Ammonia emissions from urea fertilization - Multi-annual micrometeorological measurements across Germany

Among the synthetic fertilizers used in crop production, the application of conventional urea is one of the major sources of ammonia (NH₃) emissions. However, NH₃ emission estimates based on existing emission factors (EFs) are subject to significant uncertainties due to limited underlying data, obtained under various conditions and applied methods. To assess the accuracy of current EFs for different regions within Germany, NH₃ emissions were measured using the integrated horizontal flux (IHF) method employing ALPHA passive samplers following urea application in 2021, 2022, and 2023 across six agroecological regions of Germany. Measurements were conducted under winter wheat cropping, with total nitrogen (N) rates of 145–230 kg N ha⁻¹ , split into two or three applications, resulting in 51 measurement campaigns. Measured N input related NH₃ emission (NH_3Ninput) ranged from 1.1 % to 20.7 % (median 4.8 %, mean 8.5 %), significantly lower than the 2019 IPCC (14.2 %) and 2023 EMEP (16.1 % for pH<7) estimates for urea. No consistent trend in NH_3Ninput was observed between fertilizer applications and regions, though NH_3Ninput tended to be higher at sites with higher sand contents than sites with higher clay contents. Rainfall was negatively correlated with NH_3Ninput, while N application rates had no effect. The current EFs overestimate the NH₃ emissions from urea applied to winter wheat under the conditions tested in Germany. The observed emissions may deviate from long-term regional trends due to inter-annual variability and complex environmental interactions. Nevertheless, establishing a national EF for Germany could enhance the accuracy of NH₃ emission estimates and improve assessments of mitigation measures' potential to reduce emissions.