Emission of gaseous nitrogen oxides from an extensively managed grassland in Ne Bavaria, Germany. I. Annual budgets of N2O NOx emissions

In a one-year field study (June 1998 to April 1999), we quantified N2O and NOx emissions from an extensively managed grassland in NE Bavaria (Germany) in unfertilized controls and after application of slurry or mineral N (calcium ammonium nitrate), respectively. Emissions were measured every 2–4 weeks, with additional daily measurements for 10 days after each fertilizer application. The closed chamber method was used for N2O and the open chamber method for NOx measurements. Fertilizer applications resulted in significantly increased N2O emission rates in comparison to the low annual mean of the control plots (1.4 mol m–2 h–1). Episodical emission peaks during the summer were attributed to high microbial activity after rainfall. Mineral N fertilization resulted in the highest emission rates. Cumulative annual N2O emissions were 11.2 kg N ha–1 a–1 for the mineral N, 8.8 kg N ha–1 a–1 for the slurry and 3.4 kg N ha–1 a–1 for the control plots. This represents 10.5% (mineral N) and 7.2% (slurry) of the applied nitrogen. The fertilizer-induced N2O emission factors on this extensively managed grassland are high in comparison to emission factors on intensively managed grassland and substantially higher than the 1.5% estimate used by the global emission inventory. NOx emissions increased after the first fertilizer application in summer, but not after the two following fertilizations in fall and spring. Differences between treatments were not significant. Annual NOx emissions were 1.9 kg N ha–1 a–1 for both, mineral N and slurry plots and 1.5 kg N ha–1 a–1 for the controls, representing 0.5% of the N applied with each fertilizer. The ratio of emitted NOx to N2O was 1:4.7 for both fertilized treatments (based on N-atoms