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
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.