A rotational field experiment was established in the year 2002 at the experimental farm Etzdorf in the Hercynian dry region of central Germany. Since 2005 field measured datasets were used to determine the effect of different preceding crop combinations and different nitrogen (N) fertilisation treatments on the seed yield, oil content, oil yield and N-use efficiency of oilseed rape (Brassica napus L.). The preceding crop combinations compared were winter wheat (Triticum aestivum L.)-winter wheat (WW), WW-oilseed rape (OSR), OSR-OSR and an OSR monoculture. In addition to the preceding crop combination, N fertiliser treatments with either 120 kg N ha–1 or 180 kg N ha–1 were established in the year 2013.
Overall the results demonstrated that seed yield, oil yield and N-use efficiency all declined with an increased cropping intensity for the period 2005–2012. Higher N rates in the 2013–2014 seasons increased seed yield and oil yield when OSR followed WW-WW pre-crops. OSR monoculture had lowest yield independent of applied N. Seed yield declined from 4.61 t ha–1 (OSR following WW-WW) to 4.28 t ha–1 in the OSR monoculture with 120 kg N ha–1, and from 4.81 t ha–1 (following WW-WW) to 4.42 t ha–1 in the OSR monoculture with 180 kg N ha–1. Higher N rates generally reduced N-use efficiency, with highest N-efficiency for WW-WW-OSR (38.4 kg kg–1), and lowest for continuous OSR receiving 180 kg N ha–1 (24.5 kg kg–1).
These results emphasise the importance of crop rotation to maintain seed yield and oil yield of oilseed rape, and to maximise the response to applied N. A reduced N rate increased N-use efficiency and reduced the risk of high N surpluses without a significant/equivalent decrease of the seed yield when the rotation was optimised.
Winter oilseed rape (WOSR) is the major oil crop cultivated in Europe and the most important feedstock for biodiesel. Up to 90% of the greenhouse gas (GHG) emissions from biodiesel production can occur during oilseed rape cultivation. Therefore, mitigation strategies are required and need to focus on direct nitrous oxide (N2O) emission as one of the largest GHG contributors in biodiesel production. Earlier studies show that nitrification inhibitors (NIs) can reduce N2O emissions derived from N-fertilization. Since information on the effect of biogas digestates with or without NIs on N2O emissions from WOSR fields is scarce, the aim of this study was to evaluate their effects on N2O emissions, mineral N dynamics, and oil yield in WOSR production fertilized with digestate. The study was conducted at five sites across Germany over three years resulting in 15 full site-years data sets. Across all sites and years, N2O emission from WOSR fertilized with biogas digestate (180 kg NH4+-N ha−1yr−1) ranged between 0.2 and 3.5 kg N2O–N ha−1 yr−1. Due to the reduction of the nitrate concentrations following digestate application, application of NI significantly reduced annual N2O emission by 36%. Our results demonstrate that NI can be an effective measure for reducing N2O emissions from digestate application, but its effectiveness depends on soil and weather conditions, and ultimately on the site-specific potential for N2O production and release. There was no effect of NI application on grain and oil yield.
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