The nitrogen cascade

Nitrogen is now known to be unusual among the elements that have had their cycles significantly perturbed by human action. The great significance of nitrogen is that it is linked to so many of the major global and regional environmental challenges that we face today: ozone layer depletion, acidification of soils and surface waters, global warming, surface and groundwater pollution, biodiversity loss, and human health and vulnerability. As nitrogen moves along its biogeochemical pathway, the same atom can contribute to many different impacts. This sequence of effects has been termed the 'nitrogen cascade' (Galloway and Cowling, 2002). It is depicted in Figure 4.3.

Nitrogen Cascade

Figure 4.3 The nitrogen cascade, illustrating the movement of human-produced reactive nitrogen (Nr) as it cycles through the atmosphere, terrestrial ecosystems and aquatic ecosystems

Source: Galloway and Cowling (2002), with permission from the Royal Swedish Academy of Sciences

Figure 4.3 The nitrogen cascade, illustrating the movement of human-produced reactive nitrogen (Nr) as it cycles through the atmosphere, terrestrial ecosystems and aquatic ecosystems

Source: Galloway and Cowling (2002), with permission from the Royal Swedish Academy of Sciences

The concept of the cascade, and the extensive research that underlies it, has allowed us not only to determine the linkages among the various aspects of the nitrogen cycle, but also to begin to assess how changes in one part of the cycle can delay or enhance the transfer of nitrogen to other parts of the cycle. The cascade continues as long as the nitrogen remains active in the environment, and it ceases only when reactive nitrogen is stored for a very long time, or is converted back to non-reactive N2.

Reactive nitrogen released into the environment in gaseous forms (NH3 and NOx) can have remote as well as local impacts; the gases may be transported thousands of kilometres from the point of emission before being deposited to ecosystems. As an example, Plate 4.1 shows the change in the rates of deposition from the atmosphere of reactive nitrogen, relative to the predominantly natural conditions prevailing at the start of the industrial era. In addition to atmospheric emission and transport, much nitrogen is transported via river systems to coastal marine waters. Human activity has had great influence on nitrogen fluxes to coastal oceans in some parts of the world, but very little effect elsewhere. The nitrogen cycle is most altered where farming and industrial activity in the river basins and watersheds is most intense (Howarth et al, 1996, 2002; Boyer and Howarth, 2002).

The sequence of events linking the addition of new reactive N into the environment specifically to N2O emissions is depicted in Figure 4.4. Here, the dominant sources are nitrogen fertilizers and manures, and the release of N from the mineralization of soil organic matter following cultivation, especially as a result of land-use change (see Chapters 5-7).

The Nitrogen Cascade

Combustion Emissions of N and N Volatilization

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Combustion Emissions of N and N Volatilization

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Storage and Management or Lvojiock Manure

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