Different pathways of formation of N2O, N2 and NO in black earth soil

The use of N-15 tracer provides a suitable technique to investigate the processes of N transformation in soils and the origin of the environmentally relevant gaseous N compounds N2O and NO from nitrification and denitrification. incubation experiments with black earth soil under two different water contents are presented here. Nitrification and denitrification proceeded simultaneously, but the importance of these two microbial processes shifted depending on the water content of the soil. Under water-unsaturated conditions the microbial oxidation of NH4+ to NO3- predominated, but a reduction of NO3- also occurred. The emission of NO exceeded the emission of N2O by a factor of up to 20 at the beginning of the experiments. Under water-saturated conditions denitrification was the dominant process of N transformation in the soil. However, nitrification also occurred to a considerable extent. The emission of N2O was greater than under unsaturated conditions. The formation of NO could hardly be observed. N loss by molecular nitrogen from denitrification could be detected under saturated conditions. The N loss amounted to 60% of NO3- and thereby the cumulative N ratio of N-2 to N2O was 3. Under either unsaturated or saturated conditions NO arose from NO2- or during the microbial oxidation of NH4+ to NO2-. However, N2O mainly formed from denitrification under both conditions. Furthermore, NO could not be observed as a precursor of N2O and the free NO2- could not be detected as a common N pool for the formation of N2O and NO. High emissions of NO could be a problem for the black earth soil in the semi-arid climate in central Germany, if there are large amounts of NH4+ in the soil after fertilisation. (C) 2000 Elsevier Science Ltd. All rights reserved.