Biochar Can Cut or Raise Nitrous Oxide Emissions

A soil additive regarded by many scientists as a potential climate solution may not deliver the same results in every setting.

New research finds that biochar – a carbon-rich substance produced by burning crop residues – can either lower or sharply raise nitrous oxide emissions, depending on the soil in which it is applied.

In a dry upland agricultural soil, the amendment reduced nitrous oxide emissions. Yet in a nearby flooded rice paddy, the identical treatment sent emissions soaring.

The findings underline the powerful influence of soil conditions on climate outcomes. A measure that benefits one field could increase emissions in another.

The study, headed by Dr. Jinbo Zhang of Hainan University, indicates that reducing agricultural greenhouse gases may demand much more precisely targeted approaches than scientists had previously expected.

Nitrous oxide’s climate impact

Nitrous oxide receives less attention than carbon dioxide, yet it is among the atmosphere’s most potent greenhouse gases.

According to the U.S. Environmental Protection Agency (EPA), once emitted, it remains in the atmosphere for an average of around 121 years and traps approximately 265 times more heat than carbon dioxide over a 100-year period.

A substantial share of human-produced nitrous oxide originates from fertilised farmland. As soil conditions change, microbes convert surplus nitrogen into a range of gases.

Cutting these emissions requires agricultural methods that direct nitrogen away from nitrous oxide, while maintaining crop yields and using fertiliser efficiently.

What biochar does in soil

Biochar affects more than the amount of carbon held in soil. Its porous form can retain water, alter soil acidity and supply additional carbon to microbes.

These effects can change the way soil microbes process nitrogen. Under certain conditions, nitrogen is directed towards harmless nitrogen gas; under others, nitrous oxide production can rise instead.

As different feedstocks and burning temperatures produce biochar with varying chemical properties, outcomes can differ greatly among soils and crops.

This variation helps account for why the same amendment lowered emissions in one field but increased them in another.

Biochar works in upland soils

In the dry upland soil, acidity made it easier for nitrous oxide to form, so the researchers tested lime alongside the char.

Biochar reduced emissions more effectively than lime, as it suppressed reactions that generated nitrous oxide while supporting those that turn it into nitrogen gas.

The soil contained fewer high-emitting fungi, while genetic indicators associated with nitrous oxide removal became more widespread.

For acidic upland fields, this pattern indicates that farmers may be able to reduce emissions while adding carbon – provided moisture remains at moderate levels.

Biochar backfires in rice paddies

In the flooded paddy soil, lime was unable to stop emissions increasing, while biochar applications drove nitrous oxide levels much higher.

At the end of the experiment, soil treated with three or five percent biochar emitted substantially more nitrous oxide than untreated soil, with emissions increasing by more than fivefold and over fourteenfold.

Several microbial pathways became more active simultaneously because the added carbon and altered chemistry supplied the flooded microbial community with more resources.

Waterlogged rice systems may therefore require different guidance, as the same amendment can increase emissions alongside existing burdens.

Microbes set the dial

Soil microbes were the decisive factor, and the team identified the pathways that generated nitrous oxide under each set of conditions.

During denitrification, when microbes convert nitrate into gases as oxygen becomes scarce, bacteria and fungi release nitrous oxide as an intermediate product.

A fungal group known as Chaetomium declined in the upland soil after biochar was added, but increased in the paddy soil.

Emissions dropped when microbes completed the process by converting nitrous oxide to nitrogen gas. Flooded soils, however, seldom supported that result.

Soil microbes drive emissions

Using biochar to meet climate goals starts with managing soil water, because wet and dry fields function as markedly different systems.

Assessing soil acidity, organic matter and irrigation practices can help identify whether the amendment is likely to favour microbes that convert nitrous oxide into harmless nitrogen gas.

“This tells us that biochar is not a one-size-fits-all solution,” said Dr. Zhang. In flooded rice paddies, that warning may mean delaying its use until field trials demonstrate a clear overall benefit.

The results are based on laboratory incubations, which show clear cause-and-effect links but cannot completely reproduce a growing season involving roots, rainfall and changing weather. A test lasting only four days can also overlook slower developments as biochar ages, microbes adjust and fertiliser schedules progress over weeks.

In real fields, ploughing, drying and re-flooding cycles also change oxygen levels and affect which gases are released from the soil.

Until long-term studies establish how these patterns occur outdoors, researchers say biochar should be viewed as a locally tested tool rather than a guaranteed climate solution.

Smarter ways to use biochar

Understanding the microbial pathways that produce nitrous oxide gave the researchers a straightforward practical aim: guide nitrogen towards harmless nitrogen gas instead.

This may involve selecting biochar types, application rates and timings that support microbes able to reduce nitrous oxide in dry soils.

“By identifying the microbial pathways behind these emissions, we can begin to design smarter soil management practices,” the researchers noted, including Zhang.

More precise targeting could retain biochar in locations where it helps to reduce emissions, while avoiding soils in which it may intensify them.

The findings indicate that biochar can reduce nitrous oxide where dry, acidic soils support microbes that complete the process by converting the gas to nitrogen. Future field research will need to identify where emissions increase or decrease, enabling climate programmes to reward the appropriate practice in the appropriate location.