A student looks at a machine to measure nitrous oxide emissions in a field of oilseed rape
Field study: a student carries out research into nitrous oxide emissions in Germany © Christof Stache/AFP via Getty Images

Nitrogen-based fertilisers are essential for maintaining agricultural production at the level required to feed a growing world population — but they are one of the biggest contributors to the greenhouse gas emissions causing global warming.

A comprehensive analysis published in February by two Cambridge university researchers, Yunhu Gao and André Cabrera Serrenho, found that synthetic fertilisers and manures generate the equivalent of roughly 2.6bn tonnes of carbon dioxide per year. That represents 5 per cent of the world’s greenhouse gas emissions — more than global aviation and shipping combined.

The surprisingly high total results from what happens to fertilisers after they have been applied, says Serrenho. Microbial activity and chemical reactions in the soil release gases — above all, 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 it lasts in the atmosphere for an average of 120 years.

“Two-thirds of emissions take place in the crops and fields after [fertiliser] application and just one-third during the production process,” Serrenho adds. “We know how to produce fertilisers with minimal emissions and we should certainly do that as quickly as possible, but we have to recognise that reducing the remaining two-thirds is mostly about changing farming practices.”

On the production side, Yara International, the Norwegian agricultural giant, says it has cut emissions from its operations by 50 per cent since 2005. The company now wants to achieve a further substantial reduction in its fertiliser carbon footprint by producing ammonia — a key ingredient — with green energy.

Workers in high-visibility jackets walk through Yara’s Porsgrunn plant
Going greener: Yara’s Porsgrunn plant will use renewable energy to produce fertiliser © Reuters

This green adaptation involves generating hydrogen by splitting water molecules using electricity from renewable sources, rather than by extracting it from a fossil fuel, such as natural gas. Birgitte Holter, head of green fertilisers at Yara, says commercial manufacturing of the low-carbon product will begin later this year at the company’s Porsgrunn plant in Norway.

Yara has essentially adjusted conventional technology to make a green fertiliser very similar to existing products made from petrochemicals, but with a carbon footprint that it says will be 80-90 per cent smaller. “The beauty is that these green fertilisers have the same physical and chemical properties as those made from petrochemicals, so that 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 outside Swindon. Its patented process uses CO₂ captured from industrial activities, such as biogas production, that would otherwise be vented into the atmosphere. This reacts with organic materials, such as sludge from sewage treatment plants or waste from food factories, to produce fertiliser pellets rich in fibrous materials as well as nitrates and other crop nutrients. The process also cuts emissions by 90 per cent compared with production methods for conventional mineral fertilisers.

Pawel Kisielewski, CCm chief executive, says the company is working with food manufacturers such as PepsiCo, whose Walkers crisp factory in Leicester generates large quantities of potato waste that makes an ideal input for its fertiliser production.

But sewage treatment is likely to be a larger source in the long term. One of the largest CCm installations, so far, is at Severn Trent Water’s Minworth plant near Birmingham. Waste streams from UK water utilities could produce up to 500,000 tonnes of fertiliser per year, equivalent to a third of the chemical fertilisers used in the UK, the company estimates.

CCm is beginning to sell its CCm Growth product into the UK market, with 3,000 tonnes likely to be available in 2023. “This year’s production is already sold out,” Kisielewski says. “Food companies are seeking to buy tens of thousands of tonnes to help reduce their carbon emissions.”

The company claims that its green fertiliser also tackles the issue of greenhouse gas emissions after application, while improving soil quality as well as delivering essential nutrients to crops. This, it says, is because its pellets have a different, more fibrous composition than conventional synthetic fertilisers, which changes the way they break down in the soil.

Yara, too, is working with farmers to reduce post-application emissions from its fertilisers, by minimising the amount of fertiliser that is not absorbed by the plant.

“We must contribute in every way we can to build a nature-positive food future, and here we increase ‘nitrogen use efficiency’ by matching the supply of nutrients to the crop’s requirements,” says Anke Kwast, who leads Yara’s drive to become carbon neutral. “We are developing tools to help farmers achieve just-in-time nutrient management.”

By getting the nitrogen use efficiency right, farmers can substantially reduce a crop’s carbon footprint, she adds. Yara supplies an app that monitors crop growth and measures the plants’ nitrogen requirement as the fertiliser spreader moves across the field, adjusting the application rate accordingly.

At Cambridge, Serrenho says one chemical means of reducing post-application emissions would be to add compounds called nitrification inhibitors, which prevent soil bacteria converting nitrogen in the fertiliser into nitrous oxide. But he points out that not enough is known about their environmental impact to be confident in their safety — and they would make fertilisers more expensive.

Instead, the single most effective way to reduce fertiliser emissions would be to cut substantially the amount used. “We are using far more than we need,” Serrenho says. “We need to find the right mix of financial, technological, and policy solutions to reduce emissions while keeping the world fed.”

The Cambridge study suggests that, using all the mitigation measures available for fertiliser manufacturing and application, emissions could be reduced by 80 per cent from today’s levels.

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