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

Abstract

The aim of this study was to investigate the effect of crop residues from winter oilseed rape on N 2 O emissions from a loamy soil and to determine the effect of different tillage practices on N 2 O fluxes. We therefore conducted a field experiment in which crop residues of winter oilseed rape (Brassica napus L., OSR) were replaced with 15 N labelled OSR residues. Nitrous oxide (N 2 O) emissions and 15 N abundance in the N 2 O 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 N 2 O fluxes in control plots where we completely removed the crop residues. High N 2 O 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 N 2 O emissions differed for distinct treatments and sub-periods, cumulative N 2 O emissions over the whole investigation period (299 days) ranged between 1.7 kg and 2.4 kg N 2 O-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 N 2 O budgets of OSR. The contribution of residue N to the N 2 O emission was low and explained by the high C/N-ratio fostering immobilization of mineral N. In total only 0.03% of the N 2 O-N emitted in the conventional tillage treatment and 0.06% in the reduced tillage treatment stemmed directly from the crop residues. The 15 N recovery in the treatments with crop residues was 62.8% (CT) and 75.1% (RT) with more than 97% of the recovered 15 N in the top soil. Despite our measurements did not cover an entire year, the low contribution of the OSR residues to the direct N 2 O 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 N 2 O emission, only during the winter period were N 2 O emissions from reduced tillage significantly higher compared to conventional tillage. Annual N 2 O emission from RT and CT did not differ.