Nitrous oxide (N₂O) is a stable, long-lived greenhouse gas produced by soil bacteria and other processes, that catalyzes the depletion of ozone in the stratosphere. Its concentration in the atmosphere is now about 20% higher than at the beginning of the industrial revolution. Approximately 40% of global N₂O emissions have anthropogenic sources, with more than half of them coming from fertilizer nitrogen inputs in agriculture.
Measuring N₂O emissions is difficult. Many countries have to rely on greenhouse gas accounting procedures that use ‘default’ global emission factors. For direct N₂O emissions from N inputs to agricultural soils, a default emission factor of 1% of N input has been widely used. In 2019, the IPCC introduced a disaggregated Tier 1 approach in which the default emission factors for synthetic nitrogen fertilizers were increased to 1.6%, whereas it was decreased to 0.6% for other nitrogen inputs such as manure. However, depending on soil type, climate, crop, cropping practices, fertilizer source and rate, there is huge variation in actual emissions. Default factors can lead to substantial over- or under-estimation of actual emissions, and thus also wrong conclusions about mitigation measures to take.
A team at the Thünen Institute conducted a detailed analysis to derive country-specific, stratified N₂O emission factors for Germany. In contrast to the IPCC, their results did not support a distinction between emission factors for synthetic and organic fertilizers. A geographical delineation resulted in district-wise emission factors that ranged from 0.38% to 0.92%. The average national implied emission factor for direct N₂O emissions from agricultural soils was only 0.62%. Consequently, the resulting new estimate of total German national greenhouse gas emissions from agriculture in 2015 was 8.59% lower than previously reported.
The paper highlights the urgent need to further refine the methodologies used for estimating direct N₂O emissions, also to allow for a better assessment of which specific management practices can contribute most to reducing them. This is an area of increasing importance to the fertilizer industry because it forms a major component of Scope 3 emissions associated with fertilizer use.
