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

Abstract

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 μmol gdw−1) and still produced N2O from denitrification. Under denitrifying incubation conditions, much more N2O was formed (0.91 μmol gdw−1 after 48 h). The isotope ratios of N2O emitted from denitrification stabilized at δ15N = −40.8 ± 5.7‰ and δ18O = 2.7 ± 6.3‰. 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 δ15N = −8.6‰ and δ18O = 34.7‰ after slurry application, δ15N = −4.6‰ and δ18O = 24.0‰ after mineral fertilizer application and δ15N = −6.4‰ and δ18O = 35.6‰ in the control plots, respectively. So, in all treatments the emitted N2O was 15N-depleted compared to ambient air N2O (δ15N = 11.4 ± 11.6‰, δ18O = 36.9 ± 10.7‰). 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 N2 and (2) that N2O emission rates had to exceed 3.4 μmol N2O m−2 h−1 to obtain reliable N2O isotope data.