Getting Our Nitrogen Fix

What We Know / by Jonah Lehrer /

Our ability to pull nitrogen from the air fed a growing human population. Can 21st century biotechnology refine the process while reducing environmental impact?

Photograph: Corbis

In 1968 the entomologist Paul Ehrlich published a foreboding manifesto called The Population Bomb, in which he argued that the explosive growth of the human population would lead, within the next decade, to mass starvation and the death of hundreds of millions of people. It was the old Malthusian trap: Population increases tend to be exponential, while increases in the food supply are linear. The tragedy was simple math.

But Ehrlich was wrong. There were plenty of horrific famines in the 1970s, especially in Sub- Saharan Africa, but there was no global food shortage. In fact, over the next few decades, the growth of the food supply consistently outstripped population growth; it was Malthus in reverse. While a long list of variables drove this trend, from the increased use of pesticides to new varieties of corn, wheat, and rice, one of the most important factors was the introduction of synthetic fertilizer. Plants thrive on nutrients in the soil, and these mass-produced fertilizers led to the doubling of crop yields between 1950 and 1990. We learned how to feed ourselves because we learned how to feed plants.

The story of modern fertilizer is really the story of nitrogen, and how humans learned to make plant food out of air and energy. Fritz Haber was there first. A German industrial chemist, he developed a method for “fixing” nitrogen around the turn of the 20th century. Although nitrogen is abundant, accounting for more than 78 percent of the Earth’s atmosphere, it is also highly inert: The gas is defined by its triple covalent bonds, which are extremely difficult to sever. Haber achieved his breakthrough by heating a mixture of nitrogen and hydrogen gas in the presence of the chemical catalyst iron oxide. When this concoction was heated to 500°C and 200 atmospheres of pressure, the result was a brick of ammonia, or NH3. With Haber, the sky was rendered solid.

Before the Haber process was perfected, fixed nitrogen, usually in the form of sodium nitrate, or “Chile saltpeter,” was a crucial natural resource. In addition to being valuable as fertilizer, it was an essential component of military ammunition — nitrogen provides the explosive lift inside the gun chamber — so controlling access to nitrogen meant controlling the ingredients of war. Indeed, the first major deployment of Haber’s chemical process had nothing to do with agriculture. Rather, it provided Germany with a seemingly infinite supply of ammonia, which meant that it could fight a world war even if the British Navy cut its access to saltpeter.

We learned how to feed ourselves because we learned how to feed plants.

Not until the war ended did the Haber process come to dominate the production of fertilizer. For the first time in human history, the natural nitrogen content of soil (or the availability of nitrogen-rich manure) ceased to be a constraint on crop yields, and from Iowa to India, harvests rose steadily. This so-called Green Revolution, however, hasn’t proved to be particularly green in the modern sense of the word, since the application of synthetic fertilizers can have devastating environmental consequences, such as the dead zones off the Gulf Coast. (Excessive amounts of nitrogen destroy the delicate equilibrium of the ocean, as they trigger toxic algal blooms that sap the sea of oxygen.) Furthermore, because the Haber process requires intense pressure and high heat, the production of fertilizer currently consumes more than 1 percent of the world’s energy supply and more than 5 percent of its natural gas. Paradoxically, just as climate change is threatening the global food harvest, our primary means of nourishing those crops only intensifies our use of fossil fuels.

What’s clearly needed is another technological breakthrough, a Haber process for the 21st century. Fortunately, natural selection has already devised the perfect solution: a bacterial enzyme known as nitrogenase, which can fix nitrogen at room temperature using only a collection of metal ions, including the unusual molybdenum ion, and a hefty dose of energy-giving ATP. Found in the common dirt-dwelling microbe called Rhizobium, nitrogenase allows a long list of plants, from peanuts to lentils to clover, to fix nitrogen in the soil. (Interestingly, Rhizobia can’t fix nitrogen independently; they must first establish themselves in the root nodules of these plants — a classic case of symbiosis). The question, of course, is how nitrogenase fixes nitrogen so efficiently. From the perspective of modern chemistry, it’s like squeezing water from a rock.

While scientists are getting closer to unlocking the secrets of nitrogenase — the lab of Richard Schrock, a Nobel laureate at MIT, recently created an artificial version of the enzyme that works, just not very well — its details continue to mystify them. How does an obscure metal allow prokaryotes to break one of the most durable bonds in nature? And how does the process unfold without an infusion of heat and pressure? Solving this biological magic trick won’t be easy. It takes place at the atomic level, as electrons are swapped from element to element. But a solution would have profound consequences. We already know how to turn air into food. What we need now is a way of growing food that doesn’t destroy the Earth.

Originally published March 4, 2009

Tags biotechnology energy scarcity

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