Cleaning up nitrogen pollution may reduce future carbon sinks

Biosphere carbon sinks are crucial for reducing atmospheric carbon dioxide (CO₂) concentration to mitigate global warming, but are substantially affected by the input of reactive nitrogen (N_r). Although the effects of anthropogenic CO₂ emission and nitrogen deposition (indicated by N_r emission to atmosphere) on carbon sink have been studied, it is unclear how their ratio (C/N) changes with economic development and how such change alters biosphere carbon sinks. Here, by compiling datasets for 132 countries we find that the C/N ratio continued to increase despite anthropogenic CO₂ and N_r emissions to atmosphere both showing an asymmetric para-curve with economic growth. The inflection points of CO₂ and N_r emissions are found at around $15,000 gross domestic product per capita worldwide. Economic growth promotes the use of N_r and energy, while at the same time increases their use efficiencies, together resulting in occurrences of inflection points of CO₂ and N_r emissions. N_r emissions increase slower but decrease faster than that of CO₂ emissions before and after the inflection point, respectively. It implies that there will be relatively more anthropogenic CO₂ emission but less N deposition with economic growth. This may limit biosphere carbon sink because of relative shortage of Nr. This finding should be integrated/included in global climate change modelling. Efforts, such as matching N deposition with carbon sequestration on regional scale, to manage CO₂ and N_r emissions comprehensively to maintain a balance are critical.