Emissions of nitrous oxide (N2O) affected by pH-related nitrite accumulation during nitrification of N fertilizers

The potential of nitrogen (N) fertilizer forms to mitigate N2O emissions from agricultural fields is not very well understood. It is known that both reduced and oxidized N forms can be beneficial, depending on site-specific edaphic and climatic conditions. In the present study four laboratory incubation experiments were conducted under conditions that should minimize denitrification to investigate N2O emissions during the nitrification of solid N fertilizers. It was found that on two loamy sand soils with pH values slightly above pH 5, urea caused 2.7-3.8 fold higher cumulative N2O emissions than ammonium sulphate (AS). N2O emission rates from urea were accompanied by elevated soil nitrite levels, while those of AS were not. The direct addition of a NaNO2 solution revealed an exponential relationship between soil nitrite levels and cumulative N2O emissions, indicating that soil nitrite induces N2O emissions, probably by nitrite reduction via nitrifier denitrification. When the soil pH was increased to 6.3 (soil from a long-term liming experiment) nitrite accumulation during nitrification was observed for both N forms. Cumulative N2O emissions did not differ between N forms on the latter soil, but N2O emission kinetics were faster for urea. Cumulative N2O emissions from AS at soil pH 6.3 were significantly higher than those of AS at pH values around 5. It was concluded that i) nitrite accumulation during nitrification of reduced N fertilizer forms can significantly contribute to N2O emissions under non-denitrifying conditions, ii) the risk of nitrite accumulation increases with higher soil pH values, and iii) that the alkalizing hydrolysis of urea increases the risk of nitrite accumulation and thus higher N2O emissions as well. This mechanism would explain why urea causes higher N2O emissions than fertilizers in the form of ammonium salts.