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