Crushed rocks and fertiliser switches can cut nitrous oxide from farms

Spreading crushed basalt on cropland and using special fertilisers that stop nitrogen loss could cut global agricultural emissions of a potent planet-warming gas by 25 per cent.

Nitrous oxide is a greenhouse gas with 270 times more warming power than carbon dioxide. Global emissions have risen by 40 per cent in the past 40 years. Agriculture is a major driver, due to increased use of nitrogen-based fertilisers and growing livestock numbers.

Microbes in the soil convert ammonium from fertilisers and animal waste into nitrate and release nitrous oxide in the process. Chemical compounds that hamper this process, called nitrification inhibitors, can be added to fertilisers to reduce nitrous oxide emissions. Spreading basalt rock dust on soils, a technique known as enhanced rock weathering (ERW), can also help by making the soil more alkaline.

But as well as being a planet-warming pollutant, nitrous oxide emissions also have a complex relationship with the ozone layer, in some circumstances aiding its recovery. It has therefore proved difficult to find the best approach to mitigate nitrous oxide emissions without inadvertently harming the ozone layer.

To address this, Maria Val Martin at the University of Sheffield, UK, and her colleagues modelled the impact of widespread use of both ERW and nitrification inhibitors on nitrous oxide emissions and the ozone layer, under two different climate scenarios.

They found a “moderate” approach, in which ERW is deployed in key regions around the world, and where most farmers except the poorest use nitrification inhibitors, could lower nitrous oxide emissions from agriculture by 25 per cent. Overall nitrous oxide emissions would be cut by 5 per cent. These gases also come from combustion engines and industry.

What’s more, up to 2 gigatonnes of additional carbon would be sequestered in the soil thanks to the ERW, and there would be no harm to the ozone layer under both scenarios, says Val Martin.

“We get the [carbon] sequestration from enhanced rock weathering; we have the decrease in nitrous oxide, which is 300 times more powerful than CO2, so we have the climate benefit from reducing nitrous oxide emissions. And then we safeguard the ozone layer,” she says.

Deploying nitrous oxide reduction efforts on this scale would cost billions of dollars. Using ERW to sequester carbon will cost $80-180 per tonne of CO2, according to previous studies, with reduction in nitrous oxide emissions a “free” co-benefit of carbon sequestration, according to Val Martin. Using nitrification inhibitors costs around $28-45 per hectare, and to cover 600 million hectares, as modelled in the study, would cost $17-27 billion annually. That area is around an eighth of all farmland.

Nevertheless, Val Martin says the scenario is deliberately cautious in its ambition, so it could be deployed in the real world. “With this work, what we wanted to do is come up with a scenario that could be realistic. So if governments want to mitigate nitrous oxide, they can do it with [these] strategies that we have in place.”

Parv Suntharalingam at the University of East Anglia, UK, says new strategies to curb nitrous oxide emissions are urgently required, and the study is especially valuable for its focus on curbing emissions without harming the ozone layer.