Concurrent Observation of Several Processes of Nitrogen Metabolism in Soil Amended with Organic Materials : IV. Regulatory Effects of Ammonium- and Nitrate-Nitrogen on Denitrification and N_2 Fixation

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The changes in the rates of denitrification in soil were essentially opposite to those of N_2 fixation. When denitrification occurred at a high rate, N_2 fixation was suppressed; and vice versa. Under upland moisture conditions (60% maximum water-holding capacity), denitrification hardly occurred when the level of NO_3-N in soil was below 5.5 μg N/g soil. and its rate substantially increased with the increase of the level of NO_3-N in the rice straw (RS)-treated soils, while in the soils without organic material amendment it only slightly increased with the increase of the level of NO_3-N. The maximum rate of denitrification was as high as 65.0 μg N_2O-N/g/d in the RS-treated soils. The N_2 fixation was also stimulated by RS amendment and the maximum rate recorded was as high as 220 ng N_2-N/g/d when the level of NH_4-N in soil was below 7.8 μg N/g soil. The high concentrations of NH_4-N, ranging from 98.2 to 298.5 μg N/g soil, inhibited the initial N_2 fixation activity. The N_2 fixation activity was restored when the level of NH_4-N in soil decreased below 51.3 g N/g. However, the maximum rate of N_2 fixation would have never reached the value of 220 ng N_2-N/g/d obtained in the RS-treated soil without the application of N fertilizer.
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