Overestimated natural biological nitrogen fixation translates to an exaggerated CO2 fertilization effect

Background

Biological nitrogen fixation (BNF) supplies a substantial fraction of reactive nitrogen of ecosystems, typically estimated in the tens to over 100 Tg N yr⁻¹ globally, thus constraining how strongly plants can respond to rising CO2. Many Earth system models (ESMs) parameterize BNF such that nitrogen does not strongly limit CO2 fertilization, potentially inflating modelled land carbon sinks and underestimating future atmospheric CO2 concentrations. Quantifying this bias is important for realistic carbon budget and mitigation assessments.

Goals and Methods

This study compares BNF rates used in ESMs to observationally constrained estimates and explores how correcting BNF estimates affects simulated CO2 fertilization and land carbon uptake. The authors analyzed model configurations where BNF exceeds observational ranges by on the order of tens of Tg N yr⁻¹ and then adjusted nitrogen inputs to align with data-based estimates. They run CO2 forcing experiments to evaluate how changes in BNF translate into differences in net primary productivity, land carbon storage, and atmospheric CO2 over multi-decadal simulations.

Conclusions and Takeaways

This analysis highlights that overestimating BNF can substantially inflate projected land carbon sinks by several tens of petagrams of carbon by 2100, compared to the smaller carbon sink estimates that observed nitrogen inputs actually sustain. Such overestimation misrepresents Earth’s capacity to absorb CO₂ and exaggerates the strength of the CO₂ fertilization effect, giving a false sense of climate mitigation potential. When BNF is realistically constrained, model simulations show weaker land sinks and higher atmospheric CO₂ concentrations, ultimately shrinking the remaining carbon budget. Therefore, the authors emphasize that credible climate projections must explicitly integrate nitrogen limitations, warning that unrealistic assumptions about nutrient availability could undermine effective climate policy and overstate the role of land-based sequestration in achieving net-zero targets.