Human-era increases in the global CO2 burden of new nitrogen creation: An alternative hypothesis

Background/Question/Methods

For billions of years of Earth history, the essential nutrient nitrogen (N) entered ecosystems and nourished life at a naturally growth-limiting pace. Today, post-industrial activities (i.e., Haber-Bosch fertilizer production, fossil fuel combustion) have more than doubled N circulation on land, substantially increasing N deposition inputs to freshwater, marine and terrestrial ecosystems. While producing many unwanted side-effects, the increased anthropogenic N in bulk precipitation has been purported to slow the pace of CO2-induced climate change, specifically via its ability to stimulate terrestrial plant growth, photosynthesis and ecosystem C sequestration. Here, I use stoichiometric principles to place constraints on the CO2 footprint of major N input pathways (natural biological fixation, lightening, fossil-fuel, Haber-Bosch), including the CO2 released to the atmosphere during new N creation vs. the CO2 consumed via N inputs to forest vegetation. I compile CO2 life-cycle data from the literature to estimate gross CO2 production associated with N inputs, and evaluate the effective role of various N input pathways on net C sequestration using a simple biogeochemical model.

Results/Conclusions

This analysis shows that N fixed by fossil combustion carries a substantial C burden (C/N (g/g) > 300/1 ), much higher than N inputs via symbiotic (C/N = 8/1) and asymbiotic fixation (C/N = 50/1), lightening-strikes (C/N = 0), and human-derived Haber-Bosch N (C/N ~ 0.5/1). Accounting for the contribution of fossil fuel combustion and Haber-Bosch fixation to deposition further indicates that the C/N of N inputs to natural ecosystems has increased substantially (especially in the temperate zone) since the industrial revolution. Application of these data in a simple heuristic model therefore reveals that symbiotic N fixation and asymbiotic fixation need only increase by a small fraction to compensate for any NOx emissions reductions and the potential loss of C storage in forest vegetation. Further, reduction of anthropogenic N deposition will help abate biodiversity declines, water quality and eutrophication risks, and soil acidification and forest dieback.