Unlike organic fertilisers, which come from plant or animal material, synthetic fertilisers are made by humans using chemical processes.
Production and transportation cause carbon emissions, while agricultural use of these fertilisers leads to the release of nitrous oxide (N₂O) – a greenhouse gas 265 times more potent than carbon dioxide (CO₂) over a century.
The research team – from the Greenpeace Research Laboratories at the University of Exeter, and the University of Turin – found that the synthetic nitrogen fertiliser supply chain was responsible for emitting the equivalent of 1.13 gigatonnes of CO₂ in 2018.
This is more than 10% of global emissions from agriculture, and more than the emissions from commercial aviation in that year.
The top four emitters – China, India, USA and the EU28 (European Union countries plus the UK) – accounted for 62% of the total.
“There is no doubt that emissions from synthetic nitrogen fertilisers need to be reduced – instead of increasing, as is currently predicted,” Dr Reyes Tirado, from the Greenpeace Research Laboratories.
“The global agri-food system relies on synthetic nitrogen to increase crop yields, but use of these fertilisers is unsustainable.
“Emissions could be reduced without compromising food security.
“At a moment when synthetic fertiliser prices are skyrocketing, mirroring the energy crisis, reducing their use could both benefit farmers and help us tackle the climate crisis.”
When nitrogen fertiliser is applied to soil, some is taken up by plants and some is used by soil micro-organisms, which produce N₂O as a by-product of their metabolism. Nitrogen can also end up leaching from the site.
The researchers say the most effective strategy for cutting emissions is to reduce over-fertilisation – which currently happens in most cases.
“We need a comprehensive scheme to reduce overall use of fertilisers and to increase efficiency of nitrogen recycling in agricultural and food systems,” said Dr Stefano Menegat, from the University of Turin.
“We can produce enough food for a growing population with a much smaller contribution to global greenhouse gas emissions, without compromising yields.
“Shifting dietary patterns towards less meat and dairy products could play a central role.
“Three quarters of nitrogen in crop production (expressed in terms of protein and including bioenergy by-products) is currently devoted to livestock feed production globally.”
The study’s data, from 2018, showed North America had the highest annual per person nitrogen fertiliser use (40kg) followed by Europe (25-30kg). Africa had the lowest use (2-3kg).
The research team developed the largest field-level dataset available on N₂O soil emissions.
Using this, they estimated national, regional and global N₂O direct emission factors, while they used existing literature to find emission factors for indirect N₂O soil emissions, and for nitrogen fertiliser manufacturing and transportation.
The paper, published in the journal Scientific Reports, is entitled: “Greenhouse gas emissions from global production and use of nitrogen synthetic fertilisers in agriculture.”