Peak emissions of NO and N 2 O are often observed after wetting of soil. The reactions to sudden changes in the aeration of cultures of nitrifying and denitrifying bacteria with respect to NO and N 2 O emissions were compared to obtain more information about the microbiological aspects of peak emissions. In continuous culture, the nitrifier Nitrosomonas europaea and the denitrifiers Alcaligenes eutrophus and Pseudomonas stutzeri were cultured at different levels of aeration (80 to 0% air saturation) and subjected to changes in aeration. The relative production of NO and N 2 O by N. europaea, as a percentage of the ammonium conversion, increased from 0.87 and 0.17%, respectively, at 80% air saturation to 2.32 and 0.78%, respectively, at 1% air saturation. At 0% air saturation, ammonium oxidation and N 2 O production ceased but NO production was enhanced. Coculturing of N. europaea with the nitrite oxidizer Nitrobacter winogradskyi strongly reduced the relative levels of NO and N 2 O production, probably as an effect of the lowered nitrite concentration. After lowering the aeration, N. europaea produced large short-lasting peaks of NO and N 2 O emissions in the presence but not in the absence of nitrite. A. eutrophus and P. stutzeri began to denitrify below 1% air saturation, with the former accumulating nitrite and N 2 O and the latter reducing nitrate almost completely to N 2. Transition of A. eutrophus and P. stutzeri from 80 to 0% air saturation resulted in transient maxima of denitrification intermediates. Such transient maxima were not observed after transition from 1 to 0%. Reduction of nitrate by A. eutrophus continued 48 h after the onset of the aeration, whereas N 2 O emission by P. stutzeri increased for only a short period. It was concluded that only in the presence of nitrite are nitrifiers able to dominate the NO and N 2 O emissions of soils shortly after a rainfall event.

The contribution of nitrifiers and denitrifiers to the nitrous oxide production in slurries of calcareous silt loam and river bank sediment at different oxygen concentrations was determined using acetylene as nitrification inhibitor. The addition of 10 Pa acetylene resulted in inhibition of nitrous oxide production at oxic conditions, but strongly enhanced the nitrous oxide production at oxygen-poor and anoxic conditions. Inhibition of nitrification by short exposure (1 to 24 h) to high concentrations of acetylene (100 Pa to 10 kPa) was tested using the same samples. After the removal of acetylene, nitrification was inhibited almost completely (82% to 89%) for at least 6 days whereas nitrous oxide reduction was restored within a day. It was concluded that the 'short exposure' inhibition method resulted in adequate nitrification inhibition without repressing the nitrous oxide reduction in anoxic nitrate-rich microsites and that the method was suitable for assessing the nitrifier contribution to the nitrous oxide emission of intact soil and sediment cores.

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The effectiveness of acetylene (C2H2) as inhibitor of nitrification was studied in relation to the decomposition of C2H 2. This was done by examining the effects of single and multiple additions of different C2H 2 concentrations (10, 100, 1000 Pa) on mineral N and NO~-N production in samples of the organic (FH) and upper mineral (Ah) layer of an acid oak-beech forest soil. The decomposition of C2H 2 was much faster in Ah samples than in FH samples. A single addition of 10 Pa C2H 2 was not sufficient for complete inhibition of nitrification in the Ah samples. Nitrification was blocked completely by all other C2H 2 treatments in both FH and Ah samples. Addition of C2H 2 decreased net mineral N production in Ah samples but not in FH samples. Addition of carboxymethyl-cellulose and chitin to Ah soil had no affect on the rate of decomposition of C2H 2. Chitin had a negative effect on net NO3-N production.

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