Nitrous oxide emissions from crop rotations including wheat, oilseed rape and dry peas

Abstract. Approximately 65% of anthropogenic emissions of N₂O, a potent greenhouse gas (GHG), originate from soils at a global scale, and particularly after N fertilisation of the main crops in Europe. Thanks to their capacity to fix atmospheric N₂ through biological fixation, legumes can reduce N fertilizer use, and possibly N₂O emissions. Nevertheless, the decomposition of crop organic matter during the crop cycle and residue decomposition, and possibly the N fixation process itself, could lead to N₂O emissions. The objective of this study was to quantify N₂O emissions from a dry pea crop (Pisum sativum, harvested at maturity) and from the subsequent crops in comparison with N₂O emissions from wheat and oilseed rape crops, fertilized or not, in various rotations. A field experiment was conducted over 4 consecutive years to compare the emissions during the pea crop, in comparison with those during the wheat (fertilized or not) or oilseed rape crops, and after the pea crop, in comparison with other preceding crops. N₂O fluxes were measured using static chambers. In spite of low N₂O fluxes, mainly due to the site's soil characteristics, fluxes during the crop were significantly lower for pea and unfertilized wheat than for fertilized wheat and oilseed rape. The effect of the preceding crop was not significant, while soil mineral N at harvest was higher after the pea crop. These results should be confirmed over a wider range of soil types. Nevertheless, they demonstrate the absence of N₂O emissions linked to the symbiotic N fixation process, and allow us to estimate the decrease in N₂O emissions by 20–25% through including one pea crop in a three-year rotation. On a larger scale, this reduction of GHG emissions at field level has to be added to the decrease due to the reduced production and transport of the N fertilizer not applied to the pea crop.