NRES 201 Lecture 34 Outline of Last Lecture I. Nitrogen in plantsII. Natural occurrence of nitrogenIII. Nitrogen in soilsIV. The nitrogen cyclea. Biological Fixationb. Atmospheric Depositionc. Mineralizationd. Immobilization Outline of Current Lecture V. The Nitrogen Cyclea. Nitrificationb. Denitrification c. Plant Uptaked. Ammonium Fixatione. Ammonia Volatilization f. Leaching of NitrateVI. Nitrogen Fertilizer SourcesCurrent LectureNitrification 2 Step Oxidation - - Step 1: 2NH4 + 3O2 = 2NO2 + 2H2O + 4H- Step 2: 2NO2 + O2 = 2NO3Carried out by soil bacteria - - Step 1 is due to Nitrosomonas - Step 2 is due to Nitrobacter - Both groups are chemoautotrophs o C from CO2o Energy from oxidizable NOpposing 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 forNH4 over NO3These 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:- Alkalinityo Optimum pH: 8.0 - 8.5o Limiting promoted nitrificationo Nitrification produces acidity- Soil aeration - both steps require O2- Adequate Soil Moisture o Field capacity optimalo Some nitrification at the wilting point and also in flooded soilsDenitrification - 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 subsoilo Supply of NO3 and NO2 NO2 favored over NO3 High NO3 increases N2O/N2 ratioo Warm Temperatureso Alkalinity Like nitrification Acidity increases the N2O/N2 ratio- Environmental implications - o Beneficial effect: Less NO3 leachingo Detrimental effect: Emission of N2O (greenhouse gas that promotes destructionof stratospheric ozone)Strategies for controlling denitrification:- Tile drainageo Improves Aeration but also promotes NO3 leaching- Delay N fertilization until the crop is growingo 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 byprolonging high NH4 concentrationAmmonium 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 subsoilAmmonia 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 WeatherFactors affecting the extent of leaching of nitrate:- The soil typeo Moisture propertieso Organic matter content- The weather conditionso 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- Pollutiono Growth and decay of aquatic plantso Oxygen depletion in coastal waters- Health Hazardso Blue baby syndromeo Stomach cancerNitrogen 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 soilBiotic Processes - N2 Fixation, Mineralization, Immobilization, Nitrification, DenitrificationAbiotic Processes - Atmospheric Deposition, NH4 Fixation, NH3 Volatilization, NO3
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