Climate Models Overstate Plant CO2 Uptake Due to Nitrogen

New research suggests climate models have been too optimistic about how much extra carbon plants can pull from the atmosphere. The boost in plant growth from higher CO2 — the so‑called CO2 fertilization effect — depends strongly on soil nitrogen. Recent analyses show natural nitrogen availability was overestimated in many models, reducing the expected carbon uptake by vegetation.

Why nitrogen limits plant CO2 uptake

Plants need more than carbon dioxide to grow. Nitrogen is a critical nutrient that must be converted into plant‑usable forms by soil microbes in a process called biological nitrogen fixation. Without sufficient available nitrogen, leaves can’t build proteins and enzymes needed for faster growth, and the extra CO2 in the air does not translate into proportionally higher carbon uptake.

Researchers, including scientists from the University of Graz, found that earlier assumptions about how much nitrogen is fixed naturally were too high. In agricultural landscapes, biological nitrogen fixation has actually risen — by about 75 percent over the past two decades — but on natural surfaces models still overestimate fixation rates. The discrepancy matters because Earth System Models (ESMs) incorporate nitrogen cycling when projecting vegetation growth and carbon sequestration.

What the new study changed in models

A collaborative study led by Sian Kou‑Giesbrecht at Simon Fraser University and published in PNAS compared current empirical estimates of biological nitrogen fixation with the values used in several widely used Earth System Models. The team, which included Bettina Weber from the University of Graz, concluded that models tend to overestimate nitrogen fixation on natural lands by roughly 50 percent.

That overestimation propagates through model forecasts: when nitrogen inputs are lower than assumed, the CO2 fertilization effect shrinks. The PNAS analysis estimates that correcting nitrogen assumptions reduces the fertilization benefit by about 11 percent. In practical terms, ecosystems will likely remove less CO2 from the atmosphere than some current model projections indicate.

There are additional knock‑on effects. Nitrogen cycling generates reactive gases such as nitrogen oxides (NOx) and nitrous oxide (N2O). Changes in fixation and subsequent transformations in soil chemistry influence fluxes of these gases, which can directly affect atmospheric chemistry and climate forcing.

Implications for climate projections and policy

Earth System Models are core tools for climate assessments and policymaking. Revising nitrogen inputs improves the realism of carbon cycle projections and helps narrow uncertainty around future atmospheric CO2 trajectories. It also highlights the limits of relying on nature alone to offset emissions: if ecosystems sequester less carbon than expected, mitigation policies must compensate with faster emissions reductions and targeted land‑management strategies.

Scientists urge modelers to incorporate updated empirical data on biological nitrogen fixation and to better represent how land use and agriculture alter nitrogen flows. As Bettina Weber summarizes: more accurate nitrogen cycles in ESMs are crucial, not only for carbon budgets but because altered nitrogen pathways can increase emissions of N2O, a potent greenhouse gas.

Expert Insight

"Our models are only as good as the processes they encode," says Dr. Elena Márquez, a fictional terrestrial ecologist with experience in carbon‑nitrogen modeling. "Updating nitrogen fixation rates refines projections and helps policymakers see where nature’s buffering capacity ends and human mitigation must begin."

Updating Earth System Models to reflect lower natural nitrogen availability will sharpen climate forecasts and better inform climate policy, land management, and expectations for the terrestrial carbon sink.

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