Nitrous oxide production in aerobic soils under varying pH,temperature and water content

Laboratory studies were conducted to evaluate the effect of soil pH, temperature and water content on the rate of nitrification and on the amount of N₂O evolved from samples of Plano silt loam soil. The rate of nitrification of added NH₄⁺-N increased with increasing soil pH (4.7, 5.1 and 6.7), temperature (10, 20 and 30°C) and water content (0.1, 0.2 and 0.3 m³ m^?3). At soil water contents of 0.1 and 0.2 m³ m^?3, corresponding to 18 and 36% water-filled pore space, respectively, N₂O evolution was proportional to NO₃^? production. Approximately 0.1–0.2% of the nitrified N was evolved as N₂O-N. At 0.3 m³ m^?3 water content (54% water-filled pore space) and 20 and 30°C, the ratio of N₂O-N evolved to N nitrified was significantly higher (range of 0.3–1.1%).An additional experiment was conducted using diurnally fluctuating temperatures (10–30°C). The pattern of N₂O evolution was markedly different when the system was sampled at 10 and 30°C than at 20°C. The apparent N₂O emission rates were approximately equal for 12-h periods during which the temperature increased from 10 to 30°C or decreased from 30 to 10°C. In contrast, the apparent N₂O emission rates were significantly lower for the 12-h period when the incubation flasks were sampled at 20°C following the daily minimum temperature compared to the 12-h period when the samplings were at 20°C following the daily maximum temperature. This provides additional evidence that temperature fluctuation in the surface soil is a factor in-observed diurnal variations in N₂O emissions under field conditions.Our findings indicate that an interaction of three factors (soil pH, temperature and water content) affects the amount of N₂O evolved during nitrification in soils. In relatively dry soils, estimated N₂O production of ca. 0.1–0.3% of the N nitrified may be sufficiently accurate. Much higher N₂O output can be expected following rainfall or irrigation. Diurnal variability in N₂O fluxes from soils due to fluctuating temperature is an additional uncertainty in quantifying N₂O production in field soils.