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THE NITROGEN CASCADE:IMPACTS OF FOOD AND ENERGY PRODUCTION ON THE GLOBAL NITROGEN CYCLE
Box 2: What is reactive nitrogen?

Most of the nitrogen in the environment is in the global atmosphere in the form of atmospheric nitrogen (di-nitrogen or N2), where it makes up 78 per cent of gases. This form of nitrogen is ‘non-reactive’ and unusable by almost all living things, but can be made usable, or ‘reactive’, through both natural and artificial processes. In the natural process, bacteria (such as Rhizobium and cyanobacteria) transform N2 from the air into ammonia (NH3) through biological nitrogen fixation.

Humans create reactive nitrogen from N2 in several ways: through the NH3-producing Haber-Bosch process (used mostly for fertilizer); through agriculture that induces biological nitrogen fixation (such as soybean cultivation); and as a byproduct of burning fossil fuels, which converts N2 and fossil N to reactive nitrogen oxides (NOX). Globally, about five times more reactive nitrogen results from food production than from energy production.

Source: Galloway and others 2003 

Human activities have greatly increased the amount of reactive nitrogen that circulates through the earth’s land, air, and water each year (Galloway and others 2003). The main reason for this is the manufacture of reactive nitrogen as a fertilizer to increase food production (Box 2). Nitrogen is necessary to increase crop yields, but plants are inefficient at taking it up and often more fertilizers and animal wastes are added than the plants need. As a result, only a fraction of the nitrogen applied to soils actually ends up in crops; in some regions it is less than 20 per cent (Smil 1999). The rest ends up moving freely through the environment where it may have serious impacts on the air, land, freshwater and oceans, as well as on human health.

However, some parts of the world, notably Africa, suffer the opposite problem – a deficiency of reactive nitrogen in the soil. This contributes to low crop yields and to food insecurity in the region.
Another main source of excess reactive nitrogen in the environment has been its inadvertent creation, in gaseous forms such as nitrogen oxides, from burning coal, oil, and natural gas. Finally, untreated or partially treated human and animal wastes add reactive nitrogen to aquatic ecosystems, contributing to degradation and human health risks.

 


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