One unwelcome byproduct of agricultural fertiliser use could accelerate global warming by a factor of hundreds. But smart agronomists are looking at a way to cut its impact – while at the same time boosting crop yields. By Thin Lei Win.

Rome, June 2021. Much of the world’s agriculture is underpinned by the use of nitrogen-based fertilisers. Hundreds of millions of tonnes of the stuff are spread on fields worldwide. A stealth byproduct of nitrogen fertiser use is the gas nitrous oxide – which as a greenhouse gas is 300 times more potent than carbon dioxide. And the quantity in the atmosphere is rising.

Normal soil processes driven by microbes in the soil produce nitrous oxide (N2O), also known as laughing gas, which is then released into the atmosphere in small amounts. More worryingly, the use nitrogen based fertiliser creates large quantities of nitrous oxide when warm moist soils are while are exposed to excess reactive nitrogen – fertiliser, in other words.

Researchers published a report recently in the journal Nature that showed levels in the atmosphere - where it remains for more than 100 years - have risen 20% since 1750, while human-attributed N2O emissions have increased by 30% since 1980. China alone emitted some 550 million tonnes of H2O in 2016, and that figure has likely increased.

The problem is compounded by the availability of high subsidies for nitrogen fertilisers to farmers in countries like India. Worse, there is currently no replacement for nitrogen fertilisation, notes Pep Canadell, of Australia’s Commonwealth Scientific and Industrial Research Organisation.

But a more recent paper from PNAS has suggested a possible solution that could achieve a triple win – one that reduces N₂O emissions, boosts yields by up to 50%, and helps save land from deforestation.

“Global and U.S. nitrogen pollution have been growing relentlessly, causing ocean dead zones, unhealthy air and large greenhouse gases,” said paper co-author Tim Searchinger working out of Princeton University alongside Canadell. But there could be a solution.

Together, Searchinger and G.V. Subbarao from Japan International Research Center for Agricultural Sciences describe a strategy to breed crops that suppress nitrous oxide emissions by making use of a trait called biological nitrification inhibition (BNI) that is naturally present in many crops such as rice, wheat, maize and sorghum,

The key is that BNI capability limits the nitrification of ammonium nitrate fertilisers (NH₄⁺) into nitrate (NO₃⁻). “Once nitrogen nitrifies into NO₃⁻, it has a good chance of polluting the environment unless crops or grasses quickly take it up,” explain the researchers. The excess NO₃⁻ can also easily leach into groundwater and waterways and release N₂O that way.

So anything that can help cut down nitrification of fertilisers is good, reason Searchinger and his research team. If agronomists could cross-breed existing high yield staple crops such as rice, wheat, maize and sorghum with either wild species of these staples or little used varieties that have stronger BNI traits, the result would be less N2O emissions.Tim Searchingerleach into groundwater and waterways

This approach, says Searchinger, could significantly reduce emissions of the “laughing gas” while studies have also shown that adding more ammonium (non-nitrified fertiliser) increased wheat growth by 54% and maize by more than 80%.

This could mean even better yields from existing land – so less need to chop down more trees to produce more food.

“The insight of the Nature article is to combine three different strands of fairly obscure research that are under-appreciated by themselves and whose potential joint implications have not previously been recognised,” said Searchinger.

“First is the evidence that no matter how efficiently farmers apply nitrogen fertiliser, there’s still going to be large-scale nitrogen pollution and nitrous oxide emissions because of soil processes in cropland.

“Second is the potential recently discovered to breed crops in ways that enhance natural properties to suppress the microbial activity in soils (nitrification) that leads to these nitrogen losses and pollution. 

“And third is an obscure line of agronomy that has found that if nitrogen in soils could remain in a more balanced form between ammonium and nitrate – what you would get if you suppressed nitrification – grain crops could have very large yield gains.”

All of which is good news for both food security and reductions in greenhouse gas pollution.