Nitrous oxide emission associated with autotrophic ammonium oxidation in Acid coniferous forest soil.

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 mug 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.