Nitrous Oxide Emission Associated with Autotrophic Ammonium Oxidation in Acid Coniferous Forest Soil

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Abstract
Aerobic N 2 O production was studied in nitrifying humus from urea-fertilized pine forest soil. Acetylene and nitrapyrin inhibited both NH 4 + oxidation and N 2 O production, indicating that N 2 O production was closely associated with autotrophic NH 4 + oxidation. N 2 O production was enhanced by low soil pH; it was negligible above pH 4.7. When soil pH decreased from 4.7 to 4.1, the relative amount of N 2 O-N produced from NH 4 + -N oxidized increased exponentially to 20%. There was also some evidence that N 2 O formation was stimulated by salts (potassium sulfate and sodium phosphates). The maximum rate of N 2 O-N production was 0.17 μg of N 2 O-N per g of soil per h. When humus was treated with NO 2 − , N 2 O evolved immediately, indicating chemical formation, but no N 2 O was formed on the addition of NO 3 − . The amount of N 2 O-N evolved was 0.6 to 4.6% of NO 2 − -N added. A high concentration of NO 2 − and low soil pH enhanced chemical production of N 2 O. There was no accumulation of NO 2 − during nitrification. The calculations indicated that chemical formation of N 2 O was not the main source of N 2 O during NH 4 + oxidation. After the addition of inhibitors of NH 4 + oxidation the soils contained NO 3 − , but no N 2 O was produced. The results suggest that enhanced autotrophic NH 4 + oxidation is a potential source of N 2 O in fertilized acid forest soil.