Is nitrogen the next carbon?

Key Points The rapid growth of anthropogenic reactive nitrogen production now makes it unquestionably dominant relative to the total of natural sources Anthropogenic production of reactive nitrogen has increased almost five‐fold in the last 60 years. This anthropogenic activity is a massive perturbation of a global geochemical cycle in a relatively short period of time. Just as carbon fueled the Industrial Revolution, nitrogen has fueled an Agricultural Revolution. The use of synthetic nitrogen fertilizers and the cultivation of nitrogen‐fixing crops both expanded exponentially during the last century, with most of the increase occurring after 1960. As a result, the current flux of reactive, or fixed, nitrogen compounds to the biosphere due to human activities is roughly equivalent to the total flux of fixed nitrogen from all natural sources, both on land masses and in the world's oceans. Natural fluxes of fixed nitrogen are subject to very large uncertainties, but anthropogenic production of reactive nitrogen has increased almost fivefold in the last 60 years, and this rapid increase in anthropogenic fixed nitrogen has removed any uncertainty on the relative importance of anthropogenic fluxes to the natural budget. The increased use of nitrogen has been critical for increased crop yields and protein production needed to keep pace with the growing world population. However, similar to carbon, the release of fixed nitrogen into the natural environment is linked to adverse consequences at local, regional, and global scales. Anthropogenic contributions of fixed nitrogen continue to grow relative to the natural budget, with uncertain consequences. Plain Language Summary Almost 50 years ago, C. C. Delwiche (1970) wrote in Scientific American “Of all man's recent interventions in the cycles of nature, the industrial fixation of nitrogen far exceeds all the others in magnitude.” Since then, the climate change impacts of human contributions to the carbon cycle have come under intense scrutiny and debate. Meanwhile anthropogenic production of reactive nitrogen has continued to increase with associated adverse environmental consequences. As in the case of carbon dioxide (CO2), some impacts of reactive nitrogen are difficult to measure on a short timescale. Reactive nitrogen also has a place in the agricultural revolution, which is analogous to the role of carbon from fossil fuels in the Industrial Revolution. This leads us to ask the question: is the anthropogenic a