Earth system models overstate carbon removal: New findings suggest nitrogen fixation is 50% lower than thought

[NickB79]

https://phys.org/news/2025-11-earth-overstate-carbon-nitrogen-fixation.html

High levels of atmospheric carbon dioxide intensify climate change, but high carbon dioxide levels can also stimulate plant growth. Plant growth removes carbon dioxide from the atmosphere, partially mitigating the effects of climate change. However, plants only grow faster in the presence of high levels of carbon dioxide if they can also acquire enough nitrogen from the atmosphere to do so.

The actual amount of nitrogen acquired from the atmosphere was reassessed in a previous study co-led by Columbia faculty that was published this summer; it was shown to be significantly lower than previously estimated.

The climate implications of the overestimation of nitrogen fixation are the focus of a new study published in the Proceedings of the National Academy of Sciences. This new study found that, in Earth system models, the amount of natural nitrogen fixation directly correlates to future plant growth. Therefore, the lower amount of natural nitrogen fixation than previously thought means that Earth system models have been overestimating future plant growth.

[mike_c]

1. this is an odd press release

The actual model they're describing is that increased plant growth in a high CO₂ atmosphere can sequester additional carbon in biomass only if there is sufficient soil nitrate available to support the increased growth. This won't happen in nutrient poor soils. Atmospheric N₂ does not affect plant growth directly, at least not in the sense described in the OP. Plants do not acquire nitrogen from air. The press release seems a bit confused about that.

[OKIsItJustMe]

2. I am wary of phys.org

Original Press Release: https://www.uni-graz.at/en/news/stickstoff-als-schluessel-ueberschaetzter-effekt-von-co2-als-duenger-von-pflanzen/
Study: https://www.pnas.org/doi/10.1073/pnas.2514628122

[sue4e3]

3. don't be

phys.org is a site that provides DOI and quite a bit of information about how and where it gets it's information. this may have been an older release but it was probably brought back up when the study or the article was updated. Here is their source, it's also what the press release you linked is based on https://www.pnas.org/doi/10.1073/pnas.2514628122

[OKIsItJustMe]

5. The editorial process

Researchers write papers, which “Public Relations” department use to write “Press Releases.” phys.org (and other sites) use the “Press Releases” to write articles. They make little changes, sometimes simply for “style.” Occasionally, meanings are changed, or an attempt to “clarify” clouds the waters.

In this case, the “Public Relations” department at Columbia took their hand at things, but at least one of the authors was at their campus. Let’s just look at the evolution of a headline. First the paper:

https://doi.org/10.1073/pnas.2514628122

Overestimated natural biological nitrogen fixation translates to an exaggerated CO2 fertilization effect in Earth system models

Now, the first university press release:
https://www.uni-graz.at/en/news/stickstoff-als-schluessel-ueberschaetzter-effekt-von-co2-als-duenger-von-pflanzen/

Nitrogen as the key: the overestimated effect of CO2 as a plant fertiliser

Now, Columbia’s reworking:
https://news.columbia.edu/news/scientific-models-overestimate-natural-processes-mitigate-climate-change

Scientific Models Overestimate Natural Processes That Mitigate Climate Change

Finally, phys.org:
https://phys.org/news/2025-11-earth-overstate-carbon-nitrogen-fixation.html

Earth system models overstate carbon removal: New findings suggest nitrogen fixation is 50% lower than thought

OK, was the headline clarified in the editorial process? Where did that 50% lower figure come from anyway?


In the paper’s abstract, we find:
https://www.pnas.org/doi/10.1073/pnas.2514628122

… We find that compared to observations, ESMs underestimate agricultural BNF but overestimate natural BNF in the present day by over 50%. Natural BNF is overestimated in the most productive ecosystems that contribute most to the terrestrial carbon sink (forests and grasslands). …

Well, that’s different, now isn't it? (A 50% overestimation does not mean that the real number is 50% lower.) Also, the paper differentiates between two different types of biological nitrogen fixation - “BNF” (agricultural BNF -vs- natural BNF.)

Did the press releases make this basic mistake in handling percentages? The original press release quoted a researcher:
https://www.uni-graz.at/en/news/stickstoff-als-schluessel-ueberschaetzter-effekt-von-co2-als-duenger-von-pflanzen/

“We compared different Earth System models with current nitrogen fixation values and found that they overestimate the nitrogen fixation rate on natural surfaces by about 50 per cent,” Weber explains. Overall, this overestimation of biological nitrogen fixation leads to a reduction in the CO2 fertilisation effect of about 11 per cent.

OK, how about the folks at Columbia?
https://news.columbia.edu/news/scientific-models-overestimate-natural-processes-mitigate-climate-change

The researchers compared different Earth system models with current nitrogen fixation values and found that they have been overestimating the nitrogen fixation rate on natural land by about 50 percent. Consequently, this overestimation of nitrogen fixation means that Earth system models have been overestimating the carbon dioxide fertilization effect by about 11 percent. …

This is why I am wary of phys.org. If you can’t do simple math, you shouldn’t be rewriting press releases/

[NickB79]

4. The press release mentions bacterial N2 fixation from the atmosphere to the soil

Which is then used by plants to grow.