Influence of O2availability on NO and N2O release by nitrification and denitrification in soils

Abstract

The availability of O 2 is believed to be one of the main factors regulating nitrification and denitrification and the release of NO and N 2 O. The availability of O 2 in soil is controlled by the O 2 partial pressure in the gas phase and by the moisture content in the soil. Therefore, we investigated the influence of O 2 partial pressures and soil moisture contents on the NO and N 2 O release in a sandy and a loamy silt and differentiated between nitrification and denitrification by selective inhibition of nitrification with 10 Pa acetylene. At 60% whc (maximum water holding capacity) NO and N 2 O release by denitrification increased with decreasing O 2 partial pressure and reached a maximum under anoxic conditions. Under anoxic conditions NO and N 2 O were only released by denitrification. NO and N 2 O release by nitrification also increased with decreasing O 2 partial pressure, but reached a maximum at 0.1–0.5% O 2 and then decreased again. Nitrification was the main source of NO and N 2 O at O 2 partial pressures higher than 0.1–0.5% O 2. At lower O 2 partial pressures denitrification was the main source of NO and N 2 O. With decreasing O 2 partial pressure N 2 O release increased more than NO release, indicating that the N 2 O release was more sensitive against O 2 than the NO release. At ambient O 2 partial pressure (20.5% O 2) NO and N 2 O release by denitrification increased with increasing soil moisture content. The maximum NO and N 2 O release was observed at soil moisture contents of 65–80% whc and 100% whc, respectively. NO and N 2 O release by nitrification also increased with increasing soil moisture content with a maximum at 45–55% whc and 90% whc, respectively. Nitrification was the main source of NO and N 2 O at soil moisture contents lower than 90% whc and 80% whc, respectively. Higher soil moisture contents favoured NO and N 2 O release by denitrification. Soil texture had also an effect on the release of NO and N 2 O. The coarse‐textured sandy silt released more NO than N 2 O compared with the fine‐textured loamy silt. At high soil moisture contents (80–100% whc) the fine‐textured soil showed a higher N 2 O release by denitrification than the coarse‐textured soil. We assume that the fine‐textured soil became anoxic at a lower soil moisture content than the coarse‐textured soil. In conclusion, the effects of O 2 partial pressure, soil moisture and soil texture were consistent with the theory that denitrification increasingly contributes to the release of NO and in particular N 2 O when conditions for soil microorganisms become increasingly anoxic.