Soil is losing nitrogen, along with its climate-fighting power

Plants can’t pull carbon from the air without nitrogen in the ground. A new global study finds that forests, prairies, and other natural areas are receiving far less new nitrogen than scientists previously believed. This shortfall could weaken nature’s ability to slow climate change.

Meanwhile, farms worldwide are increasing biological nitrogen inputs to boost crop growth. But when those nutrients spill over, they threaten water, air, and biodiversity.

How the study was conducted

The analysis was led by Carla Reis Ely while she was a postdoctoral scholar in the Oregon State University College of Forestry. The project involved 24 scientists from around the world.

The team re-examined field measurements of biological nitrogen fixation, the microbial process that turns nitrogen gas in the air into plant‑usable forms. In doing so, they uncovered a major blind spot.

Past studies focused on places where nitrogen‑fixing organisms were especially common. These hotspots were represented in the data about 17 times more often than they occur across natural ecosystems globally. Correcting for that bias drove estimated natural nitrogen inputs sharply downward.

Nitrogen fuels plant growth

Nitrogen makes up more than three‑quarters of Earth’s atmosphere (most of the rest is oxygen), yet most organisms can’t use it in that gaseous form (N₂).

Nitrogen‑fixing bacteria step in. They convert N₂ into ammonia and related compounds that plants can take up through their roots.

Plants need nitrogen to build proteins and chlorophyll – the green pigment that powers photosynthesis. This allows them to grow, stay healthy, reproduce, and convert atmospheric carbon dioxide into biomass.

“In natural ecosystems, nitrogen fixation improves soil fertility and supports plant growth, thereby increasing carbon storage,” said Reis Ely.

“However, our new estimate of natural nitrogen fixation, based on improved scientific understanding, suggests less new nitrogen enters natural ecosystems.”

Less nitrogen, less carbon

If soils in wild landscapes are getting less fresh nitrogen, plants in those systems may be more nutrient‑limited than expected. That could mean slower growth and, over time, less CO₂ capture than many global carbon budgets assume.

According to Reis Ely, one implication of the revised numbers is that natural ecosystems may take up less carbon dioxide than scientists had projected, which in turn lowers the expected capacity of unmanaged lands to buffer climate change.

While nature’s nitrogen input shrinks on paper, agriculture’s slice is growing. Farmers are planting more legumes – soybeans, alfalfa, peanuts, and clovers – that host nitrogen‑fixing bacteria in root nodules.

That living fertilizer reduces the need for synthetic nitrogen, supports yields, and can help rebuild tired soils.

“Using nitrogen-fixing crops in rotation can support long-term soil health and reduce environmental damage from chemical fertilizers,” Reis Ely said.

Farms flood soil with nitrogen

There is a tipping point, though. Reis Ely explained that too much nitrogen can throw off the overall balance of nutrients in the soil, and excess nitrogen can leach into groundwater or run off into lakes and streams, causing algae blooms and harming aquatic life.

Surplus nitrogen can also transform into nitrous oxide, a potent greenhouse gas, and can tilt plant communities toward fast‑growing invasive species that crowd out natives and reduce biodiversity.

“High agricultural nitrogen fixation is a complex issue. We do benefit from it, but along with applications of synthetic nitrogen fertilizer, it contributes to nitrogen pollution and climate change,” noted Reis Ely.

“Measuring and monitoring biological nitrogen fixation need to be ongoing to help us ensure we have all the nitrogen in the ground that we need without it becoming too much of a good thing.”

Carbon math needs updating

The study’s recalibration matters beyond academic interest. Governments and companies are banking on land-based carbon removal – through reforestation, restoration, and soil carbon programs – to help meet climate targets.

Those plans rest on assumptions about how much plants can grow under rising CO₂. If nitrogen limits are tighter, projected carbon gains may need adjusting.

Likewise, nutrient management rules, water quality programs, and climate policies that address nitrous oxide all depend on accurate knowledge of nitrogen levels. It’s essential to know where nitrogen is scarce and where it exists in excess.

Keep nitrogen on the farm

The path forward is not to abandon nitrogen‑fixing crops but to use them wisely. Better crop rotations, timing, and residue management can keep nitrogen cycling on the farm and out of rivers.

Cutting food waste – one of the biggest hidden sources of nitrogen loss across the production‑to‑plate chain – would reduce pollution pressure without sacrificing food supply.

In addition, continued monitoring of biological nitrogen fixation in both natural and managed systems will help keep the global nitrogen ledger in balance.

Nature’s soils seem to be running leaner than we hoped; our farms, richer than we can afford to waste. Bridging that divide – feeding people while protecting water, air, and climate – will require paying closer attention to the invisible flow of nitrogen that underpins life on Earth.

The study is published in the journal Nature.

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