Environmental Engineering Reference
In-Depth Information
Nitrate ion can be converted into nitrogen by denitrifying bacteria in a process called
deni-
trification
(Equation 2.11):
−
+
2NO
+
12H
→+
N
6H O
[2.11]
3
2
2
The nitrification and denitrification reactions are affected by side reactions that produce
nitrous oxide (N
2
O) as a side product (Socolow, 1999).
Alteration of the nitrogen cycle
The nitrogen cycle on Earth has been altered by human activities. The alteration is due to
increasing amounts of reactive nitrogen that cycle among the biosphere, lithosphere, hydro-
sphere, and atmosphere. The alteration of the nitrogen cycle are the large-scale use of nitrogen
fertilizers, power generation, industries, fossil fuel burning for heat and in automobiles, defor-
estation, and disruption of natural ecosystems. Table 2.2 compares the amount of reactive
nitrogen circulating in the system before and after human intervention.
Before the Haber-Bosch process was developed in early 1900s, which allowed an unlimited
availability of assimilable nitrogen, nitrogen was a limited nutrient in soils that was replenished
mainly by biological fixation and some natural fertilizers, such as guano and nitrate deposits
(Galloway and Cowling, 2002). Modern nitrogen fertilization is a wasteful process with many
intermediate steps. At the end, only 14 percent of the nitrogen is consumed by those who
follow vegetarian diets and only 4 percent for carnivorous ones (Galloway and Cowling,
2002). The rest is lost to the environment. In addition, large areas of diverse natural vegetation
have been replaced with nitrogen-fixing monocultures of legumes and forages (e.g., soybean,
alfalfa, peas, and such), which accelerate nitrogen fixation.
Similarly to what happens with the carbon cycle, burning fossil fuels releases nitrogen to
the atmosphere in the form of nitric oxide, which had been stored in geological formations for
millions of years. Burning nonfossil materials (e.g., biomass, organic matter from soils, and
grasslands) also releases reactive nitrogen that was stored in these materials. In addition, the
burning process itself, especially at high temperatures, transforms atmospheric nitrogen into
reactive species generically designed as nitrogen oxides, or NOx (Vitousek et al., 1997).
The impacts of altering the nitrogen cycle include increased global concentration of nitrous
oxide that is a potent greenhouse gas, the release of NOx that is part of the formation of smog,
acidification of soils and water streams, loss of soil nutrients, loss of biodiversity in terrestrial
ecosystems, and alteration of aquatic systems (Vitousek et al., 1997). (Chapter 3 presents a
more detail discussion of the consequences of altering the nitrogen cycle.)
Table 2.2
Nitrogen fixation before and after human intervention.
Teragrams/year
Natural fixation before human intervention
90-140
After human intervention:
Haber-Bosch
80
Nitrogen-fixing crops
32-53
Fuel burning
20
Release from natural reservoirs (biomass burning,
wetlands draining, land clearing for crops)
70
1 teragram = 1 million metric tons of nitrogen.
Adapted from Vitousek et al., 1997.
