Nitrous oxide emissions after incorporation of winter oilseed rape (Brassica napus L.) residues under two different tillage treatments.

The aim of this study was to investigate the effect of crop residues from winter oilseed rape on N2O emissions from a loamy soil and to determine the effect of different tillage practices on N2O fluxes. We therefore conducted a field experiment in which crop residues of winter oilseed rape (Brassica napus L., OSR) were replaced with N-15 labelled OSR residues. Nitrous oxide (N2O) emissions and N-15 abundance in the N2O were determined for a period of 11 months after harvest of OSR and in the succeeding crop winter wheat (Triticum aestivum L.) cultivated on a Haplic Luvisol in South Germany. Measurements were carried out with the closed chamber method in a treatment with conventional tillage (CT) and in a treatment with reduced soil tillage (RT). In both tillage treatments we also determined N2O fluxes in control plots where we completely removed the crop residues. High N2O fluxes occurred in a short period just after OSR residue replacement in fall and after N-fertilization to winter wheat in the following spring. Although N2O emissions differed for distinct treatments and sub-periods, cumulative N2O emissions over the whole investigation period (299 days) ranged between 1.7 kg and 2.4 kg N2O-N ha(-1) with no significant treatment effects. More than half of the cumulative emissions occurred during the first eight weeks after OSR replacement, highlighting the importance of this post-harvest period for annual N2O budgets of OSR. The contribution of residue N to the N2O emission was low and explained by the high C/N-ratio fostering immobilization of mineral N. In total only 0.03% of the N2O-N emitted in the conventional tillage treatment and 0.06% in the reduced tillage treatment stemmed directly from the crop residues. The N-15 recovery in the treatments with crop residues was 62.8% (CT) and 75.1% (RT) with more than 97% of the recovered N-15 in the top soil. Despite our measurements did not cover an entire year, the low contribution of the OSR residues to the direct N2O emissions shows, that the current IPCC tier 1 approach, which assumes an EF of 1%, strongly overestimated direct emissions from OSR crop residues. Furthermore, we could not observe any relationship between tillage and crop residues on N2O emission, only during the winter period were N2O emissions from reduced tillage significantly higher compared to conventional tillage. Annual N2O emission from RT and CT did not differ