Carbon Footprint and Decarbonization Pathways for Mineral Fertilizers

Introduction

Since the mid-19th century, the advent of synthetic fertilizers, combined with advances in agricultural practices, genetics, and the use of crop protection products, has led to a considerable increase in yields and, consequently, in global agricultural production. Globally, in 2022, 109 million metric tons of nitrogen (N), 44 million metric tons of phosphorus (P), and 35 million metric tons of potassium (K) were applied to farmland—six times more than in 1961^[1]. The proportion of these nutrients derived from synthetic fertilizers is increasing.

Global agriculture has thus become heavily dependent on the availability of synthetic fertilizers, and the European Union—like France—is no exception. The production of synthetic fertilizers, particularly nitrogen fertilizers, relies primarily on natural gas. Consequently, most fertilizer plants are located in natural gas-producing countries (China, Russia, the United States, Egypt, etc.). Fertilizer prices—and consequently food prices—are therefore closely linked to the price of natural gas. Furthermore, its availability depends on the geopolitical context: for example, the halt in Russian fertilizer exports following Russia’s invasion of Ukraine has disrupted international markets.^[2].

Beyond economic considerations, fertilizers contribute significantly to global warming. In fact, they are currently a major source of greenhouse gas emissions, particularly due to the release of nitrous oxide (N₂O), a greenhouse gas with a global warming potential nearly 300 times greater than that of carbon dioxide. As a result, fertilizers account for 5% of global greenhouse gas emissions^[3], which is as much as the airline industry^[4].

Since the postwar period, fertilizers have been an essential part of conventional agriculture. Their current use has a significant impact on the environment and contributes to the exceeding of several planetary boundaries, particularly that of climate change. Let’s try to better understand the sources of emissions linked to mineral fertilizers.

To what extent are fertilizers responsible for climate change?

Depending on the geographic region where the fertilizer is produced, the share of greenhouse gas (GHG) emissions associated with manufacturing ranges from 30 to 50 percent of total fertilizer-related emissions, with the remainder primarily attributable to nitrous oxide (N₂O), a gas emitted when fertilizers are applied to crops.

Let's try to understand the origins of these emissions in a little more detail.

Emissions from fertilizer production

Greenhouse gases are emitted during fertilizer production: when the raw materials used to make the fertilizer are produced and transported; and through the energy consumption (in the form of electricity or gas) at fertilizer production plants.

If we trace the production chain back, we can see that All fertilizers are produced from four primary chemicals : ammonia (notably from dihydrogen and diazot), sulfuric acid (from sulfur), phosphoric acid (from phosphate rocks) and thepotassium chloride (from potassic rocks) as shown on the Figure 1. Secondary reactions followed by blending enable fertilizer manufacturers to produce the fertilizer formulations sold on the market, such as ammonium nitrate (a mixture of calcium carbonate and ammonium nitrate), urea, or sulfate ammonium (a mixture of ammonia and sulfuric acid).

The production of synthetic ammonia is based on the Haber-Bosch reaction, a process that requires both fossil fuels and hydrogen—the production of which also relies primarily on fossil fuels—and directly generates nitrous oxide emissions.Note: As expected, the result is very high in carbon: an average of 5 kgCO2e per kilogram of ammonia^[5].

Emissions from nitrogen fertilizer production therefore vary significantly depending on the type of energy used in the industrial process—and thus on the energy mix of the country of production. For example, the production of ammonium nitrate in China using coal as an energy source emits three times more greenhouse gases than ammonium nitrate produced in Europe.^[6]. Special attention must therefore be paid to the source of nitrogen fertilizers when assessing the greenhouse gas emissions associated with their production.

For sulfuric acid, phosphoric acid, and potassium chloride, the situation is slightly different because they are produced from mineral resources (sulfur and phosphate or potash ores): emissions stem from the method used to extract these mineral resources and from the energy used in the processing stage (electricity or even steam for potassium chloride, for example).

We can thus see these various raw materials in the main components of the carbon footprint of the most commonly used mineral fertilizers (see Figure 2).

We will discuss the emissions and possible decarbonization pathways for these raw materials in more detail in our next article.

Emissions from the use of fertilizer

The significant portion of greenhouse gas emissions associated with fertilizer use is due to the release of nitrous oxide into the air, a gas with a global warming potential approximately 270 times greater than that of carbon dioxide: emitting 1 kg of N₂O into the atmosphere is equivalent to emitting 270 kgCO₂e.

When fertilizer is applied, various chemical reactions take place, and these reactions result in various emissions into the air, water, and soil ((see Figure 3) : the direct emissions of nitrous oxide (N2O) associated with the denitrification process and the indirect nitrous oxide emissions related to leaching and volatilization processes (see box below).

The Emissions associated with denitrification account for nearly 80% of emissions related to fertilizer use (See Figure 4). Emissions from volatilization, on the other hand, depend on the form of the fertilizer; for example, they are higher for fertilizers with a high proportion of urea nitrogen, such as urea.

Emissions by Crop

When examining the breakdown of emission sources in agricultural production, emissions related to the manufacture and use of fertilizers very often account for the largest share of greenhouse gas emissions.

For cereal crops, which are the primary crops in France and around the world, emissions related to fertilizers account for approximately 80% of greenhouse gas emissions, with 60% of total emissions coming from fertilizer application and 20% from fertilizer production.

However, this breakdown varies depending on the type of crop and the production method. For example, the energy required to heat agricultural greenhouses can account for up to 97% of greenhouse gas emissions from crops such as tomatoes, eggplants, and peppers, significantly reducing the share of emissions from fertilizers in the crop’s total emissions.

For rice, the world’s third-largest crop, irrigation has a significant impact on greenhouse gas emissions^[8]. The breakdown of emissions is therefore different, but fertilizers still account for the largest share (65%).

How should greenhouse gas emissions from fertilizers change?

There is no specific target for reducing emissions associated with fertilizers.

In France, emissions related toThese businesses are grouped into two business segments.

Recently, France has updated its national strategy, releasing a third version, the SNBC 3^[10], which is still under discussion, sets out the short-term goals (by 2030) and refines the guidelines for achieving these objectives. The long-term goals (by 2050), which involve achieving carbon neutrality across France, remain unchanged for the time being. Three guidelines directly or indirectly concern fertilizers:

• A a target of reducing N2O emissions by 24% by 2033 (end of the SNBC's 4th carbon budget period) compared to 2015;
• The reduction in the use of mineral nitrogen fertilizers : -26% in 2030 compared to 2015;
• Thechanges in production methods including, in particular, the shift in field crops toward low-input systems (50% by 2030), the expansion of legumes (2 Mha by 2030), and the growth of catch crops (4.8 Mha by 2030).

At the European level, the The European Commission has published a “Farm-to-Table” strategy in 2020^[11] which calls for a 50% reduction in pesticide use and a 20% reduction in fertilizer use by 2030. Some Prospective studies have also made it possible to outline a potential reduction target consistent with low-carbon production and use of fertilizers. With regard to the agricultural sector, the European TYFA project led by IDDRI aims to reduce emissions by 40% between 2010 and 2050. At the global level, there is no clear target. However, the GlobAgri-Agrimonde-Terra model developed by Agrimonde Terra proposes various scenarios for the agricultural sector that incorporate different levels of GHG emission reductions (based on IPCC scenarios).

To achieve these ambitious reductions, it is necessary to implement a range of mitigation measures, which we plan to describe in upcoming articles.

To read our previous article on the fertilizer paradox, click here