Ammonia-oxidizing bacteria and fungal denitrifier diversity are associated with N₂O production in tropical soils

Nitrous oxide (N₂O) production in tropical soils cultivated with sugarcane is associated with ammonia-oxidizing bacteria (AOB) and fungal denitrifiers. However, the taxonomic identities and the community diversities, compositions, and structures of AOB and fungal denitrifiers in these soils are not known. Here, we examined the effects of applying different concentrations of an organic recycled residue (vinasse: regular non-concentrated or 5.8-fold concentrated) on the dynamics of AOB and fungal denitrifier community diversity and composition and greenhouse gas emissions during the sugarcane cycle in two different seasons, rainy and dry. DNA was extracted from soil samples collected at six timepoints to determine the dynamics of amoA-AOB and nirK-fungal community diversity and composition by amplicon sequencing with gene-specific primers. Bacterial and archaeal amoA, fungal and bacterial nirK, bacterial nirS and nosZ, total bacteria (16S rRNA) and total fungi (18S rRNA) were quantified by real-time PCR, and N₂O and CO₂ emissions were measured. The genes amoA-AOB and bacterial nirK clade II correlated with N₂O emissions, followed by fungal nirK. The application of inorganic nitrogen fertilizer combined with organic residue, regardless of concentration, did not affect the diversity and structure of the AOB and fungal denitrifier communities but increased their abundances and N₂O emissions. Nitrosospira sp. was the dominant AOB, while unclassified fungi were the dominant fungal denitrifiers. Furthermore, the community structures of AOB and fungal denitrifiers were affected by season, with dominance of uncultured Nitrosospira and unclassified fungi in the rainy season and the genera Nitrosospira and Chaetomium in the dry season. Nitrosospira, Chaetomium, Talaromyces purpureogenus, and Fusarium seemed to be the main genera governing N₂O production in the studied tropical soils. These results highlight the importance of deciphering the main players in N₂O production and demonstrate the impact of fertilization on soil microbial N functions.