The nitrification inhibitor DMPP preferentially suppresses ammonia-oxidising bacteria and reduces nitric oxide emissions in an archaeal-dominated grassland soil.

Aims Although ammonia-oxidising archaea (AOA) dominate nitrifier communities in many agricultural soils, the contribution of the less abundant ammonia-oxidising bacteria (AOB) to nitrogen oxide emissions, particularly nitric oxide, remains poorly understood. This study investigated the roles of AOA and AOB in regulating nitric oxide (NO) and nitrous oxide (N2O) emissions from a urea-fertilised grassland soil and evaluated the nitrification inhibitors dicyandiamide (DCD) and 3,4-dimethylpyrazole phosphate (DMPP). Methods A 28-day soil incubation experiment was conducted using a denitrification incubation system to continuously monitor NO and N2O emissions. In parallel, a destructive incubation experiment was performed to determine the dynamics of ammonium (NH-N) and total oxidised nitrogen (TOxN) and to assess changes in the abundance of AOA and AOB following urea application with or without DCD or DMPP. Results AOA accounted for approximately 66% of the total ammonia-oxidising community. Neither inhibitor significantly affected AOA abundance, whereas both suppressed AOB, with DMPP showing stronger and more persistent inhibition than DCD. DMPP reduced NO emissions by 16 to 75%, whereas N2O emissions decreased by only 0.2 to 27%. The greater reduction in NO, together with the preferential suppression of AOB, suggests that low-abundance AOB contributed disproportionately to NO production despite the numerical dominance of AOA. Conclusions These findings suggest that AOB can contribute disproportionately to NO emissions despite their lower abundance relative to AOA. The preferential suppression of AOB by DMPP was associated with a substantial reduction in NO emissions, highlighting AOB-associated processes as a potential target for mitigating reactive N losses from fertilised soils.