Global Population and the Nitrogen Cycle Part II

As per promise here is Part II

During the early 1960s, affluent nations accounted for over 90 percent of all fertilizer consumption, but by 1980 their share was down below 70 percent. The developed and developing worlds drew level in 1988. At present, developing countries use more than 60 percent of the global output of nitrogen fertilizer. Just how dependent has humanity become on the production of synthetic nitrogen fertilizer? The question is difficult to answer because knowledge remains imprecise about the passage of nitrogen into and out of cultivated fields around the globe. Nevertheless, careful assessment of the various inputs indicates that around 175 million tons of nitrogen flow into the world’s croplands every year, and about half this total becomes incorporated into cultivated plants. Synthetic fertilizers provide about 40 percent of all the nitrogen taken up by these crops. Because they furnish—directly as plants and indirectly as animal foods— about 75 percent of all nitrogen in consumed proteins (the rest comes from fish and from meat and dairy foodstuffs produced by grazing), about one third of the protein in humanity’s diet depends on synthetic nitrogen fertilizer. This revelation is in some ways an overestimate of the importance of the Haber-Bosch process. In Europe and North America nitrogen fertilizer has not been needed to ensure survival or even adequate nutrition. The intense use of synthetic fertilizer in such well-developed regions results from the desire to grow feed for livestock to satisfy the widespread preference for high-protein animal foods. Even if the average amount of protein consumed in these places were nearly halved (for example, by persuading people to eat less meat), North Americans and Europeans would still enjoy adequate nutrition. Yet the statement that one third of the protein nourishing humankind depends on synthetic fertilizer also underestimates the importance of these chemicals. A number of land-scarce countries with high population density depend on synthetic fertilizer for their very existence. As they exhaust new areas to cultivate, and as traditional agricultural practices reach their limits, people in these countries must turn to ever greater applications of nitrogen fertilizer—even if their diets contain comparatively little meat. Every nation producing annually in excess of about 100 kilograms of protein per hectare falls in this category. Examples include China, Egypt, Indonesia, Bangladesh, Pakistan and the Philippines. Too Much of a Good Thing Massive introduction of reactive nitrogen into soils and waters has many deleterious consequences for the environment. Problems range from local health to global changes and, quite literally, extend from deep underground to high in the stratosphere. High nitrate levels can cause life-threatening methemoglobinemia (“blue baby” disease) in infants, and they have also been linked epidemiologically to some cancers. Leaching of highly soluble nitrates, which can seriously contaminate both ground and surface waters in places undergoing heavy fertilization, has been disturbing farming regions for some 30 years. A dangerous accumulation of nitrates is commonly found in water wells in the American Corn Belt and in groundwater in many parts of Western Europe. Concentrations of nitrates that exceed widely accepted legal limits occur not only in the many smaller streams that drain farmed areas but also in such major rivers as the Mississippi and the Rhine. Fertilizer nitrogen that escapes to ponds, lakes or ocean bays often causes eutrophication, the enrichment of waters by a previously scarce nutrient. As a result, algae and cyanobacteria can grow with little restraint; their subsequent decomposition robs other creatures of oxygen and reduces (or eliminates) fish and crustacean species. Eutrophication plagues such nitrogen-laden bodies as New York State’s Long Island Sound and California’s San Francisco Bay, and it has altered large parts of the Baltic Sea. Fertilizer runoff from the fields of Queensland also threatens parts of Australia’s Great Barrier Reef with algal overgrowth. Whereas the problems of eutrophication arise because dissolved nitrates can travel great distances, the persistence of nitrogen-based compounds is also troublesome, because it contributes to the acidity of many arable soils. (Soils are also acidified by sulfur compounds that form during combustion and later settle out of the atmosphere.) Where people do not counteract this tendency by adding lime, excess acidification could lead to increased loss of trace nutrients and to the release of heavy metals from the ground into drinking supplies. Excess fertilizer does not just disturb soil and water. The increasing use of nitrogen fertilizers has also sent more nitrous oxide into the atmosphere. Concentrations of this gas, generated by the action of bacteria on nitrates in the soil, are still relatively low, but the compound takes part in two worrisome processes. Reactions of nitrous oxide with excited oxygen contribute to the destruction of ozone in the stratosphere (where these molecules serve to screen out dangerous ultraviolet light); lower, in the troposphere, nitrous oxide promotes excessive greenhouse warming. The atmospheric lifetime of nitrous oxide is longer than a century, and every one of its molecules absorbs roughly 200 times more outgoing radiation than does a single carbon dioxide molecule. Yet another unwelcome atmospheric change is exacerbated by the nitric oxide released from microbes that act on fertilizer nitrogen. This compound (which is produced in even greater quantities by combustion) reacts in the presence of sunlight with other pollutants to produce photochemical smog. And whereas the deposition of nitrogen compounds from the atmosphere can have beneficial fertilizing effects on some grasslands or forests, higher doses may overload sensitive ecosystems. When people began to take advantage of synthetic nitrogen fertilizers, they could not foresee any of these insults to the environment. Even now, these disturbances receive surprisingly little attention, especially in comparison to the buildup of carbon dioxide in the atmosphere. Yet the massive introduction of reactive nitrogen, like the release of car- Emissions of carbon dioxide, and the accompanying threat of global warming, can be reduced through a combination of economic and technical solutions. Indeed, a transition away from the use of fossil fuels must eventually happen, even without the motivation to avoid global climate change, because these finite resources will inevitably grow scarcer and more expensive. Still, there are no means available to grow crops—and human bodies—without nitrogen, and there are no waiting substitutes to replace the Haber-Bosch synthesis. Genetic engineers may ultimately succeed in creating symbiotic Rhizobium bacteria that can supply nitrogen to cereals or in endowing these grains directly with nitrogen-fixing capability. These solutions would be ideal, but neither appears imminent. Without them, human reliance on nitrogen fertilizer must further increase in order to feed the additional billions of people yet to be born before the global population finally levels off. An early stabilization of population and the universal adoption of largely vegetarian diets could curtail nitrogen needs. But neither development is particularly likely. The best hope for reducing the growth in nitrogen use is in finding more efficient ways to fertilize crops. Impressive results are possible when farmers monitor the amount of usable nitrogen in the soil so as to optimize the timing of applications. But several worldwide trends may negate any gains in efficiency brought about in this way. In particular, meat output has been rising rapidly in Latin America and Asia, and this growth will demand yet more nitrogen fertilizer, as it takes three to four units of feed protein to produce one unit of meat protein. Understanding these realities allows a clearer appraisal of prospects for organic farming. Crop rotations, legume cultivation, soil conservation (which keeps more nitrogen in the soil) and the recycling of organic wastes are all desirable techniques to employ. Yet these measures will not obviate the need for more fertilizer nitrogen in land-short, populous nations. If all farmers attempted to return to purely organic farming, they would quickly find that traditional practices could not feed today’s population. There is simply not enough recyclable nitrogen to produce food for six billion people. When the Swedish Academy of Sciences awarded a Nobel Prize for Chemistry to Fritz Haber in 1919, it noted that he created “an exceedingly important means of improving the standards of agriculture and the well-being of mankind.” Even such an effusive description now seems insufficient. Currently at least two billion people are alive because the proteins in their bodies are built with nitrogen that came— via plant and animal foods—from a factory using his process. Barring some surprising advances in bioengineering, virtually all the protein needed for the growth of another two billion people to be born during the next two generations will come from the same source—the Haber-Bosch synthesis of ammonia. In just one lifetime, humanity has indeed developed a profound chemical dependence.

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