New N2O emission factors for crop residues and fertiliser inputs to agricultural soils in Germany
Mathivanan, G.P., Eysholdt, M., Zinnbauer, M., Rösemann, C., Fuß, R. 2021. New N2O emission factors for crop residues and fertiliser inputs to agricultural soils in Germany. Agriculture, Ecosystems and Environment 322, 107640
Summary by Dobermann, A.
Atmospheric concentrations of nitrous oxide (N₂O), a stable, long-lived gas produced by soil bacteria and other processes, are now about 20% higher than at the beginning of the industrial revolution. Nitrous oxide is a strong greenhouse gas and it catalyzes the depletion of ozone in the stratosphere. Approximately 40% of global N2O emissions have anthropogenic sources, with more than half of these 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 (EFs). 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 EF for synthetic nitrogen fertilizers was 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 of 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 being made between EFs 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 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.
Classic Plant Nutrition Paper
Production of Nitrous Oxide by Ammonia-Oxidizing Chemoautotrophic Microorganisms in Soil
Blackmer, A.M, Bremner, J.M., Schmidt, E.L. 1980. Production of nitrous oxide by ammonia-oxidizing chemoautotrophic microorganisms in soil. Applied and Environmental Microbiology 40: 1060–1066.
Summary by Dobermann, A.
In the late 1960s it became known that N2O diffused from the soil and other sources through the atmosphere all the way into the stratosphere, where it is partly converted to nitrogen oxides (NO, NO2). In 1970, Paul Crutzen linked this process to ozone depletion, work for which he received the Nobel Prize in Chemistry in 1995. He also noted that the increasing use of fertilizers might have led to an increase in N2O emissions over the natural background, which would in turn result in a further depletion of ozone in the stratosphere.
The prevailing thought throughout the 1970s was that N2O is produced in soils only through denitrification of nitrate (NO3) under anaerobic conditions. This landmark paper, led by Alfred Blackmer at Iowa State University, challenged that assumption. In 1978, the authors had already shown in a paper in Science that N2O emissions from well-aerated soils treated with ammonium forms of nitrogen exceeded those from soils treated with nitrate, and that they could be reduced by adding nitrapyrin, a compound that inhibits nitrification (the conversion of ammonium to nitrate in soils by soil microorganisms).
Here they provide more conclusive evidence for this process by adding ammonium sulfate to a sterilized soil and inoculating it with pure cultures of ammonia-oxidizing microorganisms. They concluded that, at least under some conditions, most of the N2O evolving from soils treated with ammonium-based fertilizers is generated by nitrifying microorganisms during oxidation of ammonium to nitrite. This provides a strong justification for utilizing technologies such as nitrification inhibitors or controlled release fertilizers, or for N sources and application practices that avoid high ammonium concentrations in the root zone.