus sp。的宏基因组分析。Sol14”是Nitrosococcus谱系中的一个新物种，它揭示了多种基因组适应性，预计将促进这个极端栖息地的渗透和 pH 稳态，包括直接 Na ⁺挤出/H ⁺进口和通过积累相容的溶质来增加细胞内渗透压的能力。比较基因组分析表明，耐盐机制的变化是盐碱土壤中氨氧化剂生态位分化的主要驱动力。^{这些结果表明氨氧化剂如何适应 Na +}含量过高的盐碱土壤，并为极端陆地生态系统中的氮循环提供新的见解。