27 -1 27.1 Biological Nitrogen Fixation Richa Anand 27.1.1 Introduction Nitrogen is abundant in the atmosphere, lithosphere, and hydro- sphere of the earth. However, 99% of nitrogen is in the form of dinitrogen, which is inert and cannot be used by most living organisms. In order for living cells to use it for synthesis of vital bioorganic molecules like proteins, nucleic acids, and vitamins, molecular nitrogen (dinitrogen) has to be in its reduced or ixed form. Dinitrogen ixation, the process by which dinitrogen is reduced to ammonia, is therefore a very important process for the sustenance of life. hree processes are responsible for most of t he dinitrogen ixation in the biosphere. Atmospheric ixation by lightning con- tributes approximately 5%–8% (5 × 10 9 kg N year −1 ) of total ixed nitrogen (Myrold and Bottomley, 2007). he enormous energy contained in lightning breaks dinitrogen molecules and enables their atoms to combine with oxygen in the air forming nitrogen oxides that dissolve in rain. hese oxides of nitrogen t hen form nitrates that are carried to the earth in rainfall. Industrial nitro- gen ixation occurs through a process called the Haber–Bosch industrial process that was established in 1913. his process uses a catalytic agent (iron with a small amount of aluminum added) at high pressure (as much as 5.06 × 10 7 Pa) and high tem- perature (600–800 K) and consumes large amounts of fossil fuel. Ammonia produced through this highly expensive process is combined with other elements to produce nitrogenous fertiliz- ers like urea and ammonium nitrate. Although the use of these fertilizers is inevitable in meeting rising food demand to sustain the growing global population, their indiscriminate use has set of very negative efects on water resources and the environment. Approximately 1.1 × 10 11 kg N (Myrold and Bottomley, 2007) are manufactured by ammonia industry annually. Increasing fossil 27 Nitrogen Transformations 27.1 Biological Nitrogen Fixation ................................................................................................ 27-1 Introduction • Nitrogen-Fixing Organisms • Chemistry of Nitrogen Fixation • Genetics of Nitrogen Fixation • Methods of Studying Biological Nitrogen Fixation • Nitrogen Fixation in Agriculture and Forestry • Biological Nitrogen Fixation: Economy and Environment • Factors Limiting Biological Nitrogen Fixation References........................................................................................................................................... 27-6 27.2 Nitrogen Mineralization–Immobilization Turnover ....................................................... 27-8 Introduction • Biochemistry and Physiology of Mineralization • Biochemistry and Physiology of Immobilization • Diversity of Enzymatic Functions • Substrate Quality Efects on Mineralization–Immobilization Turnover • Process Rates and Kinetics • Simulation Models of Mineralization–Immobilization Turnover • Concluding Remarks References......................................................................................................................................... 27-15 27.3 Nitriication .......................................................................................................................... 27-19 Introduction • Nitrifying Microorganisms • Nitriier Activity and Nitriication Rates • Nitriier Communities and Diversity • Links between Nitriier Communities and Soil Nitriication Activity • Control and Management of Soil Nitriication • Conclusions References......................................................................................................................................... 27-29 27.4 Denitriication ...................................................................................................................... 27-32 Introduction • Denitriication Pathway • Denitrifying Organisms • Denitriication Activity in Soils • Greenhouse Gas Emissions by Denitriication • Managing Denitriication in Soils • Conclusions References.........................................................................................................................................27-40 27.5 Nitrogen in the Environment ............................................................................................27-46 Introduction • Forms of N hat Are of Concern in the Environment • Evaluating N Sources and Sinks and Hydrologic Connections in the Landscape • Conclusions References......................................................................................................................................... 27-51 Richa Anand The University of British Columbia Jean-Claude Germon Institut National de la Recherche Agronomique Peter M. Groffman University of Georgia Jeanette M. Norton Utah State University Laurent Philippot Institut National de la Recherche Agronomique James I. Prosser University of Aberdeen Joshua P. Schimel University of California