CASE 6 AGRICULTURE: FEEDING A GROWING POPULATION

Harvesting crops removes nutrients—including nitrogen—from an ecosystem. These nutrients must be returned if the land is to remain fertile. In the early days of agriculture, farmers left fields unplanted for long periods between crops. During these periods, nitrogen-fixing bacteria replenished soil nitrogen, either in symbiotic association with roots or as free-living bacteria in the soil.

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As populations grew, fields could not be left unplanted for long periods. Organic fertilizers such as crop residues, manure, and even human feces recycled some of the lost nitrogen, but unless they are brought in from off the farm, they cannot replace all the nitrogen removed during harvest.

Crop rotation has been widely practiced throughout the history of agriculture as a means of sustaining soil fertility. Every few years, a field is planted with legumes harboring nitrogen-fixing bacteria. Typically, the legume plants are not harvested, but instead are treated as “green manure” that is allowed to decay and replenish soil nitrogen. During the eighteenth and nineteenth centuries, legume rotation contributed to the higher crop yields needed to support increasing populations. Therefore, along with the mechanization of agriculture, nitrogen-fixing symbioses contributed to the social conditions that fueled the Industrial Revolution.

By the second half of the nineteenth century, more abundant and more concentrated supplies of nitrogen fertilizers were sought. Mining of guano and sodium nitrate mineral deposits helped meet the demand for the nitrogen, but with time these natural nitrogen sources were depleted. A mechanism was needed to obtain nitrogen fertilizer from the vast storehouse of nitrogen in the atmosphere.

German chemist Fritz Haber developed an industrial method of fixing nitrogen in the years 1908–1911, and Karl Bosch subsequently adapted it for industrial production. Their method allows humans to achieve what, until then, had been accomplished only by prokaryotes. Like nitrogen fixation by bacteria, the Haber–Bosch process requires large inputs of energy; unlike nitrogen fixation by bacteria, it occurs only at high temperatures and pressures. The prospect of an essentially unlimited supply of nitrogen in a form that plants can make use of has proved irresistible. Today industrial fertilizers produced by the Haber–Bosch process account for more than 99% of fertilizer use. Industrial fixation of nitrogen is approximately equal to the entire rate of natural fixation of nitrogen, which is largely due to bacteria, although a small amount of nitrogen is fixed by lightning.

As we discuss in Chapter 49, human use of fertilizer is altering the nitrogen cycle on a massive scale, posing a threat to biodiversity and the stability of natural ecosystems. Yet abundant nitrogen fertilizers are a cornerstone of modern agriculture. The high-yielding varieties of corn, wheat, and rice introduced in the mid-twentieth century achieve their high levels of growth and grain production only when abundantly supplied with nitrogen. The fact that plants such as legumes (including beans, soybeans, alfalfa, and clover) form symbiotic relationships with nitrogen-fixing bacteria suggests a possible way around this problem. If it were possible to engineer nitrogen fixation into crops such as wheat and rice, either by inducing them to form symbiotic relationships with nitrogen-fixing bacteria or by transferring the genes that would enable them to fix nitrogen on their own, would that eliminate the need for nitrogen fertilizers? The answer is yes—but with a caveat. As we have seen, nitrogen fixation requires a large amount of energy, diverting resources that could otherwise be used to support growth and reproduction.