Environmental Problems, Their Causes, and Sustainability 1

(NO 3 ), which are easily taken up by plants as anutrient.Nitrogen fixation and nitrification add inorganicammonia, ammonium ions (NH 4 ), and nitrate ions tosoil water. Then plant roots can absorb these dissolvedsubstances in a step called assimilation. Plants use theseions to make nitrogen-containing organic moleculessuch as DNA, amino acids, and proteins. Animals inturn get their nitrogen by eating plants or plant-eatinganimals.Plants and animals return nitrogen-rich organiccompounds to the environment as wastes, cast-off particles,and dead bodies. In the ammonification step, vastarmies of specialized decomposer bacteria convert thisdetritus into simpler nitrogen-containing inorganiccompounds such as ammonia and water-soluble saltscontaining ammonium ions.Nitrogen leaves the soil in the denitrification step inwhich other specialized anaerobic bacteria in waterloggedsoil and in the bottom sediments of lakes,oceans, swamps, and bogs convert NH 3 and NH 4back into nitrite and nitrate ions and then into nitrogengas (N 2 ) and nitrous oxide gas (N 2 O). These gases arereleased to the atmosphere to begin the cycle again.Global nitrogen (N) fixation(trillion grams)200150100500Nitrogen fixation by natural processesNitrogen fixation by humanprocesses1920 1940 1960 1980 2000YearFigure 4-32 Natural capital degradation: human interferencein the global nitrogen cycle. Human activities such as productionof fertilizers now fix more nitrogen than all naturalsources combined. (Data from UN Environment Programme,UN Food and Agriculture Organization, and U.S. Department ofAgriculture)How Are Human Activities Affecting theNitrogen Cycle? Altering NatureExcessive inputs of various nitrogen-containingcompounds into the environment from humanactivities is becoming a major regional and globalenvironmental problem.In the past 100 years, human activities have had severaleffects on the earth’s current nitrogen cycle.First, we add large amounts of nitric oxide (NO) tothe atmosphere when we burn any fuel. In the atmosphere,this gas can be converted to nitrogen dioxidegas (NO 2 ) and nitric acid (HNO 3 ), which can return tothe earth’s surface as damaging acid deposition, commonlycalled acid rain—more on this in Chapter 20.Second, we add nitrous oxide (N 2 O) to the atmospherethrough the action of anaerobic bacteria onlivestock wastes and commercial inorganic fertilizersapplied to the soil. This gas can warm the troposphereand deplete ozone in the stratosphere.Third, we release large quantities of nitrogenstored in soils and plants as gaseous compounds intothe troposphere through destruction of forests, grasslands,and wetlands. Fourth, we upset aquatic ecosystemsby adding excess nitrates in agricultural runoffand discharges from municipal sewage systems—more on this in Chapter 22.Fifth, we remove nitrogen from topsoil when weharvest nitrogen-rich crops, irrigate crops, and burn orclear grasslands and forests before planting crops.Sixth, inputs of nitrogen into the air, soil, and watermostly from our activities is beginning to affectthe biodiversity of terrestrial and aquatic systems byshifting their species composition towards speciesthat can thrive on increased supplies of nitrogennutrients.Since 1950, human activities have more than doubledthe annual release of nitrogen from the terrestrialportion of the earth into the rest of the environment(Figure 4-32). These excessive inputs of nitrogen intothe air and water is a serious local, regional, and globalenvironmental problem that so far has attracted fairlylittle attention compared to global environmentalproblems such as global warming and depletion ofozone in the stratosphere. Princeton University physicistRobert Socolow calls for the nations of the world towork out some type of international nitrogen managementagreement to help prevent this problem fromreaching crisis levels.How Is Phosphorus Cycled in theBiosphere? Slow Cycling without Usingthe AtmospherePhosphorus cycles fairly slowly through the earth’swater, soil, and living organisms.Phosphorus circulates through water, the earth’s crust,and living organisms in the phosphorus cycle, depictedhttp://biology.brookscole.com/miller1481