Reduced nitrification by biochar and/or nitrification inhibitor is closely linked with the abundance of comammox Nitrospira in a highly acidic sugarcane soil

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An incubation experiment was conducted to examine combined or individual impacts of biochar and/or 3,4-dimethylpyrazole phosphate (DMPP) on the net nitrification rates, the abundances of ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB, a halophilic β-AOB) and TAO100 (a non-halophilic γ-AOB), and comammox Nitrospira in a highly acidic sugarcane soil. The net nitrification rates were markedly suppressed by biochar and/or DMPP application. Urea addition stimulated but DMPP alone significantly inhibited the growth of AOB, TAO100, total comammox Nitrospira, and clade A. The application of biochar alone increased AOA and AOB abundances, but reduced those of TAO100, total comammox Nitrospira, and clade A. Likely in the combined biochar and DMPP treatment, DMPP was absorbed by biochar then reducing its efficacy in suppressing the growth of AOB, total comammox Nitrospira, and clade A. Total comammox Nitrospira and clade A abundances were significantly correlated with NO3− concentrations (P < 0.001). Furthermore, biochar and/or DMPP addition had a significant effect on the composition of comammox Nitrospira community by PCoA. Phylogenetic analysis showed that all main comammox Nitrospira community fell within the clade A. Overall, our findings proved that the dynamics of total comammox Nitrospira resulting from the application of biochar and/or DMPP played a pivotal role in controlling soil nitrification in the highly acidic sugarcane soil with high N input.