Environmental Problems, Their Causes, and Sustainability 1

mal wastes and fertilizers and discharges from sewagetreatment systems—more on this in Chapter 22.Scientists estimate that since 1900 human activitieshave increased the natural rate of phosphorus releaseinto the environment about 3.7-fold.How Is Sulfur Cycled in the Biosphere?The Sulfur CycleSulfur cycles through the earth’s air, water, soil,and living organisms.Sulfur circulates through the biosphere in the sulfurcycle, shown in Figure 4-34. Trace the flows and pathsin this diagram. Much of the earth’s sulfur is storedunderground in rocks and minerals, including sulfate2(SO 4 ) salts buried deep under ocean sediments.Sulfur also enters the atmosphere from severalnatural sources. Hydrogen sulfide (H 2 S)—a colorless,highly poisonous gas with a rotten-egg smell—is releasedfrom active volcanoes and from organic matterin swamps, bogs, and tidal flats broken down byanaerobic decomposers.Sulfur dioxide (SO 2 ), a colorless, suffocating gas,2also comes from volcanoes. Particles of sulfate (SO 4 )salts, such as ammonium sulfate, enter the atmospherefrom sea spray, dust storms, and forest fires. Plantroots absorb sulfate ions and incorporate the sulfur asan essential component of many proteins.Certain marine algae produce large amounts ofvolatile dimethyl sulfide, or DMS (CH 3 SCH 3 ). Tinydroplets of DMS serve as nuclei for the condensationof water into droplets found in clouds. Thus changesin DMS emissions can affect cloud cover and climate.In the atmosphere DMS is converted to sulfur dioxide.In the atmosphere, sulfur dioxide (SO 2 ) fromnatural sources and human activities is convertedto sulfur trioxide gas (SO 3 ) and to tiny droplets ofsulfuric acid (H 2 SO 4 ). Sulfur dioxide also reacts withother atmospheric chemicals such as ammonia toproduce tiny particles of sulfate salts. These dropletsand particles fall to the earth as components of acid deposition,which along with other air pollutants canharm trees and aquatic life—more on this in Chapter20.In the oxygen-deficient environments of floodedsoils, freshwater wetlands, and tidal flats, specializedbacteria convert sulfate ions to sulfide ions (S 2 ). Thesulfide ions can then react with metal ions to form insolublemetallic sulfides, which are deposited as rock,and the cycle continues.WaterSulfur trioxide Sulfuric acid Acidic fog and precipitationSulfur dioxideOxygenHydrogen sulfideAmmoniaAmmonium sulfatePlantsDimethyl sulfideVolcanoIndustriesAnimalsOceanSulfate saltsMetallicsulfidedepositsDecaying matterSulfurHydrogen sulfideFigure 4-34 Natural capital: simplified model of the sulfur cycle.http://biology.brookscole.com/miller1483