NRES 201 Lecture 34 Outline of Last Lecture I Nitrogen in plants II Natural occurrence of nitrogen III Nitrogen in soils IV The nitrogen cycle a Biological Fixation b Atmospheric Deposition c Mineralization d Immobilization Outline of Current Lecture V The Nitrogen Cycle a Nitrification b Denitrification c Plant Uptake d Ammonium Fixation e Ammonia Volatilization f Leaching of Nitrate VI Nitrogen Fertilizer Sources Current Lecture Nitrification 2 Step Oxidation Step 1 2NH4 3O2 2NO2 2H2O 4H Step 2 2NO2 O2 2NO3 Carried out by soil bacteria Step 1 is due to Nitrosomonas Step 2 is due to Nitrobacter Both groups are chemoautotrophs o C from CO2 o Energy from oxidizable N Opposing effects on plan N availability The downside NO3 is more prone to loss than NH4 through leaching and denitrification The upside Immobilization by soil microbes is reduced by their strong preference for NH4 over NO3 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 Favored by Alkalinity o Optimum pH 8 0 8 5 o Limiting promoted nitrification o Nitrification produces acidity Soil aeration both steps require O2 Adequate Soil Moisture o Field capacity optimal o Some nitrification at the wilting point and also in flooded soils Denitrification An Anaerobic Process N gases formed by reduction of NO3 Occurs in waterlogged soil A bacterial process that involves mostly heterotrophs an can live with or without O2 Favored by o Waterlogging Controls aeration Large losses in ponded areas Very little loss below field capacity o Decomposable organic matter Energy source for denitrifies Not much denitrification in subsoil o Supply of NO3 and NO2 NO2 favored over NO3 High NO3 increases N2O N2 ratio o Warm Temperatures o Alkalinity Like nitrification Acidity increases the N2O N2 ratio Environmental implications o Beneficial effect Less NO3 leaching o Detrimental effect Emission of N2O greenhouse gas that promotes destruction of stratospheric ozone Strategies for controlling denitrification Tile drainage o Improves Aeration but also promotes NO3 leaching Delay N fertilization until the crop is growing o Prevents pre season loss of fertilizer N Use of nitrification inhibitor o Limits the supply of NO3 as a substrate but can also promote immobilization by prolonging high NH4 concentration Ammonium Fixation Like K NH4 is fixed as an interlayer cation by vermiculite and illite Fixation is due to size Fixed NH4 is not readily available slowly available to plants and mainly in the subsoil Ammonia Volatilization Favored by high NH4 concentration from animal waste or fertilizer urea Alkalinity pH is greater than 7 5 Low soil CEC that limits adsorption of NH4 and buffer capacity Surface placement Warm and Windy Weather Factors affecting the extent of leaching of nitrate The soil type o Moisture properties o Organic matter content The weather conditions o Rainfall amount and intensity o Temperature Tillage practices Presence of plants Uptake of NO3 reduces leaching and so does uptake of water Depth of rooting A function of plant type stage of growth and soil properties N Fertilization o Over fertilization promotes leaching of NO3 Reasons for Concern Economy Pollution o Growth and decay of aquatic plants o Oxygen depletion in coastal waters Health Hazards o Blue baby syndrome o Stomach cancer Nitrogen Fertilizer Sources Anhydrous NH3 o The leading and least expensive fertilizer in the US o Produced by the Haber Bosch process Urea o The leading N fertilizer on the world market o Also present in animal urine o Converts to NH3 when applied to soil Biotic Processes N2 Fixation Mineralization Immobilization Nitrification Denitrification Abiotic Processes Atmospheric Deposition NH4 Fixation NH3 Volatilization NO3 Leaching
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