Effects of nitrification inhibitors on gross N nitrification rate, ammonia oxidizers, and NO 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 (n) rate and NO 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 NO emissions, n and net N nitrification (n) 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 n and n rates and NO production rate from the two pasture soils but to different extents. The inhibitory rates of NIs on n and n reached 6.80-63.8 and 5.91-62.3%, respectively, whereas that on NO production rate totaled 4.5-41.4% in the tested soils. NIs reduced nitrification and NO 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 NO emission by applying NIs in pasture soils.