The emission factor paradox: N2O emissions from organic fertilizer exceed those from synthetic N fertilizers on Danish agricultural soils

N₂O is currently the single most damaging of all ozone-depleting greenhouse gases (GHG) associated with climate change, and agriculture is the primary source of this extremely potent GHG. Direct N₂O emissions from agricultural fields constitute about 21 % of all greenhouse gases emitted by agriculture in Denmark. Serious efforts for N₂O mitigation must be taken to limit global warming, and rigorous monitoring and correct documentation of national greenhouse gas emissions are at the forefront of this endeavor. The Danish National Inventory Report for greenhouse gases uses the Tier 1 default emission factor (EF) of 1% for mineral soils, assuming 1% of N input as fertilizer is emitted as N₂O. A refinement provided by the IPCC in 2019 suggests using specific land-use categories: 1.6% for synthetic fertilizers and 0.6% for organic fertilizers in wet climates like Denmark. However, studies have shown that this distinction is unsuitable for Danish agricultural conditions, and that especially emissions from synthetic fertilizer are overestimated with both approaches.

We conducted 28 individual field trials under common Danish agricultural management throughout the country from 2022 to 2024, measuring N₂O emissions in spring barley and winter wheat during their growing seasons. We were specifically interested in comparing N₂O emissions from synthetic vs. organic fertilizers. The average cumulative N₂O emissions of synthetic fertilizers ranged from 0.12 to 1.05 kg N₂O-N ha^-1 in spring barley, and from 0.08 to 1.17 kg N₂O-N ha^-1 in winter wheat. Average cumulative N₂O emissions of organic fertilizers ranged from 0.95 to 1.41 kg N₂O-N ha^-1 in spring barley, and from 0.19 to 1.30 kg N₂O-N ha^-1 in winter wheat. All emissions were comparably low throughout trials, treatments and years. Average EF (± S.E.) for synthetic fertilizers were 0.10 ± 0.04 % (spring barley) and 0.16 ± 0.05 % (winter wheat), and for organic fertilizers 0.38 ± 0.03 % (spring barley, cattle slurry), 0.38 ± 0.06 % (winter wheat, pig slurry), and 0.37 ± 0.06 % (winter wheat, digestate) during the growing season. Our results contradict both the default and refined Tier 1 EF provided by the IPCC. In agreement with other studies, we found that N₂O EF for synthetic fertilizers were lower than EF for organic fertilizers. Possible explanations for, and implications of this paradox will be discussed.