Different pathways of formation of N2O, N-2 and NO in black earth soil

The use of N-15 tracer provides a suitable technique to investigate the pro cesses of N transformation in soils and the origin of the environmentally r elevant gaseous N compounds N2O and NO from nitrification and denitrificati on. incubation experiments with black earth soil under two different water contents are presented here. Nitrification and denitrification proceeded si multaneously, but the importance of these two microbial processes shifted d epending on the water content of the soil. Under water-unsaturated conditio ns 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 f actor of up to 20 at the beginning of the experiments. Under water-saturate d conditions denitrification was the dominant process of N transformation i n the soil. However, nitrification also occurred to a considerable extent. The emission of N2O was greater than under unsaturated conditions. The form ation of NO could hardly be observed. N loss by molecular nitrogen from den itrification could be detected under saturated conditions. The N loss amoun ted 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 duri ng the microbial oxidation of NH4+ to NO2-. However, N2O mainly formed from denitrification under both conditions. Furthermore, NO could not be observ ed as a precursor of N2O and the free NO2- could not be detected as a commo n N pool for the formation of N2O and NO. High emissions of NO could be a p roblem 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) 2 000 Elsevier Science Ltd. All rights reserved.