Nitrification potential and ammonia-oxidizing archaeal diversity in three contrasting acidic soils of eastern China

Acidic soils are widespread and are further acidified by intensive nitrogen fertilization. However, the ammonia-oxidizing microorganisms that drive nitrification under low pH remain insufficiently understood. Here, we investigated nitrification potential and the responsible nitrifying microorganisms in three representative acidic soils of eastern China (forest soil, red paddy soil, and tea garden soil), sampling both surface (0-10 cm) and subsurface (10-20 cm) layers. Potential nitrification rates were quantified using microcosms, and nitrifier communities were characterized by quantitative PCR of functional genes and Illumina amplicon sequencing of 16S rRNA and archaeal amoA genes, coupled with phylogenetic and scatter plot analyses. All soils exhibited measurable nitrification potential (0.69-2.97 mg NO₃^--N kg^-1 d^-1) with pronounced variation among land uses and depths. Ammonia-oxidizing archaea (AOA) dominated ammonia oxidizers in the three sampled soils, with Nitrosotalea prevailing in forest and tea garden soils and Nitrososphaera dominating the red paddy soil. AOA amoA gene abundances were consistently orders of magnitude higher than those of ammonia-oxidizing bacteria and often increased during incubations. Nitrospira was the major nitrite-oxidizing bacterium, and scatter plot correlations supported coordinated associations between AOA and Nitrospira consistent with rapid nitrite turnover and complete nitrification. AOA communities showed strong habitat- and depth-specificity, and amoA sequencing revealed far greater diversity than 16S rRNA profiling, including a distinct AOA-like lineage potentially representing an undescribed archaeal clade. These findings highlight substantial nitrification potential in acidic soils and identify AOA-centered nitrifying consortia as key drivers with implications for nitrate loss and ongoing acidification.