NRES 201 Lecture 33 Outline of Last Lecture I Plant Roots II Algae III Fungi IV Bacteria and other soil microbes V Factors affecting microbial growth Outline of Current Lecture VI Nitrogen in plants VII Natural occurrence of nitrogen VIII Nitrogen in soils IX The nitrogen cycle a Biological Fixation b Atmospheric Deposition c Mineralization d Immobilization Current Lecture Functions of nitrogen in plants Protein constituent structural and enzymes o required for all metabolic processes Inheritance constituent of DNA and RNA Energy storage and transfer constituent of ATP and related compounds Photosynthesis constituent of chlorophyll Natural Occurrence of Nitrogen N occurs in all four spheres of the earth 1 Lithosphere 98 is in here a Due to fixed NH4 being in igneous rocks 2 Atmosphere a rest is in atmosphere b air is 78 N2 by volume c the ultimate source of N for living systems 3 Hydrosphere 4 Biosphere The Nitrogen Cycle 1 N addition These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute 2 3 4 5 6 7 Mineralization Nitrification Plant Uptake NO3 leaching Denitrification Volatilization Biological Nitrogen Fixation Overall Reaction N2 NH4 Organic N High Energy Requirement A triple bond N N must be broken Two forms o Symbiotic N2 fixation by legumes and a few non legumes Association with Rhizobium Fixation occurs in root nodules Amount of N fixed varies widely Fixation is reduced by Soil Acidity Plant Stress NO3 Availability Legumes take up soil N Non legumes Lower fixation than with nodulated legumes o Non symbiotic N2 fixation By free living bacteria Major organisms Photoautotroph and Heterotrophs Significance Very limited because of the need for Light Organic C and Fertilizer Sales Atmospheric Nitrogen Deposition Forms of Deposition As NH4 or NO3 in particulates dry deposition or in rain snowfall wet deposition Sources of Deposition Manure Combustion of Fossil Fuel Natural Fires Lightning Magnitude Greatest near animal feedlots and urban areas Mineralization Defined as organic N NH4 It is the most important N cycle process for plant growth without N2 fixation in the absence of N fertilization on fertilized soils It opens access to soil reserves of organic N It provides an ongoing supply of mineral N Decomposition Process o It occurs in stages o C and N are liberated as waste produces C as CO2 and N as NH4 o Carried out by a wide variety of heterotrophs such as fungi and bacteria o Key fraction of soil N is alkali labile decomposition by NaOH Probably derived in part from amino sugars which occur in bacteria cell walls It is promoted by o Favorable environmental conditions Adequate soil moisture warm temperatures 100F optimal good aeration absence of soil acidity o Wetting and drying cycles Drying kills some soil microbes Upon rewetting the survivors utilize the dead biomass The result is a flush of mineralization o Cultivation Improves aeration and exposes fresh organic matter to microbial attack o N Fertilization accelerates residue decomposition unless P or S is limiting Immobilization NH4 NO3 turn into Organic N A process of assimilation the reverse of mineralization except that either NH4 or NO3 may be immobilized NH4 is strongly preferred over NO3 The C N ratio decreases during decomposition because CO2 is lost and NH4 is not lost
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