Emission of gaseous nitrogen oxides from an extensively managed grassland in NE Bavaria, Germany - II. Stable isotope natural abundance of N2O

We analysed the stable isotope composition of emitted N2O in a one-year field experiment ( June 1998 to April 1999) in unfertilized controls, and after adding nitrogen by applying slurry or mineral N (calcium ammonium nitrate). Emitted N2O was analysed every 2-4 weeks, with additional daily sampling for 10 days after each fertilizer application. In supplementary soil incubations, the isotopic composition of N2O was measured under defined conditions, favouring either denitrification or nitrification. Soil incubated for 48 h under conditions favouring nitrification emitted very little N2O (0.024 mumol g(dw)(-1)) and still produced N2O from denitrification. Under denitrifying incubation conditions, much more N2O was formed (0.91 mumol g(dw)(-1) after 48 h). The isotope ratios of N2O emitted from denitrification stabilized at delta(15)N = - 40.8 +/- 5.7parts per thousand and delta(18)O = 2.7 +/- 6.3parts per thousand. In the field experiment, the N2O isotope data showed no clear seasonal trends or treatment effects. Annual means weighted by time and emission rate were delta(15)N = - 8.6parts per thousand and delta(18)O = 34.7parts per thousand after slurry application, delta(15)N = - 4.6parts per thousand and delta(18)O = 24.0parts per thousand after mineral fertilizer application and delta(15)N = - 6.4parts per thousand and delta(18)O = 35.6parts per thousand in the control plots, respectively. So, in all treatments the emitted N2O was N-15-depleted compared to ambient air N2O (delta(15)N = 11.4 +/- 11.6parts per thousand, delta(18)O = 36.9 +/- 10.7parts per thousand). Isotope analyses of the emitted N2O under field conditions per se allowed no unequivocal identification of the main N2O producing process. However, additional data on soil conditions and from laboratory experiments point to denitrification as the predominant N2O source. We concluded (1) that the isotope ratios of N2O emitted from the field soil were not only influenced by the source processes, but also by microbial reduction of N2O to N-2 and (2) that N2O emission rates had to exceed 3.4 mumol N2O m(-2) h(-1) to obtain reliable N2O isotope data.