Farming contributes to both ammonia and methane emissions. A recent guidance document maps the synergies and trade-offs in mitigating these two gases.
The Taskforce on Reactive Nitrogen has recently unveiled a policy brief and guidance document, titled “Co-mitigation of Methane and Ammonia Emissions from Agricultural Sources”. This document is set to be presented at the upcoming meeting of the executive body of the Convention on Long-Range Transboundary Air Pollution in December, aiming for adoption as an official document of the convention.
The rationale behind the document is grounded in the observation that the lion’s share of ammonia emissions and about half of anthropogenic methane emissions in the region covered by the convention1 emanate from agricultural activities. The objective? To unravel the effects of mitigation measures on both gases and explore potential interactions. While some measures show synergistic benefits, a call for further optimisation of practices echoes through the document to minimise trade-offs in mitigating these two gases.
Agricultural ammonia emissions predominantly result from livestock manure management and nitrogen fertiliser use, whereas methane emissions predominantly stem from enteric fermentation in ruminants. Rice production also contributes to both methane and ammonia emissions, albeit representing a small fraction of the overall emissions in the region.
Optimisation of animal diets and nutrient management is considered the category of measures that exhibits the most significant synergies between the mitigation of the two gases.
This includes ensuring good animal health, which enhances feed use efficiency at both the individual animal and herd levels, contributing to the mitigation of ammonia and methane emissions at the production system level.
Improved feed efficiency means reduced emissions associated with growing feed crops, while higher feed use efficiency results in less nitrogen in manure storage and decreased emissions during application.
Higher productivity in ruminant livestock is another strategy for lowering enteric methane and ammonia emissions per unit of product. This is due to a larger share of carbon and nitrogen metabolism being directed toward growth or milk production rather than animal maintenance. However, the document cautions about potential risks to animal health associated with increased productivity.
Some strategies for mitigating enteric methane emissions focus on specific feeds or feed additives. For instance, the addition of 3-Nitrooxypropanol (3-NOP) to cattle diets has shown a significant reduction in enteric methane emissions without compromising production. But the efficacy of 3-NOP decreases when not fed frequently, which is not possible during grazing. This supplement is currently only registered for dairy use, and its effects on ammonia emissions are considered negligible.
The addition of other compounds, such as fats or nitrate to cattle diets has shown lesser reductions in enteric methane. The document emphasises the need for proper dietary formulation, since adding fat may increase methane from manure storage, and adding nitrate may elevate ammonia emissions by increasing the nitrogen content of excreta.
The second most relevant category for synergies, identified in the document, is manure treatment and storage. Methane emissions from manure storage are a result of microbial decomposition under anaerobic or low-oxygen conditions. Ammonia, on the other hand, forms when urease enzymes catalyse the hydrolysis of urea. An interesting point of distinction is that while ammonia flux depends on the manure surface/gas layer interface, methane production occurs within the bulk of the manure.
Various forms of manure coverage are the most widely used methods for mitigating ammonia emissions from storage. It is common to cover the slurry storage with semi-porous materials that favour the growth of methane-oxidising microbes, which convert methane into carbon dioxide and water. However, it is vital to avoid the development of gaps in the cover, as they can compromise the effectiveness of these measures.
Another option is to enclose the slurry storage with an impervious cover. This approach may potentially increase slurry temperature and consequently methane emissions. However, a gas-tight cover opens up the possibility of flaring or biofilters to convert methane to carbon dioxide.
Over the past decade, acidification of slurry has gained popularity as a method to mitigate ammonia emissions. Since acidification has also demonstrated effectiveness in reducing methane emissions, it should be seen as a win-win technology. The acid dosage required for methane reduction is, however, lower than the typical amount used for ammonia mitigation.
Utilising animal manure in biogas production through anaerobic digestion is a popular practice, which effectively reduces methane emissions, provided that the generated methane is efficiently collected and utilised for energy or industrial purposes. In this process organic nitrogen is converted into ammonium, which enhances its efficiency for crop fertilisation.
However, the increased ammonium concentration and pH in the biogas digestate may also lead to elevated ammonia emissions during storage and field application. This in turn can be mitigated by coverage, acidification, and low-emission application technology.
Measures related to animal housing are the third most relevant area for synergies. For instance, frequent flushing of slurry from animal housing reduces methane emissions in colder climates, with an anticipated reduction in ammonia emissions. The cooling of slurry in animal housing, a recognised method to reduce ammonia emissions, also exhibits positive impacts on methane emissions.
The document emphasises the holistic consideration of the entire chain of farm operations, cautioning against fixating on one measure. Preventing ammonia and methane emissions from manure in animal housing retains more nitrogen and carbon. However, without mitigation practices, emissions may still occur during storage or application. This holistic approach underscores the need for comprehensive strategies to effectively address the complex interplay of factors contributing to ammonia and methane emissions in farming.
Kajsa Pira
1. All European countries, North America (Canada and United States), Central Asia (Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan and Uzbekistan).
The not yet adopted version of “Co-mitigation of methane and ammonia emissions from agricultural sources: policy brief and guidance” can be found here: https://unece.org/sites/default/files/2023-06/ECE_EB.AIR_WG.5_2023_5_%28...
