Impact of mineral N fertilizer application rates on N2O emissions from arable soils under winter wheat

Nitrogen fertilizers are a major source of nitrous oxide (N2O) emissions from arable soils. The relationship between nitrogen application rates and N2O 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 N2O 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 N2O 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 N2O 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 N2O emissions per unit of harvested wheat grain were calculated for two field sites to assess the environmental impact of wheat grain production. Yield-scaled N2O emissions followed a hyperbolic function with a minimum of 177 and 191 g N2O–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 N2O emissions compared to zero fertilization. Thus, improving nitrogen use efficiency may be the best management practice for mitigating yield-scaled N2O emissions.