NRES 201 Lecture 33 Outline of Last Lecture I. Plant RootsII. AlgaeIII. FungiIV. Bacteria and other soil microbesV. Factors affecting microbial growthOutline of Current Lecture VI. Nitrogen in plantsVII. Natural occurrence of nitrogenVIII. Nitrogen in soilsIX. The nitrogen cyclea. Biological Fixationb. Atmospheric Depositionc. Mineralizationd. Immobilization Current LectureFunctions 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 earth1. Lithosphere (98% is in here)a. Due to fixed NH4 being in igneous rocks2. Atmosphere a. rest is in atmosphereb. air is 78% N2 (by volume)c. the ultimate source of N for living systems3. Hydrosphere4. Biosphere The Nitrogen Cycle - 1. N additionThese 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. Mineralization3. Nitrification 4. Plant Uptake 5. NO3 leaching6. Denitrification 7. 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 legumeso 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 SalesAtmospheric 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 areasMineralization- 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 stageso 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 bacteriao 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 acidityo Wetting and drying cycles Drying kills some soil microbes Upon rewetting, the survivors utilize the dead biomass The result is a flush of mineralizationo Cultivation - Improves aeration and exposes fresh organic matter to microbial attacko N Fertilization accelerates residue decomposition unless P or S is limitingImmobilization - 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
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