Earth System Models Overstate Natural Nitrogen Fixation, Study Finds

Recent scientific research has identified that Earth system models commonly used in climate studies overstate the amount of natural biological nitrogen fixation, affecting calculations of carbon uptake by plants.

The study, published in PNAS and led by Sian Kou-Giesbrecht of Simon Fraser University, compared model estimates with observational data and found that natural nitrogen fixation is overestimated by more than half. An international working group supported by the USGS John Wesley Powell Centre contributed to the research.

According to the study, this overestimation results in an inflated CO₂ fertilization effect in models, with the effect being about 11% higher than what updated estimates suggest. The findings indicate that when legacy values for nitrogen fixation are replaced with more accurate, lower numbers, the modeled impact of carbon dioxide on plant growth is reduced by a similar margin.

The research also determined that while natural nitrogen fixation is overestimated, agricultural nitrogen fixation has increased by approximately 75% over the past two decades. The PNAS study further found that models tend to underestimate agricultural nitrogen fixation while overstating natural sources.

What the numbers show

• Earth system models overestimate natural nitrogen fixation by more than 50%
• The CO₂ fertilization effect is exaggerated by about 11% in current models
• Agricultural nitrogen fixation has risen by about 75% in 20 years
• Sampling bias in field studies overrepresents nitrogen-fixing organisms by 17 times

An earlier study published in Nature highlighted that field measurements of nitrogen fixation in natural environments are often biased toward areas with unusually high activity. This sampling approach results in global estimates that do not accurately reflect average conditions, as areas with abundant nitrogen-fixing organisms are overrepresented.

The Nature study reported that this bias causes field data to reflect locations where nitrogen-fixing organisms are about 17 times more common than the global average. This has led to inflated estimates of nitrogen availability in natural ecosystems, which in turn affects climate model projections.

Earth.com summarized that updating nitrogen fixation values in models to reflect more accurate, lower estimates reduces the calculated CO₂ fertilization effect by about 11%. This adjustment brings model projections closer to observed data and addresses discrepancies identified by recent research.

Bettina Weber of the University of Graz stated that nitrogen fixation in natural ecosystems has been substantially overestimated, while agricultural fixation has shown considerable growth over recent years. These findings underscore the need for improved measurement and modeling practices in climate science.

* This article is based on publicly available information at the time of writing.