Investigations to analyze the nature of nitrification in acid soils were carried out in the laboratory using 2 soil types from a tea field. The optimum temperature for the nitrification activity in these soils was 25~ and the activity was inhibited above 35"C, suggesting that the nitrifying bacteria became adapted to the soil conditions. The optimum NH4+-N concentrations for nitrification ranged between 20 and 200 mg N/100 g of soil and the activity decreased at a concentration above 300 mg N. The effect of acidity on nitrification was studied by using soil samples amended with various amounts of CaC03. Although the pattern of NO2-production changed, the rate of ammonia oxidation was not influenced by the addition of CaCO3. The nitrification activity was completely inhibited by the addition of nitrapyrin or acetylene. These results suggest that acid-tolerant or acidophilic autotrophic nitrification occurs in acid tea soils in Japan.

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