Effects of nitrification inhibitors on gross N nitrification rate, ammonia oxidizers, and N2O production under different temperatures in two pasture soils

Australian pasture soil for cattle and sheep industries constitutes the principal land use with considerable N fertilizer consumption, which is one of the causes of local environmental problems. Nitrification plays a key role in regulating soil inorganic N concentration and its environmental diffusion. The effects of different nitrification inhibitors (NIs) on gross N nitrification (ngᵣₒₛₛ) rate and N₂O production under different temperatures in pasture soils remain unclear. A laboratory incubation experiment was conducted to determine the effect of NIs (dicyandiamide [DCD], 3,4-dimethylpyrazole phosphate [DMPP], and 3-methylpyrazol and 1H-1,2,4-triazol [3MP + TZ]) on N₂O emissions, ngᵣₒₛₛ and net N nitrification (nₙₑₜ) rates, and the abundance of ammonia oxidizers, namely, ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB), in two Australian pasture soils incubated at temperatures of 15, 25, and 35 °C. All NIs reduced both ngᵣₒₛₛ and nₙₑₜ rates and N₂O production rate from the two pasture soils but to different extents. The inhibitory rates of NIs on ngᵣₒₛₛ and nₙₑₜ reached 6.80–63.8 and 5.91–62.3%, respectively, whereas that on N₂O production rate totaled 4.5–41.4% in the tested soils. NIs reduced nitrification and N₂O production by inhibiting the growth of AOB rather than AOA. The inhibitory effects of NIs were temperature-dependent, that is, decreasing with increasing temperature from 15 to 35 °C. In general, DMPP performed better than DCD and 3MP + TZ at 15 and 35 °C, whereas DCD performed more effectively than the other two NIs at 25 °C. Our results suggest that the utilization of NIs will depend on the conditions present, especially soil temperature. Additionally, AOB is the target of inhibition when mitigating nitrification and N₂O emission by applying NIs in pasture soils.