Terrestrial nitrogen cycling in Earth System Models: from biological nitrogen fixation to projecting pollution for planetary stewardship

Terrestrial nitrogen cycling plays a vital role in the Earth system, influencing climate change and a myriad of dimensions of human well-being, yet remains poorly constrained in Earth system models (ESMs). In particular, terrestrial biological nitrogen fixation (BNF) is the dominant natural nitrogen source to the terrestrial biosphere and can alleviate nitrogen limitation of CO₂ fertilization but is a key source of uncertainty in ESMs. When comparing terrestrial BNF from a CMIP6 ensemble of ESMs to a new global synthesis of observations across natural and agricultural biomes, ESMs are found to underestimate agricultural BNF but overestimate natural BNF by over 50% in the present day. Natural BNF is overestimated in the most productive ecosystems that contribute most to the terrestrial carbon sink (forests and grasslands). There is a positive correlation between modeled present-day natural BNF and the CO₂ fertilization effect across ESMs, suggesting that overestimated natural BNF translates to an exaggerated CO₂ fertilization effect of approximately 11%. Additionally, while the focus of terrestrial nitrogen cycling in ESMs has primarily been nitrogen limitation of CO₂ fertilization, nitrogen losses from the terrestrial biosphere to the atmosphere and hydrosphere has been neglected. These include key flows of nitrogen, such as reactive nitrogen gas emissions from soils and wildfires as well as its transport along the land to ocean aquatic continuum, that are strongly influenced by human activities. ESMs with fully interactive nitrogen cycling could both improve climate change projections and be used to project nitrogen pollution and its impacts to inform planetary stewardship over the 21st century.