Mitigating N2O Emissions and Enhancing Yields with Inhibitors and Liming
18 July 2024This article is part of the Climate Change & Carbon Research Briefings series. More articles in the series can be found below:
- Understanding Natural Capital Markets
- Collecting On-Farm Biodiversity Data with Bioacoustics
- Faba Beans for Alternative Protein and Reducing Monogastric Carbon Footprint
- Applicability of Slurry Separation and Acidification on Farms in Scotland
- Advancing Agricultural Practices, Reducing Emissions and Ensuring Sustainable Growth in the Face of Climate Challenges
- Making Sense of Soil Carbon
- Are Methane Inhibitors a Silver Bullet for the Sustainability of Beef and Dairy?
- Low Emissions Livestock Breeding - The What and the Why
This research briefing highlights the role of nitrification inhibitors, urease inhibitors, and liming in reducing agricultural greenhouse gas emissions, while increasing yields. Laboratory studies and some field experiments suggest that these treatments can mitigate ammonia and nitrous oxide emissions. However, field data is limited, and more research is needed to test their efficiency under diverse field conditions. A recent, SRUC-led, Kirkton experiment aims to address this by applying these inhibitors and liming to assess their impact on N2O fluxes in managed grassland.
Agriculture as a source of ammonia
Agricultural soils are the largest anthropogenic source of ammonia (NH3) and nitrous oxide (N2O), with N2O constituting 26% of agricultural emissions in 20226. To address this, nitrification and urease inhibitors, have been identified as top strategies to reduce N2O and NH3emissions1 and liming has also been proposed as a mitigation option5. Liming and inhibitors can also lead to higher agricultural yields. Liming does so by improving the nutrient availability in soil. Inhibitors, on the other hand, positively affect yields by slowing the release of nitrogen from fertilizers, reducing nitrogen loss through leaching or denitrification, and providing a more consistent nitrogen supply for crops2. This improves nitrogen use efficiency, supporting biomass gains and reducing surplus nitrogen in the environment.
Key sources of soil N2O emissions
The primary sources of soil N2O emissions are microbial nitrification (oxidation of ammonium to nitrate) and denitrification (reduction of nitrate to atmospheric di-nitrogen). Mineral fertilizers, particularly urea fertilizers, contribute about 17% of agricultural NH3emissions. The mechanism of NH3emissions from urea involves the hydrolysis of urea in the soil, where it reacts with water and the soil enzyme urease. This reaction rapidly converts urea to ammonium, which can subsequently be lost as NH3through volatilization.
N2O production in soil is influenced by numerous soil and environmental conditions, including soil nitrogen concentration, soil moisture, soil temperature, organic carbon concentrations, and soil pH. Low soil pH inhibits the final step of denitrification, the conversion of N2O to inert N2, thus increasing N2O emissions. Therefore, acidic soils are potential N2O sources. Intensively managed temperate grazed grasslands often have acidic soils due to high rainfall, low evapotranspiration, and acidifying nitrogen from fertilizers and animal excretion.
Reducing emissions with inhibitors and liming
Nitrification inhibitors (NIs) slow the conversion of ammonium to nitrate, reducing nitrogen loss through leaching and denitrification while extending soil nitrogen availability. Urease inhibitors (UIs) added to urea fertilizers reduce ammonia (NH3) emissions by preventing the rapid breakdown of urea into ammonium. Combined, NIs and UIs lower nitrogen leaching into surface and groundwater, reducing indirect emissions.
Liming increases soil pH, aiding the transformation of N2O to N2 and mitigating N2O emissions. Optimizing soil pH with lime enhances nitrogen use efficiency, improving nutrient regulation and reducing the need for synthetic N-fertilizers.
Co-benefits of using nitrification and urease inhibitors and liming
The adoption of inhibitors in the fertilizer industry offers several co-benefits. They contribute to reducing pollution by maintaining current business practices while improving nitrogen use efficiency, which leads to lower emissions of N2O and NH3.
Financially, urease inhibitors have the potential to increase yields, albeit outcomes vary depending on soil and temperature conditions. By enhancing nitrogen retention, inhibitors can decrease the frequency of fertilizer applications, thereby lowering greenhouse gas emissions from manufacturing fertilizer.
Additionally, reduced fertilizer use reduces fuel consumption and machinery wear associated with tractor passes, which can also improve soil structure and overall soil health. Finally, liming can enhance plant-available phosphorus, boost biomass production, and support carbon sequestration efforts.1
Limitations of nitrate and urease inhibitors and liming
The use of inhibitors in fertilizers presents several challenges. Financially, they are costlier than traditional fertilizers, yet the potential productivity gains may not always offset these higher costs. Their efficacy is variable, influenced by factors such as soil type, moisture and temperature.
Safety concerns include risks of contaminating the food chain and uncertain long-term impacts on soil and water quality, exacerbated by the potential for microplastic pollution from polymer-coated inhibitors. A negative effect of liming is the potential CO2 emissions from lime extraction, transport, and application. Furthermore, there are significant knowledge gaps regarding their overall effectiveness, highlighting a need for further research and understanding in their application.1
Investigating inhibitors and liming on N2O emissions and yields - The Kirkton experiment
To address some of the knowledge gaps, the Kirkton experiments examine how different treatments affect N2O emissions and yields in grazed grassland. One completed experiment assessed the impact of liming on N2O emissions3. A new, ongoing experiment is now testing the combination of liming with urease and nitrification inhibitors.
The experimental treatments consist of eight different combinations of lime application and nitrogen (N) fertilization. Treatments vary by the presence of lime (0 or 5 t/ha) and the type of N application (none, urea alone, urea with a urease inhibitor, or urea with both urease and nitrification inhibitors). Researchers measure N2O fluxes using the static chamber method, along with monthly soil and annual yield assessments.
Preliminary findings of the Kirkton experiment
Previous studies in the UK and Ireland have shown that microbial inhibitors can reduce N2O emissions2. However, initial results from the Kirkton experiment do not support these findings. The first experiment did not show a reduction in N2O emissions or an increase in yield from liming3. Similarly, preliminary data from the current experiment do not indicate that nitrification inhibitors, urease inhibitors, or liming reduce N2O emissions. However, the Kirkton experiment is ongoing, with final results expected in 2025.
Conclusion
Nitrification and urease inhibitors as well as liming have the potential to reduce agricultural greenhouse gas emissions. However, significant knowledge gaps remain. While preliminary results from the Kirkton experiment assesses the impact of inhibitors and liming on N2O emissions, still further research is needed to understand and quantify the long-term effects, optimal application rates, best practices, and the persistence of inhibitors in soils and food chains.
Stephanie Jones, SRUC
and
Victoria Barthelmess, SAC Consulting
Useful Resources
- Buckingham, S. et al. (2023). Greenhouse gas and ammonia emission mitigation priorities for UK policy targets. Frontiers of Agricultural Science and Engineering, 10 (2). 268-280. https://doi.org/10.15302/J-FASE-2023495.
- Cowen, N. et al. (2020). Nitrous oxide emission factors of mineral fertilisers in the UK and Ireland: A Bayesian analysis of 20 years of experimental data. Environmental International, 135 (105366). https://doi.org/10.1016/j.envint.2019.105366.
- Jones, S. et al. (2023). Can precision liming of grasslands mitigate nitrous oxide emissions? The Soil Sentinel, 5. Available at: Soil Sentinel Issue 5 | Scotland's soils (environment.gov.scot).
- Freeman, D. et al. (2020). Evidence review of the efficacy of nitrification and urease inhibitors. ClimateExchange. http://dx.doi.org/10.7488/era/449.
- Bakken, L. and Frostegård, Å. (2020). Emerging options for mitigating N2O emissions from food production by manipulating the soil microbiota. Current Opinion in Environmental Sustainability, 47, pp. 89-94. https://doi.org/10.1016/j.cosust.2020.08.010.
- UK Government (2024). 2022 UK Greenhouse Gas Emissions, Final Figures. Available at: 2020 UK Greenhouse Gas Emissions, Final Figures (publishing.service.gov.uk)
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