How nitrification-related N2O is associated with soil ammonia oxidizers in two contrasting soils in China?

As a key process contributing to N2O emissions, nitrification is regulated by soil microbes and mainly affected by soil pH, NH3 availability, temperature and O2 availability. Current knowledge gaps include how nitrification-related N2O is associated with soil microbes in different pH soils. In the current study, a microcosm incubation experiment was conducted with two contrasting soils of different pH (5.08, 8.30) under controlled conditions. The soils were amended with ammonium sulphate ((NH4)2SO4, 50 mg N kg-1) combined with or without nitrification inhibitors and incubated under 20 °C, 65% water hold capacity (WHC) for three weeks. N2O fluxes, mineral nitrogen (N) concentrations and ammonia oxidizers populations were measured during the incubation to investigate the correlations of nitrification-related N2O with ammonia oxidizers. The nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) was used to inhibit nitrification albeit to various inhibition effects with different soils. Acetylene (0.1% v/v C2H2), an inhibitor of AOA and AOB ammonia monooxygenase (AMO), was used to distinguish N2O emissions by nitrifiers and denitrifiers. 1-octyne (5 μM aqueous), a selective specific AOB inhibitor, was used to assess the relative contributions of AOA and AOB to N2O emissions. The results showed that N2O yield for AOA and AOB varied with soil pH. AOB was the key microbial player in alkaline soil, contributing about 85% of nitrification-related N2O. Conversely, about 78% of nitrification-related N2O was contributed by AOA in acidic soil. Furthermore, there was a significant and positive relationship between mineral N (NO2-, NO3-), AOA and AOB populations and nitrification-related N2O in alkaline soil. However, in acidic soil, NO3- concentration and AOA had significantly positive relationships with nitrification-related N2O. To conclude, soil pH was a key factor affecting the contribution of ammonia oxidizers to nitrification-related N2O emissions. AOA-related N2O production dominated at low pH (5.08), while AOB-related N2O was favored in alkaline soil (pH 8.3).