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Nitrogen Paradox

Carbon is the structural element of life, making up nearly half of all living matter, but nitrogen is the key. Life requires only small quantities of nitrogen, yet its shortage is often the limiting factor in crop production and human growth. Nitrogen-containing compounds control all of life's phenomena, and within these compounds, the nitrogen atom plays a vital role. Nitrogen is present in every living cell, including chlorophyll, which can be excited by light to energize photosynthesis; DNA and RNA which store and process all genetic information; amino acids which make up all proteins; and enzymes which control the chemistry of life. Nitrogen's importance in plants cannot be overstated. It is the nutrient responsible for vigorous vegetative growth and for the size and protein content of humankind's staple cereals.

Nitrogen gas is odorless, colorless, nonflammable, nonexplosive, nontoxic, and nonreactive under normal environmental conditions. Nitrogen comprises 78 percent of the earth's atmosphere as N₂, a triple-bonded molecule. N₂ molecules must be split before nitrogen atoms can be incorporated into inorganic and organic compounds, and thus its usable forms are scarce. Lightning is the only physical process that can naturally fix substantial amounts of nitrogen; it splits the tightly bonded molecules and frees the nitrogen atoms to form reactive compounds. Lightning's contribution to the nitrogen cycle, however, falls considerably short of agriculture's needs.

It took the advent of modern chemistry to begin to elucidate the complex relationships between the atmosphere, plants, and soil. In the late 1700s, scientists experimenting with gases noticed "nonvital air" left behind after the "vital air" reacted with various chemicals. The gases were termed nitrogen and oxygen, respectively, and by the 1850s, several key facts concerning nitrogen's role in agriculture were clear: its constant presence in animal and plant tissues, its necessity for vigorous plant growth, and its beneficial effects on crop yields when applied. Ammonia was soon placed with water and carbonic acid as the necessary elements of life, as well as the end products of decay.

While other plants seemed dependent on nitrogen present in the soil, it was discovered that legumes had a second source: the air. Furthermore, it was demonstrated that legumes, such as clover, could restore nitrogen to the soil. Legumes themselves do not possess the capability to obtain free nitrogen from the air, but their symbiotic microbial partners possess nitrogenase, a specialized enzyme that splits the strong dinitrogen bonds.

Presently, around 100 genera of bacteria and cyanobacteria are known to have evolved nitrogenase. Rhizobium bacteria, symbiotically associated with the roots of leguminous plants, are the most recognized. Nitrogenase can only function in the absence of oxygen; thus, the most important nitrogen-fixing microorganisms are those that live in special swellings, or nodules, on leguminous plant roots. Only cyanobacteria, such as Anabaena and Nostoc, add appreciable amounts of fixed nitrogen as free-living microbes. Plant hosts provide Rhizobium bacteria with the equivalent of 12 grams of glucose to power an enzymatic reaction that produces one gram of fixed nitrogen. This is noteworthy because legumes' productivity is so low that, combined with their lower digestibility (compared to staple grains), there is no incentive to expand their cultivation -- especially in countries supported by marginal food supplies.

Although nitrogen is fixed by combustion, volcanic action, and lightning, the most usable nitrogen is fixed by nitrogen-fixing microorganisms that employ the nitrogenase enzyme. The reduction of nitrogen gas to ammonia by nitrogenase at standard atmospheric conditions is an unbelievable accomplishment. Remarkably, there are only a few kilograms of nitrogenase on the planet at any one time, yet those few kilograms sustained the biosphere for millennia. Biology accomplished what 19th-century chemists could not replicate experimentally, and certainly not commercially, due to the tremendous heat, pressure, and energy consumption a chemical process required.

Once the need for nitrogen fertilization was clearly understood, the search was on for new sources of the nutrient. Ancient practices of cropping with leguminous plants and recycling of organic wastes were finally explained, but traditional farming could not sustain high crop yields over large cultivated lands. By the turn of the 20th century, agricultural land use had expanded almost to its limits to meet the challenge of feeding the 1.6 billion people on the planet. Cultivation needed to be intensified, since the grasslands of the Great Plains, Canadian prairies, and Australia were already converted to cropland. The planet was settled -- and it needed food.

Commercialized sources of nitrogen at that time included guano (mainly from the Chincha Islands off the coast of Peru), recovery of by-product ammonia from the coking of coal, and the exploitation of nitrate fields in Chile. Collectively, these inputs added up to a small fraction of managed nitrogen requirements worldwide, with Europe consuming a disproportionate amount. European countries, mainly Germany, relying on Chilean nitrates in a time of volatile global relationships, put growing pressure on a century-long search to take nitrogen and hydrogen and synthesize the most basic nitrogen compound, ammonia.

Fritz Haber demonstrated experimentally the conversion of nitrogen gas (N₂) to ammonia (NH₃) in 1909, and Carl Bosch brought the first ammonia plant on line in 1913. This engineering feat cannot be overlooked; even BASF, the chemical company responsible for patenting the Haber-Bosch process of ammonia synthesis, was very skeptical about running continuous catalytic synthetic reactions at such high temperatures and pressures. With unmatched speed and precision, Carl Bosch kept BASF gambling, and together they pioneered ammonia synthesis. More efficient processes are used today, but their operations are based on the same principles as the original invention and will likely operate this way for the next century at least.

In 1914 allied forces cut off Germany from the Chilean nitrates needed for their munitions. Carl Bosch and BASF began producing nitric acid for the military by 1915. The Haber-Bosch process was the breakthrough that effectively removed the key limit to crop production and turned world agriculture toward high-yielding cultivars, but the world fertilizer market sat and waited through the first half of the 20th century.

World population has risen to 6 billion, from 1.6 billion in 1900. Worldwide, and especially in areas experiencing population explosions, much of the protein needed for growth comes from the synthesis of ammonia. Only some developed countries, such as the U.S., could still feed their citizens without the addition of fertilizers, though we would have to adopt a more vegetarian diet to do so.

Now, our challenge is to deliver the nutrient more efficiently. Synthetic fertilizers have almost doubled the reactive nitrogen in the environment, dramatically altering natural nitrogen flows, often by more than a magnitude. Rivers, lakes, coastal waters, and forests are experiencing many known and unknown effects, and many are alarming. As a global community, we tied our fate to the Haber-Bosch process, as nearly half of us rely directly on the production achieved from these nitrogen inputs.

The world has evolved for millions of years with fixed nitrogen as a limiting factor to plant growth. We are loosing many of our synthetic inputs to surrounding natural communities that have never experienced such a deluge. We cannot be sure what will happen, but prudent action would eliminate all unnecessary inputs while making sure all necessary inputs do enter into crops and not natural communities. Nature is not designed to adapt in mere generations to a change in such a fundamentally important nutrient cycle. Next time you think about the importance of landing on the moon or personal computers, remember that only one 20th-century innovation feeds the world. Use it wisely.