Farmers regularly add fertilizers to their soils to provide crops with the nutrients they need to grow. While this has boosted crop production in the last century, it has also come at a cost for the planet, with fertilizers accounting for 1.4% of global CO2 emissions — more than the total emissions of countries like France or Germany — or even up to 5% by other estimates.

Reducing the emissions of fertilizers without affecting food production would be important for reducing our greenhouse gas emissions. Now, researchers have found a way to do just that.

Fertilizers and greenhouse emissions

“In order to reduce emissions, it’s important for us to identify and prioritize any interventions we can make to make fertilizers less harmful to the environment,” André Cabrera Serrenho, study co-author, said in a media statement. “But if we’re going to do that, we first need to have a clear picture of the whole lifecycle of these products.

One of the main nutrients that plants need to grow is nitrogen. But plants can’t take it from the air the way they absorb CO2. In the early 1990s, scientists invented a process to produce a nitrogen-containing compound, ammonia, that plants can absorb from the soil. But making ammonia requires a lot of energy, and this energy mostly comes from fossil fuels.

Fertilizers also produce emissions after farmers apply them to their fields, as crops only take up half of the nitrogen they get from fertilizers. The rest runs off into waterways or gets broken down into the atmosphere, releasing nitrous oxide. This greenhouse gas has a warming potential 300 times higher than carbon dioxide.

For their study, the researchers mapped the global flows of fertilizers and their emissions for 2019 along all stages of the lifecycle. They found most of the emissions from fertilizers don’t happen during production but during their use. “Only after quantifying all emissions, we can look at mitigation methods,” Serrenho explained.

The researchers listed and quantified the impact of different emissions reduction methods. While most are already known, their potential effect hadn’t been quantified. The most effective measure at the production stage would be to decarbonize the heating and hydrogen production that occurs from ammonia synthesis, they said.

Additionally, fertilizers could be mixed with a group of chemicals known as nitrification inhibitors, which prevent bacteria from forming the nitrous oxide. However, using these chemicals would make fertilizers more expensive. This would require providing a financial incentive to farmers and fertilizer-producing companies, the researchers said.

However, the most effective way to reduce fertilizer-related emissions would be to reduce the amount of fertilizer that we use. “We’re using far more than we need, which is economically inefficient and that’s down to farming practices. If we used fertilizer more efficiently, we would need substantially less fertilizer,” Serrenho said.

Implementing all the emission reduction measures analyzed in the study would allow reducing emissions in the fertilizer sector by as much as 80% by 2050, the researchers estimated. The world has to find the right mix between financial, technological and policy solutions to reduce emissions while still feeding the world, they concluded.

The study was published in the journal Nature.