Interactive effects of ammonium application rates and temperature on nitrous oxide emission from tropical agricultural soil

Emissions of nitrous oxide (N ₂ O), a potent greenhouse gas, from agricultural soil have been recognized to be affected by nitrogen (N) application and temperature. Most of the previous studies were carried out to determine effects of temperature on N ₂ O emissions at a fixed N application rate or those of N application rates at a specific temperature. Knowledge about the effects of different ammonium (NH ₄ ⁺ ) application rates and temperatures on N ₂ O emissions from tropical agricultural soil and their interactions is limited. Five grams of air-dried sandy loam soil, collected in Central Vietnam, were adjusted to 0, 400, 800 and 1200 mg NH ₄ -N kg ^–1 soil (abbreviated as 0 N, 400 N, 800 N and 1200 N, respectively) at 60% water holding capacity were aerobically incubated at 20°C, 25°C, 30°C or 35°C for 28 days. Mineral N contents and N ₂ O emission rates were determined on days 1, 3, 5, 7, 14, 21 and 28. Cumulative N ₂ O emissions for 28 days increased with increasing NH ₄ ⁺ application rates from 0 to 800 mg N kg ^–1 and then declined to 1200 mg N kg ^–1 . Cumulative N ₂ O emissions increased in the order of 35°C, 20°C, 30°C and 25°C. This lowest emission at 35°C occurred because N ₂ O production was derived only from autotrophic nitrification while other N ₂ O production processes, e.g., nitrifier denitrification and coupled nitrification-denitrification occurred at lower temperatures. More specifically, cumulative N ₂ O emissions peaked at 800 N and 25°C, and the lowest emissions occurred at 1200 N and 35°C. In conclusion, N ₂ O emissions were not exponentially correlated with NH ₄ ⁺ application rates or temperatures. Higher NH ₄ ⁺ application rates at higher temperatures suppressed N ₂ O emissions.