The atmosphere is nitrogen's largest biospheric reservoir, with stable N2 molecules forming 78 per cent of its volume. Trace amounts of NO and NO2 (designated jointly as NOx), of nitrous oxide (N2O), nitrates (NO-3) and ammonia (NH3) are also present. The N content of soils varies by more than an order of magnitude, but most of it is embodied in humus. Water stores very little nitrogen: ammonia is not very soluble, and nitrate concentrations in natural (uncontaminated) streams are very low. Most of the plant tissues are N-poor polymers, but N is present in every living cell in nucleic acids, which store and process all genetic information, in amino acids, which make up all proteins, and in enzymes. It is also a component of chlorophyl, whose excitation energizes photosynthesis.

The natural N cycle is driven overwhelmingly by bacteria. Fixation, nitrification and denitrification are the basic pathways of the cycle. Fixation, the conversion of unreactive N2 to reactive compounds, can be both abiotic and biogenic. Lightning severs the strong N2 bond, and the element then forms NO and NO2, which are eventually converted to nitrates. Biofixation, moving N2 to NH3, is performed only by bacteria thanks to nitroge-nase, a specialized enzyme no other organisms carry. Rhizobium is by far the most important symbiotic fixer, forming nodules on leguminous plant roots. There are also endophytic bacteria (living inside plant stems and leaves) and free living N-fixers, above all cyanobacteria.

Nitrifying bacteria present in soils and waters transform NH3 to NO-3, a more soluble compound plants prefer to assimilate. Assimilated nitrogen is embedded mostly in amino acids which are the building blocks of all proteins. Heterotrophs (animals and people) must ingest preformed amino acids in feed and food in order to synthesize proteins tissues. After plants and heterotrophs die, enzymatic decomposition (ammonification) moves N from dead tissues to NH3, which is again oxidized by nitrifiers. Denitrification returns the element from NO-3 (via NO-2) to atmospheric N2. However, incomplete reduction results in some emissions of N2O, a greenhouse gas about 200 times more potent than CO2.

Figure 21.2 Global nitrogen cycle

There are many leaks, detours and backtracking along this main cyclical route. Volatilization from soils, plants and animal and human wastes returns N (as NH3) to the atmosphere, to be redeposited, after a short residence, in dry form or in precipitation. Both nitrification and denitrification release NOx and N2O. Nitrogen in NOx returns to the ground in atmospheric deposition, mostly after oxidation to NO3. In contrast, N2O is basically inert in the troposphere but it is a potent greenhouse gas, and it contributes to the destruction of stratospheric ozone. Highly soluble nitrates leak readily into ground and surface waters, and both organic and inorganic nitrogen in soils can be moved to waters by soil erosion.

Pre-agricultural terrestrial fixation of N, dominated by biofixation in tropical forests, amounted to at least 150-190 million tonnes (Mt) N/year (Cleveland et al. 1999). Planting of leguminous crops, practiced by every traditional agriculture, was the first major human intervention in the cycle. It now fixes annually 30-40 Mt N. Guano and Chilean NaNO3 were the first commercial N fertilizers: their exports to Europe began before 1850. By 1910, by-product ammonia from coking, calcium cyanamide and calcium nitrate from an electric arc process also provided relatively small amounts of fixed N.

Only the synthesis of ammonia from its elements - demonstrated for the first time by Fritz Haber in 1909, and commercialized soon afterwards by the BASF under the leadership of Carl Bosch - opened the way for a large-scale, inexpensive supply of fixed N (Smil 2001). Haber-Bosch fixation expanded rapidly only after 1950 and it became particularly energy-efficient with the introduction of single-train plants equipped with centrifugal processors that were commercialized during the 1960s. The current rate of global NH3 synthesis surpasses 100Mt N/year. About four-fifths of it is used as fertilizers (mostly as a feedstock for producing urea and various nitrates, sulfates and phosphates). The rest goes into industrial process, ranging from the production of explosives and animal feed to feedstocks for syntheses of dyes, plastics and fibers (for example, nylon; Febre-Domene and Ayres 2001).

Typically no more than half of the N applied to crops is assimilated by plants. The rest is lost owing to leaching, erosion, volatilization and denitrification (Smil 1999). Uptakes are lowest (often less than 30 per cent) in rice fields, highest in well-farmed, temperate crops of North America and Northwestern Europe. Because the primary productivity of many aquatic ecosystems is N-limited, eutrophication of streams, ponds, lakes and estuaries by run-off containing leached fertilizer N promotes growth of algae and phytoplank-ton. Decomposition of this phytomass deoxygenates water and seriously harms aquatic species, particularly the benthic fauna. Algal blooms may also cause problems with water filtration or produce harmful toxins (for example, 'red tides').

Nitrogen in eutrophied waters comes also from animal manures, human wastes, industrial processes and from atmospheric deposition. There is a clear correlation between a watershed's average rate of nitrogen fertilization and the riverine transport of the nutrient. The worst affected offshore zone in North America is a large region of the Gulf of Mexico, where the nitrogen load brought by the Mississippi and Atchafalya rivers has doubled since 1965, and where eutrophication creates every spring a large hypoxic zone that kills many bottom-dwelling species and drives away fish. Other affected shallow waters include the lagoon of the Great Barrier Reef, and portions of the Baltic, Black, Adriatic and North Seas.

Combustion of fossil fuels is now the source of almost 25Mt N/year as NOx. In large urban areas these gases are essential ingredients for the formation of photochemical smog. Their eventual oxidation to nitrates is a major component (together with sulfates) of acid deposition (see more under 'Sulfur'). Atmospheric nitrates, together with volatilized ammonia, also cause eutrophication of normally N-limited forests and grasslands. In parts of eastern North America, Northwestern Europe and East Asia their deposition (up to 60kg N/hectare per year) has become significant even by agricultural standards

(Vitousek et al. 1997). Positive response of affected ecosystems is self-limiting as N saturation leads to enhanced N losses.

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