The atmospheric phase is predominant in the global nitrogen cycle, in which nitrogen fixation and denitrification by microbial organisms are by far the most important (Figure 18.21b). Atmospheric nitrogen is also fixed by lightning discharges during storms and reaches the ground as nitric acid dissolved in rainwater, but only about 3-4% of fixed nitrogen derives from this pathway. Organic forms of nitrogen are also widespread in the atmosphere, some of which results from the reaction of hydrocarbons and oxides of nitrogen in polluted air masses. In addition, amines and urea are naturally injected as aerosols or gases from terrestrial and aquatic ecosystems; and a third source consists of bacteria and pollen (Neff et al., 2002). While the atmospheric phase produces by far the most important input of nitrogen, there is also evidence that nitrogen from certain geological sources may fuel local productivity in terrestrial and freshwater communities (Holloway et al., 1998; Thompson et al., 2001). The magnitude of the nitrogen flux in stream flow from terrestrial to aquatic communities may be relatively small, but it is by no means insignificant for the aquatic systems involved. This is because nitrogen is one of the two elements (along with phosphorus) that most often limits plant growth. Finally, there is a small annual loss of nitrogen to ocean sediments.
In a model for the terrestrial part of the biosphere, nitrogen fixation accounts for the input of 211 TgNyear-1. This is the predominant annual source of nitrogen and can be compared with the total amount stored in terrestrial vegetation and soil of 296 Pg year-1 (280 Pg year-1 of which is in the soil, and 90% of this in organic form) (Lin et al., 2000).
Human activities have a variety of far-reaching effects on the nitrogen cycle. Deforestation, and land clearance in general, leads to substantial increases in nitrate flux in the stream flow and N2O losses to the atmosphere (see Section 18.2.2). In addition, technological processes yield fixed nitrogen as a by-product of internal combustion and in the production of fertilizers. The agricultural practice of planting legume crops, with their root nodules containing nitrogen-fixing bacteria, contributes further to nitrogen fixation. In fact, the human activities contribute the majority of phosphorus in inland waters...
... and cause eutrophication the nitrogen cycle has an atmospheric phase of overwhelming importance humans impact on the nitrogen cycle in diverse ways amount of fixed nitrogen produced by these human activities is of the same order of magnitude as that produced by natural nitrogen fixation. The production of nitrogenous fertilizers (more than 50 Tgyear-1) is of particular significance because an appreciable proportion of fertilizer added to land finds its way into streams and lakes. The artificially raised concentrations of nitrogen contribute to the process of cultural eutrophication of lakes.
Human activities impinge on the atmospheric phase of the nitrogen cycle too. For example, fertilization of agricultural soils leads to increased runoff as well as an increase in denitrification, and the handling and spreading of manure in areas of intensive animal husbandry releases substantial amounts of ammonia to the atmosphere. Atmospheric ammonia (NH3) is increasingly recognized as a major pollutant when it is deposited downwind of livestock farming areas (Sutton et al., 1993). Since many plant communities are adapted to low nutrient conditions, an increased input of nitrogen can be expected to cause changes to community composition. Lowland heathland is particularly sensitive to nitrogen enrichment (this is a terrestrial counterpart to lake eutrophication) and, for example, more than 35% of former Dutch heathland has now been replaced by grassland (Bobbink et al., 1992). Further sensitive communities include calcareous grasslands and upland herb and bryophyte floras, where declines in species richness have been recorded (Sutton et al., 1993). The vegetation of some other terrestrial communities may be less sensitive, because it may reach a stage where nitrogen is not limited. Increased nitrogen deposition to forests, for example, can be expected to result initially in increased forest growth, but at some point the system becomes 'nitrogen-saturated' (Aber, 1992). Further increases in nitrogen deposition can be expected to 'break through' into drainage, with raised concentrations of nitrogen in stream runoff contributing to eutrophication of downstream lakes.
There is clear evidence of increased NH3 emissions during the past few decades and current estimates indicate that these account for 60-80% of anthropogenic nitrogen input to European ecosystems, at least in localized areas around livestock operations (Sutton et al., 1993). The other 20-40% derives from oxides of nitrogen (NOJ, resulting from combustion of oil and coal in power stations, and from industrial processes and traffic emissions. Atmospheric NO% is converted, within days, to nitric acid, which contributes, together with NH3, to the acidity of precipitation within and downwind of industrial regions. Sulfuric acid is the other culprit, and we outline the consequences of acid rain in the next section, after dealing with the global sulfur cycle.
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