Ammonia oxidizing bacteria dominate soil nitrification under different fertilization regimes in black soils of northeast China

Although autotrophic ammonia oxidizing archaea (AOA) and bacteria (AOB) and comammox coexist in soils, their abundances and contributions to nitrification are not well addressed, especially in arable black soils. In this study, using the nitrification inhibitors acetylene and octyne, the contributions to nitrification potential provided by different ammonia oxidizers and heterotrophic nitrifiers in four long-term fertilization treatments, i.e. , no fertilization (NoF), manure fertilization (M), chemical fertilization (CF), and chemical fertilization plus manure (CFM), were investigated in samples collected from the northern, middle and southern black soil regions of northeast China. The amoA gene abundances of AOA, AOB, comammox Clade A and comammox Clade B in these soils were quantified by qPCR. The results showed that the amoA gene abundances of the three ammonia oxidizers tended to increase from the northern to southern sites, while the opposite trend was observed in the ratios of AOA/AOB and comammox Clade B/Clade A; the different fertilization treatments significantly influenced the amoA abundances, and the effects varied with the ammonia oxidizers and across the sites. Autotrophic ammonia oxidizers accounted for 62%–100% of the gross soil nitrification potential across all soils. Among them, AOB dominated soil autotrophic nitrification in the fertilized soils, while no fertilization may have increased the nitrification potential provided by AOA plus comammox. Pairwise analysis showed that the amoA gene abundances of AOA and AOB were significantly and positively correlated with their respective nitrification-driven potentials. Correlation heatmaps indicated that soil pH and inorganic nitrogen content were the main soil factors related to nitrification activities and the abundances of the ammonia oxidizers. Our findings highlight that AOB are the dominant ammonia oxidizers contributing to nitrification in arable fertilized black soils, although AOA and comammox are highly abundant. • Autotrophic nitrification is the main process in black soils of northeast China. • AOA and comammox dominate the autotrophic nitrification in non-fertilized acid soils. • AOB dominates the autotrophic nitrification in non-fertilized neutral soils. • Long-term fertilization increases the nitrification contributed by AOB. • The abundances and activities of AOMs are mainly driven by soil pH and inorganic N.