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

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Aerobic N2O production was studied in nitrifying humus from urea-fertilized pine forest soil. Acetylene and nitrapyrin inhibited both NH4+ oxidation and N2O production, indicating that N2O production was closely associated with autotrophic NH4+ oxidation. N2O 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 N2O-N produced from NH4+-N oxidized increased exponentially to 20%. There was also some evidence that N2O formation was stimulated by salts (potassium sulfate and sodium phosphates). The maximum rate of N2O-N production was 0.17 μg of N2O-N per g of soil per h. When humus was treated with NO2−, N2O evolved immediately, indicating chemical formation, but no N2O was formed on the addition of NO3−. The amount of N2O-N evolved was 0.6 to 4.6% of NO2−-N added. A high concentration of NO2− and low soil pH enhanced chemical production of N2O. There was no accumulation of NO2− during nitrification. The calculations indicated that chemical formation of N2O was not the main source of N2O during NH4+ oxidation. After the addition of inhibitors of NH4+ oxidation the soils contained NO3−, but no N2O was produced. The results suggest that enhanced autotrophic NH4+ oxidation is a potential source of N2O in fertilized acid forest soil.

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