Low but variable nitrous oxide emissions from rapeseed: Results from a two-year field experiment at five sites in Germany

English Abstract:

Biodiesel from rapeseed is one of the major renewable resources in Germany used for fuel production. According to the Renewables Directive of the European Union, biofuels have to fulfill strict sustainability requirements, such as reductions of 35% and 50% (starting in 2017) of greenhouse gases (GHG) in comparison to fossil fuels. Thus, the mitigation of field nitrous oxide (N₂O) emissions is still challenging and depends on numerous interacting factors, such as climatic (temperature and soil moisture) and management factors, e.g. fertilizer type (organic vs. mineral fertilizer) and fertilizer rate, as well as tillage practices. Since 2012, measurements were conducted at 5 study sites throughout Germany located in the major precipitation-frost classes identified as relevant for N₂O production (Jungkunst et al. 2006). A semi-randomized block design was established at each study site investigating the complex effects of i) crop rotation of winter rape - winter wheat - winter barley and ii) 5 fertilizer regimes (100% biogas residue application with and without nitrification inhibitor, 100% mineral fertilizer, 67% mineral fertilizer, non-N fertilized control) on N₂O emissions. At each study site, nitrous oxide field emissions were measured by the same approach daily for 3-5 days following fertilization and subsequently weekly using a manual non-flow-through non-steady-state closed chamber system and 20-minute interval sampling. We present results from cropping seasons 2012-2013 and 2013-2014, featuring two climatically contrasting winter seasons. In 2013, the weather conditions were characterized by a cold and long winter with snow until mid-spring, whereas almost no freezing events occurred in 2014. Annual N₂O emissions as well as yield-normalized N₂O emissions from rape were low in both seasons. Nevertheless, annual differences between treatments and sites occurred. For both cropping seasons, emissions were smaller than predicted by the IPCC emission factors or by the model of Stehfest & Bouwman (2006), but were still dependent on nitrogen input.