Nitrification inhibitors effectively target N₂ O-producing Nitrosospira spp. in tropical soil

The nitrification inhibitors (NIs) 3,4-dimethylpyrazole (DMPP) and dicyandiamide (DCD) can effectively reduce N₂ O emissions; however, which species are targeted and the effect of these NIs on the microbial nitrifier community is still unclear. Here, we identified the ammonia oxidizing bacteria (AOB) species linked to N₂ O emissions and evaluated the effects of urea and urea with DCD and DMPP on the nitrifying community in a 258 day field experiment under sugarcane. Using an amoA AOB amplicon sequencing approach and mining a previous dataset of 16S rRNA sequences, we characterized the most likely N₂ O-producing AOB as a Nitrosospira spp. and identified Nitrosospira (AOB), Nitrososphaera (archaeal ammonia oxidizer) and Nitrospira (nitrite-oxidizer) as the most abundant, present nitrifiers. The fertilizer treatments had no effect on the alpha and beta diversities of the AOB communities. Interestingly, we found three clusters of co-varying variables with nitrifier operational taxonomic units (OTUs): the N₂ O-producing AOB Nitrosospira with N₂ O, NO₃ ^- , NH₄ ⁺ , water-filled pore space (WFPS) and pH; AOA Nitrososphaera with NO₃ ^- , NH₄ ⁺ and pH; and AOA Nitrososphaera and NOB Nitrospira with NH₄ ⁺ , which suggests different drivers. These results support the co-occurrence of non-N₂ O-producing Nitrososphaera and Nitrospira in the unfertilized soils and the promotion of N₂ O-producing Nitrosospira under urea fertilization. Further, we suggest that DMPP is a more effective NI than DCD in tropical soil under sugarcane.