Deep Dive 2
Global mass flow of nitrogen fertilisers and mitigation options
Authors:
Yunhu Gao
André Cabrera Serrenho
1 Department of Engineering,
University of Cambridge, Trumpington Street, Cambridge, CB2 1PZ, UK.
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Nitrogen fertilisers account for 5% of global greenhouse gas (GHG) emissions. Our global mass flow model uses a whole-systems approach to explore combinations of mitigation interventions. If all the interventions we consider were applied together, GHG emissions from synthetic fertilisers could be reduced by 84% by 2050. The single most effective intervention is increasing the efficiency of nitrogen fertiliser application to plants during their use phase.
the problem
Unlike most chemical products, nitrogen fertilisers release just one-third of their GHG emissions in the production phase, with the remaining two-thirds of emissions released during the use phase. Despite this, mitigation efforts have so far focused mainly on production.
Global food production is fundamentally underpinned by emissions-intensive nitrogen fertiliser production. How do we produce enough food while also reducing emissions to meet climate change mitigation targets? So far, studies have considered GHG emissions from either the production or the use phase of nitrogen fertilisers, but there has been no complete analysis conducted to date that spans the entire life cycle. Such a joined-up analysis is important for evaluating and prioritising different combinations of the available mitigation options.
Methods
We map the global flows of mass and emissions for synthetic N-fertilisers, allowing us to quantify and evaluate mitigation interventions by comparing them to the 2050 business-as-usual (BAU) scenario.
Key Results
- In the projected BAU scenario, emissions from synthetic fertilisers are set to reach 1.66 billion tonnes of CO2 equivalent per year in 2050.
- In the production phase, the key GHG mitigation options are:
- generating hydrogen from water electrolysis powered by wind energy instead of the conventional Steam Methane Reforming (SMR) process
- using wind energy to power reactions
- fitting Carbon Capture and Storage to existing SMR and coal gasification processes
- In the use phase, the key GHG mitigation options are:
- reducing demand for fertilisers by increasing the efficiency of nitrogen use by crops
- deploying nitrification and denitrification inhibitors
- phasing out more emissions-intensive urea-based fertilisers in preference of ammonium nitrate
- The single most effective intervention is demand reduction. The efficiency of nitrogen use can be maximized by improving irrigation and crop selection, and by strategically applying the right fertilisers at the right rate, at the right times, and in the right places.
- Overall, global emissions from synthetic fertilisers can be reduced by 84% by 2050 if all the intervention measures above are combined.
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Global mass flow of nitrogen fertilisers and mitigation options
Nitrogen fertilisers account for 5% of global greenhouse gas (GHG) emissions. Our global mass flow model uses a whole-systems approach to explore combinations of mitigation interventions. If all the interventions we consider were applied together, GHG emissions from synthetic fertilisers could be reduced by 84% by 2050. The single most effective intervention is increasing the efficiency of nitrogen fertiliser application to plants during their use phase.
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Abstract
Food security relies on nitrogen fertilisers, whose production and use are estimated to account for around 5% of global GHG emissions. Rising demand for food due to global population growth may jeopardize the 1.5 ∞C climate change target. To identify and prioritise meaningful mitigation options across the supply chain of nitrogen fertilisers, we have mapped the global mass flow of synthetic nitrogen fertilisers and manure, as well as the GHG emissions from all lifecycle stages. Only one-third of the GHG emissions from nitrogen fertilisers arise from production, while two-thirds are released during the use phase. We have devised a model to quantify and prioritise mitigation options in the production and use phases. Increasing nitrogen use efficiency along the supply chain is the most effective individual strategy. A combination of mitigation options is necessary to substantially reduce the GHG emissions from synthetic nitrogen fertilisers.
Introduction
The global population is forecast to reach 9.7 billion by 2050, which implies a growing demand for food. Meeting this demand carries an environmental cost; food production currently accounts for approximately one-third of global greenhouse gas (GHG) emissions.1 Crop production heavily relies on the use of fertilisers, especially nitrogen, potassium, and phosphate. Erisman et al. estimate that around half of the global population is fed by crops based on synthetic nitrogen fertilisers.2,3 However, the combined emissions from the production and use phases of synthetic nitrogen fertilisers and manure account for around 5% of global GHG emissions. It is important to identify and prioritise mitigation options along the supply chain of nitrogen fertilisers while avoiding risks to food security.
GHGs are released during both the production and the use of nitrogen fertilisers. The synthesis of ammonia, the intermediate product for other nitrogen fertilisers, alone releases about 0.8% of global GHG emissions4 and consumes 2% of global energy.5 Natural gas, coal, and oil are the feedstock and fuels in ammonia production,6 and the extraction and combustion of these fuels produce GHG emissions. CO2 is also released as the product of chemical reactions. Additional emissions arise from electricity generation. N2O, whose global warming potential is 273 times that of CO2,7 is also released as a by-product in the production of nitric acid, which is a raw material for several nitrogen fertilisers.
Further GHG emissions are produced in the use phase of nitrogen fertilisers. Most notably, soil bacteria partially convert nitrogen to N2O by the nitrification and denitrification processes, resulting in direct N2O emissions.8,9 N2O is also released indirectly from nitrate leaching and ammonia volatilisation.10 Additionally, CO2 is released from the decomposition of urea and ammonium bicarbonate (ABC), and from the use of limestones to neutralise the soil acidification caused by nitrogen fertilisers.11
Studies have investigated the mitigation of GHG emissions from the production and use phases of nitrogen fertilisers, but a holistic global assessment of GHG emissions that spans all lifecycle stages has not yet been conducted. Such an assessment would enable the prioritisation of the mitigation interventions often only assessed in isolation by the existing literature.
We fill this gap by mapping the global mass flow of nitrogen fertilisers and GHG emissions from both the production and use phases of synthesised nitrogen fertilisers and manure, and by quantifying the mitigation potentials of various interventions.
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Fig. 1 Sankey diagram of the global mass flow of synthesized nitrogen fertilisers and manure and corresponding GHG emissions in each lifecycle stage in 2019. The horizontal flows represent the mass flows of nitrogen, and the vertical flows show the points of generation of GHG along the supply chain. In both cases, the thickness of the lines is proportional to the mass flows of nitrogen and CO2e, respectively.
Method
Current mass flow of nitrogen fertilisers and GHG emissions
The global mass flows of synthetic nitrogen fertilisers and manure across all stages of the supply chain and the associated GHG emissions in 2019 are presented in Fig. 1. A total of 2.6 Gt CO2e a-1 GHGs were released from the production and use of nitrogen fertilisers (108.5 Mt N a-1) and manure (123.9 Mt N a-1) in 2019.
In the same year, 1.31 Gt CO2e a-1 (95% confidence interval: 1.15-1.56 Gt CO2e a-1) GHGs were released from synthetic fertilisers. Unlike many other products, only approximately a third of whole lifecycle emissions is attributed to production. The other two-thirds are generated in the use phase of synthetic nitrogen fertilisers through direct and indirect N2O emissions and through CO2 emissions from limestone and the decomposition of urea and ABC.
Emissions from the production of synthetic fertilisers take place mostly (84%) in ammonia synthesis. CO2 emissions due to chemical reactions comprise 21% of the total production GHG emissions. These are the net process emissions (103 Mt CO2e a-1) because the CO2 stream used as a feedstock to produce urea and ABC is excluded. Additionally, 85 Mt CO2e a-1 as N2O are released from nitric acid production. Fig. 1 depicts the global flows; the allocation of total GHG emissions to different regions and types of nitrogen fertilisers is beyond the scope of this work but can be found in our corresponding paper.12
The global mass flow of manure and corresponding GHG emissions in 2019 are shown at the bottom of Fig. 1. Of the 123.8 Mt N a-1 of manure and 4.3 Mt N a-1 bedding materials, around 43% is applied to soils, while 41% is left on the pasture. The remaining 16% is wasted in the manure management process. Both N2O and methane are emitted from manure, accounting for a total of 1.27 Gt CO2e a-1. The global average emission factor of manure as a nitrogen fertiliser is 23 t CO2e/t N, which is 1.9 times the global average emission factor of synthetic fertilisers. As above, the allocation of GHG emissions from manure to different regions and lifecycle stages is available in our corresponding paper.12
Mitigation options
Our model of the mass flow of synthetic nitrogen fertiliser and corresponding GHG emissions has been used to quantify the potential of various mitigation interventions. According to regional forecasts by the Food and Agricultural Organization (FAO)13, global synthetic nitrogen fertiliser consumption and the related GHG emissions are expected to reach 135.7 Mt N a-1 and 1.66 Gt CO2e a-1 (95% confidence interval: 1.44-1.99 Gt CO2e a-1), respectively, in the business as usual (BAU) scenario in 2050 shown in Fig. 2a.
Different mitigation interventions can be targeted at the production and use of fertilisers. These two groups of mitigation options are proposed and quantified (details of each scenario are available in the corresponding paper12).
On the production side, most GHG emissions arise from ammonia synthesis, whose feedstocks are hydrogen and nitrogen. These feedstocks can be generated using renewable energy. Nitrogen can be separated from air using wind energy, and water electrolysis powered by renewable electricity can provide hydrogen with low emissions. Alternatively, to avoid the CO2 released from combustion, electric heating generated by wind power can replace fossil fuels to power reactions.14 In the third scenario, CCS is coupled with conventional steam methane reforming, coal gasification, and boilers to remove the CO2 emissions.
In the use phase, the mitigation potentials of three interventions are also quantified. Firstly, the demand for nitrogen fertilisers can be substantially reduced by increasing the nitrogen use efficiency in the cropland, which is the ratio between the mass of nitrogen contained in harvested crops and the total nitrogen added to soil, either artificially or by natural processes. Currently, Zhang et al. estimate that the global average efficiency of nitrogen use in crops is 42%, which should be improved to 67% to reduce the demand for nitrogen fertilisers by 2050.3
Direct and indirect N2O emissions from volatilisation and nitrate leaching account for 48% of the total GHG emissions. These are caused by the nitrification and denitrification of bacteria, processes which can be substantially reduced by the deployment of nitrification inhibitors. In addition, urea decomposes to ammonia through the urease effect before ammonia is partially converted to N2O by the nitrification effect. Thus, the combination of nitrification inhibitors and urease inhibitors is effective to reduce the GHG emissions from urea.15
Finally, not all types of fertilisers used globally produce the same total GHG emissions per unit of N. Fig. 2b shows this breakdown and reveals that urea, UAN and ABC have some of the worst emissions performances, as their decomposition in the soil also releases CO2. However, it is possible to avoid the use of these types of fertilisers and replace them with a better-performing fertiliser such as AN.
Fig. 2a shows the mitigation potentials of implementing each of the different strategies. In the production phase, the water electrolysis scenario would result in the global GHG emissions from the production of nitrogen fertilizers being significantly reduced to 178 Mt in 2050, compared to 613 Mt CO2e in BAU. However, the total emissions figure is only reduced by 27% in this scenario because of the high emissions in the use phase, as shown in Fig. 2a. In the electric heating scenario, total GHG emissions are reduced by 21% in 2050 by avoiding the GHG emissions from fuel combustion. In the CCS scenario, the total GHG emissions from synthetic nitrogen fertilizers are reduced by 25%, almost equal to the water electrolysis scenario.
Fig. 2a suggests that demand reduction has the greatest potential to reduce emissions. This is due to its potential to reduce emissions from both the production phase and the use phase. By increasing the global efficiency of nitrogen use from the current 42% to 67% by 2050, the total nitrogen demand could be reduced to 71 Mt N a-1. This corresponds to -48% emissions compared to BAU. A combination of several approaches is needed to achieve the proposed nitrogen use efficiency by Zhang et al.3 This includes proper irrigation, selecting crop breeds which utilise nitrogen fertilisers more efficiently and applying the right fertilisers at the right rate and time in the right place.3
The most promising demand reduction scenario could be combined with production and use phase interventions to enhance the total mitigation potential, as shown in Fig. 2c. The total GHG emissions from synthetic nitrogen fertilisers in 2050 can be reduced by 78%, when the deployment of water electrolysis for ammonia production, fertiliser demand reduction, and the use of nitrification inhibitors are combined in the so-called combination scenario.
In this combination scenario, the decomposition of urea, UAN, and ABC accounts for 93 Mt CO2e in 2050. Substituting these with AN could reduce the GHG emissions in the combination scenario even further, to 16% of BAU emissions. However, if this substitution were deployed in isolation, without the other interventions, it would lead to a 3% growth in emissions compared to the BAU scenario.
Sensitivity analysis
The previous quantification of mitigation options was obtained based on data with uncertainties. Thus, it is important to conduct sensitivity analyses to confirm the effectiveness of the mitigation options.
Fig. 3 reveals that the most sensitive parameter is global nitrogen use efficiency. Even an increase of 5% leads to a 19% reduction in demand for synthetic nitrogen fertilisers and the resulting GHG emissions by 2050. The total GHG emissions are significantly reduced even under the worst scenario. The details of the sensitivity analysis are described in the corresponding paper.12
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Figure 2 a) Historical and future global GHG emissions from synthesised nitrogen fertilisers. The grey area represents the 95% confidence interval. The columns show the mitigation potential of individual interventions in 2050 if applied in isolation. b) Breakdown of GHG emissions from various nitrogen fertilisers in different lifecycle stages in 2019, 2050 scenario water electrolysis and 2050 scenario combination (water electrolysis, demand reduction, and nitrification inhibitors). The colours represent the same stages of the lifecycle as in Fig. 2c. c) Global GHG emissions for different lifecycle stages for the combination of all mitigation options: use of water electrolysis for ammonia production, deployment of nitrification inhibitors, and the reduction of demand of fertilisers by increasing the nitrogen efficiency (green line), and additionally the replacement of urea, UAN and ABC by AN (blue line). The error bars represent the 95% confidence interval.
Figure 3 Impact of the factors on global GHG emissions from nitrogen. The solid lines show the projected emissions from fertilisers in 2050 under scenario combination + fertiliser substitution (water electrolysis, demand reduction, nitrification inhibitor and fertiliser substitution). The first column shows the global GHG emissions with embedded GHG in wind power ranging from 0 t CO2e/GWh to 45 t CO2e/GWh. The second column indicates the effect of global nitrogen use efficiency ranging from 61% to 72% on GHG emissions. The third column shows the impact of nitrification inhibitors on direct N2O emission reduction from 42% to 55%.
Results & Discussion
The results show that up to 84% of global GHG emissions from synthetic nitrogen fertilisers can be reduced by 2050. This would require the decarbonisation of fertiliser production by the electrification of heating and the deployment of water electrolysis for ammonia production. Additionally, this would need to be combined with an increase in nitrogen use efficiency in croplands to the maximum potential values, a deployment of nitrification inhibitors, and a shift in the mix of fertilisers used globally. These are options with high technology readiness levels.
While about one-third of the emissions from synthetic nitrogen fertilisers come from production, two-thirds of the emissions are attributed to the use phase. This highlights the importance of mitigation interventions in the use phase, despite the current focus of attention on the decarbonisation of ammonia synthesis.
Manure is currently not a good substitute for synthetic nitrogen fertilisers, although the total volume of nitrogen in manure is comparable with synthetic fertilisers. The average GHG emission factor of manure applied in the soils is 1.9 times the average emission factor of synthetic fertiliser. Moreover, it is difficult to decarbonise the supply chain of manure. In contrast, we have much more control over the production of synthetic fertilisers. It is therefore easier to deploy mitigation options on synthetic fertilisers rather than manure.
The demand for crops and resulting requirements for nitrogen fertilisers can be further reduced by increasing the nitrogen use efficiency along the food supply chain (from crops to food) and changing diets.16,17
This study quantifies the mitigation potential of interventions with universal effects in all world regions. However, the complexity of agricultural production in different regions will render certain other mitigation options effective under the regionally specific conditions determined by the type of soil,18 agricultural practices,19 and climate. For example, since legume crops fix nitrogen through symbiotic relationships with bacteria, their expansion could be another strategy to further reduce the demand for nitrogen fertilisers and the corresponding emissions. The new agricultural practice of no-till farming may also reduce the direct N2O emission in particular regions.19
The barriers to applying the mitigation options evaluated above will vary in different lifecycle stages according to the costs and the vintage of existing global facilities. Entirely new plants need to be built to provide hydrogen by water electrolysis and electric heating for reactors. By contrast, coupling CCS with existing ammonia plants requires less equipment and would allow the industry to take full advantage of existing ammonia plants, which have a typical lifetime of 50 years.5
Collective actions from all stakeholders are necessary to realise the mitigation potentials. Policymakers can encourage greater nitrogen use efficiency and the application of cost-effective, mature mitigation technologies by taxing food staples with high fertiliser requirements, reducing taxes for farms with high nitrogen use efficiency, and regulating the production of fertilisers. Farmers can increase nitrogen use efficiency by adopting the so-called “4R” guidance on fertiliser use that fosters the application of the right type of nitrogen fertilisers at the right time, in the right place, and at the right rate. Other crop growth conditions, such as irrigation, type of soil, and climate, should also be controlled to increase nitrogen use efficiency.
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- Gao, Y. & Serrenho, A. C. How to feed the global population with less greenhouse gas emissions from nitrogen fertilisers? (2022).
- Alexandratos, N. & Bruinsma, J. WORLD AGRICULTURE TOWARDS 2030/2050: The 2012 Revision. (2012).
- van Geem, K. M., Galvita, V. V. & Marin, G. B. Making chemicals with electricity. Science (1979) 364, 734–735 (2019).
- Li, T. et al. Enhanced-efficiency fertilizers are not a panacea for resolving the nitrogen problem. Glob Chang Biol 24, 511–521 (2018).
- Kanter, D. R. et al. Nitrogen pollution policy beyond the farm. Nat Food 1, 27–32 (2020).
- Yetkin Özbük, R. M. & Coşkun, A. Factors affecting food waste at the downstream entities of the supply chain: A critical review. J Clean Prod 244, 118628 (2020).
- Akiyama, H., Yan, X. & Yagi, K. Evaluation of effectiveness of enhanced-efficiency fertilizers as mitigation options for N2O and NO emissions from agricultural soils: Meta-analysis. Glob Chang Biol 16, 1837–1846 (2010).
- Omonode, R. A., Smith, D. R., Gál, A. & Vyn, T. J. Soil Nitrous Oxide Emissions in Corn following Three Decades of Tillage and Rotation Treatments. Soil Science Society of America Journal 75, 152–163 (2011).
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