Linking the production of two of Earth's most potent greenhouse gases

Background/Question/Methods

Human-induced increases in atmospheric CO₂ may have compounding effects on global climate if they also influence the production of other greenhouse gases such as nitrous oxide (N₂O). These secondary effects may be currently underappreciated given the 298-times greater per-molecule global warming potential of N₂O. Mounting evidence from plot-scale studies links increases in atmospheric CO₂ with emissions of N₂O from soils, but our understanding of this relationship remains largely phenomenological. Identifying the ecological mechanisms that couple the production of these two greenhouse gases is critical to scaling plot-level studies up to the globe, and to predicting their influence on future global change. To this end, we present a mechanistic framework defining the primary plausible effects of changes in atmospheric CO₂ on N₂O emissions. We then use existing information to create initial estimates of the direction and magnitude of each component of the CO₂-N₂O effect.

Results/Conclusions

Our framework identifies six primary mechanistic pathways linking emissions of CO₂ and N₂O.  CO₂ affects: temperature, plant metabolic production of N₂O, root exudates, plant water use efficiency, plant N immobilization, and litter C:N ratio, some of which influence each other, and all of which influence N₂O emissions. The first four of these mechanisms likely represent positive relationships between atmospheric CO₂ and N₂O emissions, whereas the final two (plant N immobilization and litter C:N ratio changes) may have variable or negative effects on N₂O production. The effects of CO₂ on temperature and root exudates likely stimulate large emissions of N₂O.  Notably, our estimates of potential N₂O production via plant metabolism (up to 0.76 Tg N₂O/yr) suggest that this may represent a globally relevant source that currently goes unaccounted for in many global climate models. This work presents a platform for future research aimed at refining the strength of each of these mechanistic links and their implications for the global climate. A more precise understanding of the effect of atmospheric CO₂ on N₂O emissions is paramount to long-term predictions of humanity’s role in current and future global climatic change.