Effect of soil temperature, moisture and NH₄⁺-N concentration on nitrification and nitrification-induced N₂O emission

Nitrification is a major source of N₂O. However, there is little information on how environmental and soil variables affect N₂O emission during nitrification. A fixed fraction rate is often used to estimate N₂O emission from soil induced by nitrification in most available models. To that end, an incubation experiment was conducted to investigate the effect of soil moisture, temperature and NH₄⁺-N concentration on nitrification and nitrification-induced N₂O emission in acidic sandy-loam soils in southeastern Australia. The Michaelis-Menten equation was used to express nitrification dynamics while the Least Square method was used to derive the maximum velocity of nitrification and N₂O fraction of nitrification. A series of algorisms were proposed to describe the relationships between nitrification velocity/N₂O production and the driving factors of NH₄⁺-N concentration, soil moisture and temperature. Results show exponentially enhanced nitrification velocity with increasing soil temperature. Nitrification velocity increases when soil water-filled porosity (WFPS) increases from 20% to 40%, reaches its peak at around 40%, and then declines at 60% WFPS. NH₄⁺-N concentration is negatively correlated with nitrification velocity. By fitting with Least Square, a maximum reaction velocity (Vmax) is achieved at 6.67 mg·kg^-1·d^-1 for the sandy-loam soil. N₂O emission fraction of nitrification declines with increasing incubation temperature. Soil NH₄⁺-N concentration is slightly positively correlated with soil nitrification emitted N₂O. Under 20% and 40% WFPS, measured N₂O emission fraction of nitrification range is 0.43%~1.50%, with a maximum fraction of 3.03% obtained by fitting Least Square. However, this method cannot reliably assess the impact of soil WFPS on N₂O emission fraction of nitrification,because N₂O emission increases exponentially when WFPS increases to 60%, indicating that soil denitrification might occur at 60% WFPS.