NITROUS-OXIDE PRODUCTION AND NITRIFICATION IN ACIDIC SOIL FROM A DUTCH CONIFEROUS FOREST

The production of nitrous oxide and nitrification were studied using aerobic incubations of soil sampled from the F-layer of a nitrogen-saturated Douglas fir stand (The Netherlands). The nitrous oxide production (3 ng N2O-N g-1 dry soil h-1) in the acid soil (pH 3.7) could be inhibited by 90% using low (3 Pa) or high (2.5 kPa) acetylene concentrations. The experiments with aerobic soil slurries showed that N2O production at pH 4 was 4-8 times higher than at pH 6. Also the nitrification rate was higher at pH 4 than at pH 6. The ratio of N2O-N to NO3--N was higher at pH 4 (0.33-0.47%) than at pH 6 (0.04-0.18%). Acetylene (2.5 kPa) or nitrapyrin (10 mug ml-1) inhibited N2O (40-96%) and NO3- production (79-100%) in the soil slurries indicating the importance of chemolithotrophic bacteria. The results showed that N2O is produced in a nitrogen-saturated acid forest soil in North Western Europe even if well-aerated. The aggregated ammonium oxidizers (Nitrosospira) that are responsible for ammonium oxidation in these acid soils may have importance in N2O production. The possible significance of the aggregation of Nitrosospira cells for N2O production is discussed.