How Much Nitrous Oxide Comes From Nitrification Under Different Conditions.

Nitrous oxide (N₂O) is primarily produced by the microbially-mediated nitrification and denitrification processes in soils. It is commonly accepted that nitrous oxide is primarily produced in soil by the anaerobic activities of microorganisms during denitrification. There is very limited information about how much N₂O derived from nitrification. It has been suggested that up to 70% N₂O in Australian soils are derived from nitrification. Due to lack of experimental data most of processed based models use the fixed proportion nitrification rate to estimate nitrification induced N₂O emissions, for example 0.24% by DNDC and 2% by DayCent Model. However, it is evident that N₂O emission from nitrification in soil is not a fixed factor of nitrification and it is highly variable both in time and space. It is influenced by soil physical, chemical and microbial properties as well as soil moisture and temperature. It is necessary to quantify the effects of soil and environmental variables on N₂O production from nitrification. This information will help to more accurately evaluate and predict N₂O emission from the field following the addition of NH₃ based fertilizers and will help to develop management practices that reduce emissions. Therefore, we undertook a laboratory incubation experiment to investigate how much N₂O is produced from nitrification, and how do soil moisture and temperature influence N₂O leakage from nitrification.

In a laboratory incubation study, we examined N₂O production under different temperatures (15◦C, 25◦C and 35◦C) and moistures (40%, 60% and 85% water-filled pore space (WFPS)) using ¹⁵N labeled ammonium and nitrate, acetylene (1% vol/vol) was used as nitrification inhibitor to block nitrification process.

The results showed that at 60%WFPS, the contribution of nitrification to N₂O production from nitrification increased as temperature increased from 7.2%-28.2%, however, at 85%WFPS it decreased as moisture increased from 28.1%- 6.4%. The ratio of NO₃^- and N₂O increased as temperature and moisture increased, except 35◦C, 85% WFPS. At 35◦C, 85%WFPS, denitrification dominated over nitrification for N₂O production due to presumably low O₂ availability.