Nitrogen-based fertilizers are essential for keeping agricultural production at the level required to feed a growing world population, but they are a major contributor to the greenhouse gas emissions that cause global warming.
A complete analysis published in February by two researchers from the University of Cambridge, Yunhu Gao and André Cabrera Serrenho, found that synthetic fertilizers and manure generate the equivalent of about 2.6 billion tons of carbon dioxide a year. This represents 5% of global greenhouse gas emissions, more than global aviation and shipping combined.
The surprisingly high total comes from what happens to fertilizers after they’re applied, Serrenho says. Microbial activity and chemical reactions in the soil release gases, especially nitrous oxide, which has a very powerful warming effect. Each molecule of nitrous oxide is 265 times more potent than CO₂ as a greenhouse gas and remains in the atmosphere for an average of 120 years.
“Two-thirds of emissions occur in subsequent crops and fields [fertiliser] application and only a third during the production process,” adds Serrenho. “We know how to produce fertilizers with minimal emissions and we should certainly do it as quickly as possible, but we must recognize that the reduction of the remaining two thirds is mainly about changing agricultural practices.” .
As for manufacturing, Yara International, the Norwegian agricultural giant, says it has reduced emissions from its operations by 50 percent since 2005. The company now wants to achieve a further substantial reduction in the carbon footprint of fertilizers by producing ammonia, a key ingredient, with green energy.
Greener: Yara Porsgrunn plant to use renewable energy to produce fertilizer © Reuters
This green adaptation involves generating hydrogen by splitting water molecules using electricity from renewable sources, rather than extracting it from a fossil fuel, such as natural gas. Birgitte Holter, head of green fertilizers at Yara, says commercial production of the low-carbon product will begin later this year at the company’s Porsgrunn plant in Norway.
Yara has essentially adapted conventional technology to create a green fertilizer very similar to existing petrochemical-based products, but with a carbon footprint it says will be 80-90% lower. “The beauty is that these green fertilizers have the same physical and chemical properties as those made from petrochemicals, so farmers can use them in the same way,” Holter explains.
Others are introducing more extensive innovations. One is CCm Technologies, a UK start-up based out of Swindon. Its patented process uses CO₂ captured from industrial activities, such as biogas production, that would otherwise be released into the atmosphere. This reacts with organic materials, such as sludge from wastewater treatment plants or waste from food factories, to produce fertilizer pellets rich in fibrous materials, nitrates and other nutrients for crops. The process also reduces emissions by 90% compared to conventional mineral fertilizer production methods.
Pawel Kisielewski, chief executive of CCm, says the company is working with food producers such as PepsiCo, whose Walkers potato chip factory in Leicester generates large quantities of potato waste which is an ideal input for its fertilizer production.
But wastewater treatment is likely to be a bigger source in the long run. One of the largest CCM installations to date is at the Minworth factory at Severn Trent Water near Birmingham. Waste streams from UK water services could produce up to 500,000 tonnes of fertilizer a year, equivalent to a third of the chemical fertilizers used in the UK, the company estimates.
CCm is starting to sell its CCm Growth product in the UK market, with 3,000 tonnes likely to be available in 2023. “This year’s production is already sold out,” says Kisielewski. “Food companies are looking to buy tens of thousands of tons to help reduce their carbon emissions.”
The company says its green fertilizer also addresses the issue of greenhouse gas emissions after application, while improving soil quality and providing essential nutrients to crops. That, he says, is because his pellets have a different, more fibrous composition than conventional synthetic fertilizers, which changes how they break down in the soil.
Yara is also working with farmers to reduce the post-application emissions of its fertilizers by minimizing the amount of fertilizer that is not absorbed by the plant.
“We must contribute in every way possible to building a nature-positive food future, and here we increase ‘nitrogen use efficiency’ by matching nutrient inputs to crop needs,” says Anke Kwast, who leads the Yara’s drive to become carbon neutral. “We are developing tools to help farmers achieve just-in-time nutrient management.”
By getting the right nitrogen use efficiency, farmers can substantially reduce a crop’s carbon footprint, he adds. Yara provides an app that monitors crop growth and measures the plants’ nitrogen needs as the spreader moves across the field, adjusting the application rate accordingly.
At Cambridge, Serrenho says a chemical means of reducing post-application emissions would be the addition of compounds called nitrification inhibitors, which prevent soil bacteria from converting the nitrogen in the fertilizer into nitrous oxide. But he points out that not enough is known about their environmental impact to be sure of their safety, and they would make fertilizer more expensive.
Instead, the single most effective way to reduce fertilizer emissions would be to cut substantially the amount used. “We’re using way more than we need to,” Serrenho says. “We need to find the right mix of financial, technological and political solutions to reduce emissions while keeping the world fed.”
The Cambridge study suggests that, using all available mitigation measures for fertilizer production and application, emissions could be reduced by 80% from today’s levels.
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