CLIMATE

Looking Beyond Carbon

We Also Have to Think About the Nitrogen Cycle

Nitrogen may be the most important nutrient you’ve never heard of. Despite its significance to the climate and food production, the nitrogen cycle has typically played second fiddle to its much better known cousin, the carbon cycle—presumably because the latter has now become so inextricably linked to climate change. Now two review articles published in a recent issue of Science hope to alter that perception by casting new light on this vitally important cycle: exploring its gradual transformation over a period of unprecedented fossil fuel consumption and its future impacts on the global environment. Needless to say, some of the potential scenarios they lay out fall short of ideal. Like atmospheric carbon, nitrogen is a key component of greenhouse gases. One nitrogen compound, nitrous oxide, is a common by-product of agriculture, and has a global warming potential 296 times larger than that of carbon dioxide. Agriculture and fossil fuel use are now believed to add 1.5 times more reactive nitrogen to ecosystems than do all combined terrestrial processes.

Before delving into nitrogen’s environmental impacts, however, it might help to first take a step back and ask: Why exactly should we care about the nitrogen cycle? Well, for one thing, most of the air we breathe is nitrogen gas: Fully 78 percent of the atmosphere is made up of nitrogen—by comparison, oxygen, the gas most of us associate with life, constitutes only 20.9 percent of the atmosphere [1]. While most of it is unavailable for use by organisms, nitrogen is ubiquitous in our natural environment and is vital to all life processes. The forms of the element we use in our bodies, which must be converted through a process called fixation, are collectively known as “reactive” nitrogen; examples of these abound and include ammonia, proteins, and amino acids.

Until the 20th century—and the advent of the modern industrial revolution—there existed only a limited number of microorganisms capable of fixing non-reactive gaseous nitrogen into reactive forms.

Until the 20th century—and the advent of the modern industrial revolution—there existed only a limited number of microorganisms capable of fixing non-reactive gaseous nitrogen into reactive forms. As a result, even though the amount of reactive nitrogen produced was limited—especially in light of the needs of our rapidly growing population—the global nitrogen cycle remained balanced. The invention of the Haber-Bosch process in the early part of the century, which provided an industrial-scale method to produce reactive nitrogen for agricultural purposes (mainly in the form of fertilizers), would revolutionize the pace of global development—helping sustain ever-higher population levels—and fundamentally alter the natural nitrogen cycle. The biogeochemical gears this invention set in motion would be further exacerbated by the world’s increased reliance on fossil fuels.

In 1970, Constant Delwiche, then a professor of soil biogeochemistry at the University of California, Davis, cautioned that, “The ingenuity that has been used to feed a growing world population will have to be matched quickly by an effort to keep the nitrogen cycle in reasonable balance” [2]. Since Delwiche’s prescient observations almost four decades ago, the world population—buoyed by the advances made under the “Green Revolution”—has increased by 78 percent. At the same time, the creation of reactive nitrogen has jumped by 120 percent. Thanks in large part to this industrial breakthrough, the most dire predictions made by Thomas Malthus and his modern adherents—predicated upon the belief that population growth would far outpace agricultural growth—never came to pass.

How do we manage to both significantly reduce the amount of nitrogen in some parts of the world while greatly increasing it in others?

While there is no denying that increased nitrogen production has been a boon for the world community—helping accommodate growing populations and alleviating poverty and hunger worldwide—it has also incurred many significant environmental costs. Air pollution, coastal eutrophication (which over-saturates waters with minerals and drives out animal life), and accelerated global warming trends, are just a few. Indeed, James Galloway, a professor of environmental sciences at the University of Virginia and the lead author on one of the articles, calls this the “cascade” effect [3]: the notion that every atom of reactive nitrogen can spur a cascading sequence of events which harms ecosystem and human health. These events can trigger terrestrial and aquatic ecosystem-wide shifts in biodiversity, facilitating the introduction of invasive species and corrupting the systems’ underlying stability. Worse still, as mentioned above reactive nitrogen can also alter the delicate balance of other greenhouse gases—including carbon dioxide and methane—and speed up ozone depletion.

Despite this growing imbalance, many areas of the world remain severely nitrogen-limited—primarily large regions in Africa and Latin America, where more cropping depletes more reactive nitrogen than fertilizers replenish. According to Galloway and his colleagues, 800 million individuals, or close to 15 percent of the world’s population, suffer from this “fertilizer deficit.” This then raises a difficult paradox: How do we manage to both significantly reduce the amount of nitrogen in some parts of the world while greatly increasing it in others? Or, as Galloway puts it, how do we, “maximize the benefits of anthropogenic Nr while minimizing its unwanted consequences”? Evidently, no single strategy will be sufficient to resolve these vexing issues.

Governments in the developed world should adopt a multifaceted, targeted approach to reducing the amount of excess reactive nitrogen by concentrating on the following objectives: reducing the amount of nitrogen oxide emissions produced during fossil fuel combustion; developing renewable energy technologies; increasing the nitrogen-uptake efficiency of crops; and improving animal management strategies. While recognizing that developing countries are primarily focused on increasing food production, the developed world should encourage them to mitigate their negative impacts while ensuring their access to fertilizers and clean energy technologies. When combined, these interventions would represent a substantial decrease in the amount of reactive nitrogen produced every year; implementing this approach would help offset some of the increases necessary to foster continued growth in agricultural production. Because these processes transcend both political and geographical boundaries, it will become necessary for governments to cooperate and to develop an integrated approach to managing nitrogen production.

Getting there won’t be easy. Put aside the sheer challenge of mounting such a global effort, and you’re still left with the equally daunting task of convincing people they should care as much about their nitrogen footprint as they do their carbon footprint. Given the right incentives, there is no question people can and will adapt—how and when they choose to do so, however, will be key.

Jeremy Jacquot is a graduate student in marine environmental biology at the University of Southern California and is the Los Angeles correspondent for TreeHugger.com.

Notes

[1] Carpenter, E.J. & Capone, D.G. (1983). Nitrogen in the Marine Environment. Academic Press: New York.

[2] Delwiche, C.C. (1970). The nitrogen cycle. Scientific American. 223, 137.

[3] Galloway, J.N. et al. (2003). The nitrogen cascade. BioScience, 53: 153 – 226.

Tags: chemistry, Climate Change

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