Page 145 - Environment: The Science Behind the Stories

Figure 5.19 Specialized
bacteria live in nodules
on the roots of this leg-
ume plant. In the process
of nitrogen fixation, the
bacteria convert nitrogen
to a form that the plant
can take up into its roots.

Root Nitrogen-fixing
nodules bacteria

ions of ammonium (NH ) can be taken up by plants. Nitrogen and hydrogen gases to synthesize ammonia, a key ingredient
+
4
fixation can be accomplished in two ways: by the intense in modern explosives and agricultural fertilizers, and Carl
energy of lightning strikes, or when air in the top layer of Bosch devised methods to produce ammonia on an industrial
soil comes in contact with particular types of nitrogen-fixing scale. The haber-Bosch process enabled people to overcome
bacteria. These bacteria live in a mutualistic relationship the limits on productivity long imposed by nitrogen scarcity
(p. 98) with many types of plants, including soybeans and in nature. By enhancing agriculture, the new fertilizers con-
other legumes, providing them nutrients by converting nitro- tributed to the past century’s enormous increase in human
gen to a usable form. Some farmers nourish soils by plant- population. Farmers, homeowners, and golf course managers
ing crops that host nitrogen-fixing bacteria among their roots alike all took advantage of fertilizers, dramatically altering
(Figure 5.19). the nitrogen cycle. Today, using the Haber-Bosch process, our
species is fixing at least as much nitrogen as is being fixed
Nitrification and denitrification Other types of spe- naturally. We have effectively doubled the rate of nitrogen fix-
cialized bacteria then perform a process known as nitrification. ation on Earth, overwhelming nature’s denitrification abilities.
In this process, ammonium ions are first converted into nitrite By fixing atmospheric nitrogen with fertilizers, we
ions (NO ), then into nitrate ions (NO ). Plants can take up increase nitrogen’s flux from the atmosphere to Earth’s sur-
−
−
3
2
these ions, which also become available after atmospheric face. We also enhance this flux by cultivating legume crops
deposition on soils or in water or after application of nitrate- whose roots host nitrogen-fixing bacteria. Moreover, we
based fertilizer. reduce nitrogen’s return to the air when we destroy wetlands
Animals obtain the nitrogen they need by consuming that filter nutrients; wetland plants host denitrifying bacteria
plants or other animals. Decomposers obtain nitrogen from that convert nitrates to nitrogen gas, so wetlands can mop up a
dead and decaying plant and animal matter and from ani- great deal of nitrogen pollution.
mal urine and feces. Once decomposers process nitrogen- When our farming practices speed runoff and allow soil
rich compounds, they release ammonium ions, making these erosion, nitrogen flows from farms into terrestrial and aquatic
available to nitrifying bacteria to convert again to nitrates and ecosystems, leading to nutrient pollution, eutrophication, and
nitrites. hypoxia. These impacts have become painfully evident to
The next step in the nitrogen cycle occurs when oystermen and scientists in the Chesapeake Bay, but hypoxia in
denitrifying bacteria convert nitrates in soil or water to gase- waters is by no means the only human impact on the nitrogen
ous nitrogen via a multistep process. Denitrification thereby cycle. When we burn forests and fields, we force nitrogen out
completes the cycle by releasing nitrogen back into the of soils and vegetation and into the atmosphere. When we burn
atmosphere as a gas. fossil fuels, we release nitric oxide (NO) into the atmosphere,
where it reacts to form nitrogen dioxide (NO ). This compound
2
is a precursor to nitric acid (HNO ), a key component of acid
3
We have greatly influenced precipitation (pp. 491–493). We introduce another nitrogen-
the nitrogen cycle containing gas, nitrous oxide (N O), when anaerobic bacteria
2
break down the tremendous volume of animal waste produced
Historically, nitrogen fixation was a bottleneck, a step that in agricultural feedlots (pp. 268–269). Oddly enough, the
limited the flux of nitrogen out of the atmosphere. This overapplication of nitrogen-based fertilizers can strip the soil
changed with the research of two German chemists early in of other essential nutrients, such as calcium and potassium,
144 the 20th century. Fritz Haber found a way to combine nitrogen because fertilizer flushes them out. As these examples show,

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