NITROGEN CYCLE

Only three elements cycled through the biosphere form stable compounds that can be transported both in water and air: carbon, sulfur, and nitrogen (N). The N cycle is peculiar, not only because the element's largest biospheric reservoir (nitrogen gas, N₂, in the atmosphere) exists in an unreactive form, but also because it is largely mediated by bacterial metabolism. A small number of bacterial species are the only organisms that can fix N–that is, convert it from inert N₂ to reactive ammonia (NH₃). Bacteria also convert NH₃ to soluble nitrates (nitrification), which are the main source of the nutrient for plants; decompose organic matter (ammonification); and close the N cycle by reducing nitrates back into N₂ (denitrification). N, together with phosphorus and potassium, is one of the three plant macronutrients and its shortages were the most common cause of low crop yields in all pre-industrial agricultures. N is also a key constituent of amino acids, the building blocks of proteins, adequate intake of which is essential for human growth and health.

Traditional farmers could supply the nutrient by either recycling organic materials (crop residues, manures, human waste) or by planting leguminous crops, which contain symbiotic Rhizobium bacteria, and can break down the inert atmospheric N₂ and synthesize reactive ammonia, NH₃. However, even the most assiduous recycling of organic wastes and the highest practicable planting of legumes in climates that allow year-round cultivation would result in crop yields that could support no more than five or six persons per hectare on an almost totally vegetarian diet. Guano and Chilean nitrate, commercially introduced after 1840, offered only a limited expansion of supply of reactive N.

The barrier to agricultural productivity growth, and hence to population carrying capacity, imposed by limits on the availability of nitrogen was broken by German chemist Fritz Haber's discovery of the synthesis of NH₃ from its elements (in 1909) and its speedy conversion to a large-scale industrial process by the BASF (Badische Anilin & Soda Fabrik) chemical company under the leadership of German industrial chemist Carl Bosch (in 1913). Thereafter, production of synthetic nitrogenous fertilizer became a major worldwide industry. Widespread use of N fertilizer accelerated after the mid-1960s with the introduction of new high-yield varieties of wheat and rice. American agronomist Norman Borlaug, one of the key architects of this Green Revolution, concluded that N fertilizer was responsible for its forward movement.

In the early twenty-first century, nearly 90 million tons of nitrogen are applied to crops each year, mostly to cereals and other annuals. In affluent nations, these applications help to produce an excess of food in general, and of animal foods in particular, and they boost agricultural exports. At least one-third of the world's population is alive at the beginning of the twenty-first century because of the additional food produced by application of N fertilizers–the nitrogen in the world's population dietary proteins comes from inorganic fertilizers. Moreover, given the balance of population numbers and cultivable land, and disregarding food imports from other countries, most of the anticipated future population growth in Asia could not take place without proteins synthesized by using N from urea, the world's leading N fertilizer.

The rising dependence on inorganic N represents the most pronounced human interference in the biospheric N cycle. More than half of all N fertilizer leaches into waters, volatilizes as NH₃, or is denitrified before it can be assimilated by plants. Environmental consequences of these losses include eutrophication of both aquatic and terrestrial ecosystems, contamination of waters with nitrates, and generation of nitrous oxide, a potent greenhouse gas, from imperfect denitrification. These effects can be reduced by adopting appropriate agronomic practices, as well as by more efficient feeding of animals, proper treatment of urban sewage, and reduction of the intake of animal foods.

NITROGEN CYCLE

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