Environmental Problems, Their Causes, and Sustainability 1

wildlife habitats, alter temperature and precipitationpatterns, and raise the average sea level in variousparts of the world—more about this in Chapter 21.How Is Nitrogen Cycledin the Biosphere? Bacteria in ActionDifferent types of bacteria help recycle nitrogenthrough the earth’s air, water, soil, and livingorganisms.Nitrogen is the atmosphere’s most abundant element,with chemically unreactive nitrogen gas (N 2 ) makingup about 78% of the volume of the troposphere. Nitrogenis a crucial component of proteins, many vitamins,and the nucleic acids DNA and RNA. However, N 2cannot be absorbed and used (metabolized) directly asa nutrient by multicellular plants or animals.Fortunately, two natural processes convert N 2 gasin the atmosphere into compounds that can enter foodwebs as part of the nitrogen cycle, depicted in Figure4-31. Trace the flows and paths in this diagram.One of these processes is atmospheric electrical dischargein the form of lightning. This causes nitrogen(N 2 ) and oxygen (O 2 ) in the atmosphere to react andproduce nitrogen oxide (NO). Try to write and balancethe chemical equation for this reaction.The other process is carried out by certain types ofbacteria in aquatic systems, in the soil, and in the rootsof some plants that can convert or “fix” N 2 into compoundsuseful as nutrients for plants and animals.The nitrogen cycle consists of several major steps(Figure 4-31). In nitrogen fixation, specialized bacteriain the soil convert (“fix”) gaseous nitrogen (N 2 ) to ammonia(NH 3 ) that can be used by plants. See if you canwrite a balanced chemical equation for the reaction ofN 2 with H 2 to form NH 3 .Ammonia not taken up by plants may undergonitrification. In this process, specialized aerobic bacteriaconvert most of the ammonia in soil to nitriteions (NO 2 ), which are toxic to plants, and nitrate ionsNitrogenFixationby industryfor agricultureGaseous Nitrogen (N 2 )in AtmosphereFood Webson LandFertilizersuptake byautotrophsexcretion, death,decompositionuptake byautotrophsNitrogen Fixationbacteria convert N 2 toammonia (NH 3 ); thisdissolves to formammonium (NH 4+ )Nitrogenous Wastes,Remains in SoilNO 3–in SoilDenitrificationby bacteriaNH 3 , NH 4+in SoilAmmonificationbacteria, fungi convert theresidues to NH 3 ; thisdissolves to form NH 4+2. Nitrificationbacteria convert NO 2–to nitrate (NO 3– )loss byleaching1. Nitrificationbacteria convert NH 4+to nitrite (NO 2– )NO 2–in Soilloss byleachingFigure 4-31 Natural capital: simplified model of the nitrogen cycle in a terrestrial ecosystem. Nitrogen reservoirsare shown as boxes; processes changing one form of nitrogen to another are shown in unboxed print.This cycle helps keep you alive. (From Cecie Starr and Ralph Taggart, Biology: The Unity and Diversity of Life,9th ed., Wadsworth © 2001)80 CHAPTER 4 Ecosystems: What Are They and How Do They Work?