Impact of mineral N fertilizer application rates on N 2 O emissions from arable soils under winter wheat

Nitrogen fertilizers are a major source of nitrous oxide (N 2 O) emissions from arable soils. The relationship between nitrogen application rates and N 2 O emissions was evaluated during the growth period of winter wheat (~140 days) at six field sites in north-western Germany. Nitrogen was applied as calcium–ammonium–nitrate, with application rates ranging between 0 and 400 kg N ha −1 . One trial was conducted in 2010, three trials in 2011 and two trials in 2012. Additionally, post-harvest N 2 O emissions were evaluated at two field sites during autumn and winter (2012–2013). The emission factors (during the growth period) varied between 0.10 and 0.37 %. Annual N 2 O emissions ranged between 0.46 and 0.53 % and were consistently lower across all sites and years than to the IPCC Tier 1 default value (1.0 %). Across all sites and years, the relationship between N 2 O and N application rate was best described by linear regression even if nitrogen amounts applied were higher than the nitrogen uptake of the crop. Additionally, annual N 2 O emissions per unit of harvested wheat grain were calculated for two field sites to assess the environmental impact of wheat grain production. Yield-scaled N 2 O emissions followed a hyperbolic function with a minimum of 177 and 191 g N 2 O–N t grain yield −1 at application rates of 127 and 150 kg N ha −1 , followed by an increase at higher N application rates. This relationship indicates that wheat crop fertilization does not necessarily harm the environment through N 2 O emissions compared to zero fertilization. Thus, improving nitrogen use efficiency may be the best management practice for mitigating yield-scaled N 2 O emissions.