Fertiliser emissions could be cut to ‘one-fifth of current levels’ by 2050

Fertiliser emissions could be reduced to approximately one-fifth of current levels by 2050 without the need for developing new technologies, a new study finds.

The research, published in Nature Food, looks at the production, trade and use of nitrogen fertilizers around the world to quantify their total greenhouse gas emissions.

The study finds that, on its own, making nitrogen use in crops more efficient could halve the demand for synthetic fertilizers, which would reduce their overall emissions. Some of the other proposed actions include using renewable and nuclear energy to power production of fertilizers.

By combining all available interventions, the researchers outline that emissions from nitrogen fertilizers could be cut by up to 84% by 2050, a higher figure than previously estimated.

However, a co-author on the research tells Carbon Brief that this figure would be “very challenging” to achieve, “both economically and in terms of practical implementation”.

Nitrogen fertilizers have been called “the most important invention of the 20th century” by the environmental scientist Prof Vaclav Smil in an essay published in Nature in 1999.

They broadly fall into two categories – organic and inorganic. (The latter is sometimes referred to as chemical, mineral or synthetic fertiliser.)

Organic fertilizers are mostly made of crop residues or animal waste or remains, such as manure or compost. Inorganic, or synthetic, fertilizers are made from chemically manufactured materials. Both types of fertilizers are used to encourage plant growth and boost crop yields.

Their widespread use has increased global food production, but these fertilizers have also led to a host of negative impacts on the environment, including causing harmful algal blooms in nearby watercourses, soil acidification and greenhouse gas emissions.

When nitrogen fertilizers are applied to a field, some of the element is lost to the environment – leached into soils, washed into nearby water or released as vapour into the atmosphere.

Microbes in soil break down nitrogen fertilizers to produce nitrous oxide (N2O) – a greenhouse gas nearly 300 times as powerful as CO2. In addition, producing fertilizers is an energy-intensive process, on its own responsible for almost 1.5% of total global CO2 emissions.

A 2020 study found that the global use of nitrogen fertilizers for food production could threaten efforts to keep global warming below 2C.

The new research estimates that the production and use of nitrogen fertilizers in food-growing accounts for around 5% of global greenhouse gas emissions.

The study examines the global flows of both synthetic fertilizers and organic ones, looking at their emissions in 2019. It finds that two-thirds of the emissions result from the use of nitrogen fertilizers while just one-third occurs from the production stage.

To track nitrogen-fertilizer mass flows, the researchers used data on the production and consumption of fertilizers in nine different regions of the world, taking into account how emissions intensive fertilizer production is in each region.

They estimate that emissions of 2.6bn tonnes of CO2-equivalent (CO2e) occurred from the production and use of synthetic nitrogen fertilizers and manure in 2019.

The researchers find that manure applied to soils has, on average, nearly double the emissions of synthetic fertilizers. They write that, therefore, manure is “currently not an appropriate substitute for synthetic fertilizers” in terms of greenhouse-gas mitigation.

The study examines the potential emissions impacts of different interventions, such as using renewable-powered electricity for production, reducing overall demand for fertilizers and improving their efficiency.

The chart below shows different future emission scenarios achievable through interventions such as increasing nitrogen efficiency and changing methods of ammonia production.

Improving efficiency is the “single most effective strategy to reduce emissions”, the researchers say.

Currently, the global efficiency of nitrogen use in crops is 42% – meaning that only 42% of the nitrogen applied to the soil is taken up by plants. By increasing this efficiency to 67% – roughly the level achieved in the US and Canada, according to a 2015 study – the new research finds that total fertilizer demand could be reduced by 48% by 2050.

A combination of approaches is required to achieve better efficiency, including proper irrigation, breeding crops that can better utilise nitrogen fertilizers and applying the right fertilizers at the correct rate, time and place.

In much of the world, the amount of nitrogen used on crops – both through artificial and natural sources – is “much greater than the nitrogen requirements of crops” at the moment, the study adds.

The map below, first published in a 2022 Carbon Brief article about fertilisers and climate change, identifies the amount of excess nitrogen per hectare of cropland across the globe.

Both production and usage also need to be decarbonised in the effort to reduce fertilizer emissions, the study finds. A more electrified system powered by renewables can have a sizeable impact on the production emissions.

Nitrogen and hydrogen are required to produce the compound ammonia, which makes up the backbone of nitrogen fertilizers. Hydrogen is currently produced using fossil gas, but emissions from this process could be reduced if it was instead made through water and electricity (a process called electrolysis) powered by renewables or nuclear power.

Looking at fertilizer use, the research examines three possible routes for combatting emissions: reducing demand, reducing nitrous oxide emissions directly and changing the mix of fertilizer types.

A co-author on the study, Dr André Cabrera Serrenho, an assistant professor at the University of Cambridge, says the 84% reduction figure can be achieved partly by changing the forms of nitrogen used in fertilizers. He tells Carbon Brief:

“So if instead of using urea, we use ammonium nitrate, we can then increase slightly further the potential reduction, from 80% to 84%.

“But changing the mix of fertilizers itself only leads to emissions savings after we do everything else. In the current conditions, if we do that substitution we end up actually putting more emissions into the atmosphere than otherwise.”

Substituting the type of nitrogen in fertilizers does not in itself reduce emissions, the study says – on its own, it actually leads to a 3% increase in emissions. The decrease is only seen when the replacements are made alongside other interventions.

Ammonium nitrate and urea are both sources of nitrogen – but the emissions from using ammonium nitrate can be significantly lower than those from using urea, a 2021 study says.

Improved nitrogen-use efficiency would help to reduce demand for nitrogen fertilizers; this reduced demand would in turn significantly lower emissions.

The study also says that the use of “nitrification inhibitors” can reduce total emissions.

Nitrification inhibitors (pdf) are chemicals added to fertilizers to limit nitrogen loss when the fertiliser is applied to cropland. Nitrogen loss can lead to soil acidification; limestone is commonly used to combat this acidification. So reducing nitrogen loss also reduces the requirements for limestone to correct acidification, thereby reducing total emissions. (When lime reacts with soil acidity, it produces CO2.)

The chart below shows the “business-as-usual” scenario for synthetic fertilizer emissions up to 2050 (left) and the emission impacts of different interventions considered in the study (right).

Serrenho says cleaner energy sources are an “extremely important” factor in decarbonising fertilizers, adding:

“We know that one of the actions that we have to implement to decarbonise society involves electrifying all our energy uses and producing electricity from either renewables or nuclear power.

“In the case of ammonia production and fertilizers, if we are going to produce hydrogen that we use for ammonia using electrolysis we are going to require a substantial amount of electricity. That electricity needs to come from renewables or nuclear, otherwise we will still have substantial amounts of emissions associated with ammonia production.”

Many of these actions to reduce emissions can be deployed at the same time, “thus enhancing the total mitigation potential”, the researchers write.

Prof Mark Sutton, an environmental physicist at the UK Centre for Ecology and Hydrology, welcomed the “optimism” of the mitigation routes, adding that the research comes at a useful time following the setting of a global target to reduce nutrient loss.

The target, agreed under a wider framework (pdf) at the COP15 biodiversity summit in Montreal, aims to halve nutrient loss to the environment by 2030. Sutton tells Carbon Brief:

“I think what is really new [in the study] is this very high level of saying ‘we can do it’, and that’s why it’s a challenge to the community, for people who say we can’t go quite as far as this.

“I’ve not seen the whole set of emissions from all sources combined showing that level of ambition.”

Some combinations of interventions yield very high reduction rates by mid-century. One scenario the researchers studied suggests that total 2050 emissions can be reduced by up to 78% when water electrolysis for ammonia production is combined with both a reduced demand for fertilizer production and the use of nitrification inhibitors.

The study adds that all of the interventions examined have “high technology-readiness levels”, meaning that they could be deployed on a large scale in the near future.

But mitigation options may not be easy to deploy everywhere, the study says. For example, newer production facilities may be able to apply water electrolysis and electric heating for ammonia synthesis. But in older facilities, it may be easier to couple carbon capture and storage (CCS) to the existing equipment.

Geographical variations on suitable carbon capture and storage basins or wind power availability may also impact local adaptation of measures. For the electric heating scenarios in the study, wind power is assumed to provide heat for production, but the extent to which this is possible differs across the world.

Sutton says that this research should “play into the debate about the measures” needed to cut emissions from fertilizers and improve overall efficiency, adding:

“The step beyond is how to get there. How to get business to move and to show that there’s an opportunity for business there if they are the leaders in change, rather than trailing.”