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Questions and Answers
Which of the following microbes is specific to nitrogen fixation in alfalfa?
What percentage of nitrogen is typically required between mid tillering and mid stem elongation?
How much urea should be applied per hectare if the nitrogen dose calculated is 37.59 kg N/ha?
Which cover crop yields the highest nitrogen biomass per hectare according to the given data?
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What is an important consideration when applying organic fertilizers for crop nitrogen requirements?
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What is the estimated nitrogen dose per hectare if the crop yield is 1500 kg/ha with 2% nitrogen content and 20% efficiency?
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Which process is primarily responsible for converting ammonium into nitrate in the soil?
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What role do Rhizobium species play in nitrogen cycling?
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Which process involves the conversion of ammonia to nitrite?
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What is the role of Rhizobium bacteria in the nitrogen cycle?
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Which enzyme is involved in the process of ammonification?
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What happens during the denitrification process?
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Which of the following is a primary function of Nitrobacter in the nitrogen cycle?
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In what environmental condition do heterotrophic anaerobic bacteria thrive for denitrification?
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What is the end product of the Haber-Bosch process?
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Which type of bacteria are primarily responsible for nitrification?
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What is a major source of greenhouse gas emissions during the denitrification process?
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What type of bacteria perform biological nitrogen fixation in the rhizosphere of grasslands?
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What is the primary form of nitrogen that is readily available for plant uptake in the nitrogen cycle?
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Which process in the nitrogen cycle involves the conversion of atmospheric nitrogen into a form usable by plants?
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What role do microbes primarily play in the ammonification process of the nitrogen cycle?
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Which of the following processes is directly responsible for the loss of nitrogen from soil as nitrogen gas?
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During nitrification, which compound is produced after the oxidation of ammonium ($NH_4^+$)?
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What is the difference between total nitrogen and available nitrogen in soil?
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Which process leads to the conversion of nitrite ($NO_2^-$) into nitrate ($NO_3^-$)?
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Which form of nitrogen is primarily lost through leaching in the nitrogen cycle?
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Study Notes
Nitrogen Cycle: Industrial Fixation
- The Birkeland–Eyde process, developed in 1903, involves a series of reactions beginning with the oxidation of nitrogen gas by oxygen to form nitric oxide.
- The Haber–Bosch process, established in 1910, is a more efficient method to fix atmospheric nitrogen.
- It converts nitrogen gas and hydrogen gas directly into ammonia under specific conditions: 500°C and 250 atmospheres.
- This reaction is exothermic with a change in enthalpy of -92.4 kJ·mol−1.
- The Haber–Bosch process has been crucial for producing ammonia, a key ingredient in fertilizers, since its development.
- Other products derived from the Haber–Bosch process include ammonium sulfate and urea.
Nitrogen Cycle: Biological Fixation
- Rhizobium bacteria establish a symbiotic relationship with legume plants, resulting in the formation of root nodules.
- These bacteria contain the nitrogenase enzyme, which catalyzes the conversion of atmospheric nitrogen into ammonia.
- Cyanobacteria are photosynthetic organisms that also fix nitrogen. They possess the nitrogenase enzyme and are present in various habitats.
- Azotobacter are free-living bacteria found in the rhizosphere of grasslands, contributing to nitrogen fixation in soils.
Nitrogen Cycle: Ammonification
- Ammonification, also known as mineralization, involves the decomposition of organic matter by saprotrophic microbes.
- These microbes are chemoorganotrophs, meaning they obtain energy from the breakdown of organic compounds.
- The decomposition process releases a significant amount of energy for the microbes.
- Aminotransferases and ureases are enzymes involved in the process.
- Aminotransferases catalyze the conversion of amino acids to ammonia, while ureases break down urea into ammonia and carbon dioxide.
Nitrogen Cycle: Nitrification
- The process of nitrification was discovered by Sergei Wingradsky in 1888.
- It involves the oxidation of ammonia to nitrate by specific chemoautotrophic bacteria, namely Nitrosomas and Nitrobacter.
- Nitrosomas convert ammonia into nitrite, a reaction that releases energy for the bacteria.
- The specific reaction catalyzed by Nitrosomas is 2NH4+ + 3O2 → 2NO2− + 2H2O + 4H+ (+ 276kJ).
- Nitrobacter further oxidize the nitrite to nitrate, releasing an additional 73 kJ of energy.
- This reaction can be represented as 2 NO2− + O2 → 2NO3−.
Nitrogen Cycle: Denitrification
- Denitrification is carried out by heterotrophic anaerobic bacteria, particularly by Pseudomonas species.
- These bacteria are facultative aerobes, meaning they can survive in both oxygen-rich and oxygen-poor environments.
- Nitrates and nitrites serve as their oxygen source.
- Denitrifying bacteria convert nitrates and nitrites to nitrogen gas, which is released into the atmosphere.
- Additionally, a significant portion of the denitrification process results in the production of nitrous oxide (N2O), a potent greenhouse gas.
Nitrogen in Soil Fertility: Pools of N
- The primary pools of nitrogen in the soil are organic nitrogen, inorganic nitrogen, and atmospheric nitrogen.
- The inputs of organic nitrogen come from sources like manures and crop residues.
- Inputs of inorganic nitrogen can be from biological fixation or from fertilizers.
- Atmospheric nitrogen is a major source of nitrogen and input depends on factors like atmospheric fixation and denitrification
- The amount of nitrogen present in a soil depends on the specific soil type and nutrient availability.
Nitrogen in Soil Fertility: Fertilizer Application Strategy
- Fertilizer application should be timed strategically to optimize nutrient uptake by crops.
- The recommendation is to apply 70-80% of the required nitrogen for crops between mid tillering and mid stem elongation, and the remaining amount at flag leaf emergence.
Nitrogen in Soil Fertility: Green Manure and Cover Crops
- Green manure and cover crops contribute to soil fertility by adding organic matter and nitrogen.
- Different crops vary in their biomass and nitrogen content.
- Some common examples include:
- Sweet clover (Biomass: 4.32 tons ha−1, N: 134 kg ha−1)
- Berseem clover (Biomass: 2.71 tons ha−1, N: 78 kg ha−1)
- Crimson clover (Biomass: 3.46 tons ha−1, N: 112 kg ha−1)
- Hairy vetch (Biomass: 4.32 tons ha−1, N: 123 kg ha−1)
Nitrogen in Soil Fertility: Organic Amendments
- Organic amendments are valuable for improving soil fertility and nutrient cycling.
- They contribute to soil organic matter, thus enhancing soil structure, water holding capacity, and nutrient retention.
- They also provide a slow and steady release of nutrients, promoting plant growth and health.
Nitrogen in Soil Fertility: Organic Fertilizer Recommendations
- Organic fertilizer recommendations can be based on fixed doses or empirical tables specific to crops.
- For example, a recommendation for a specific crop might suggest a 10 T/ha application.
- Another approach is to calculate the nitrogen dose based on the estimated crop yield and the amount of nitrogen required by the crop.
- This calculation can be further adjusted by factoring in the efficiency of the fertilizer in providing nitrogen to the crop.
- For instance, if a crop requires 131.58 kg N/ha, and the compost used has 2% N, then the application rate would be 6579 kg Compost / ha.
Nitrogen in Soil Fertility: Rhizobium Inoculation of Legumes
- Rhizobium bacteria are essential for biological nitrogen fixation in legumes.
- There are different species of Rhizobium, each adapted to specific legume crops.
- Inoculating legumes with Rhizobium strains can increase nitrogen fixation and improve crop yields.
- Some common Rhizobium species include:
- R. meliloti – alfalfa
- R. trifolii – clover
Nitrogen in Soil Fertility: Soil Test
- Soil testing is a crucial aspect of crop management, as it provides valuable information about nutrient availability in the soil.
- For readily available nitrogen, a soil test measuring soil nitrate content is recommended. This can be done by extracting nitrate with a KCl solution.
- To assess total nitrogen content in the soil, a Kjeldahl digestion is used.
- This method determines the total nitrogen content in all forms, organic and inorganic.
- It's important to note that about 98% of nitrogen in the soil is present in organic forms, and only a fraction is immediately available to plants.
Nitrogen in Soil Fertility: Total N is different from available N
- The total nitrogen content in the soil is not the same as the nitrogen readily available for plant uptake.
- Total nitrogen includes all forms of nitrogen, while available nitrogen refers to the nitrogen that is readily accessible to plants.
- Factors influencing available nitrogen include:
- Microbial activity
- Soil organic matter
- Nitrogen fixation
- Nitrification
- Denitrification
- Leaching
- It is crucial to manage these factors to optimize nutrient availability in the soil.
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Description
Explore the processes involved in the nitrogen cycle, focusing on industrial nitrogen fixation through the Haber–Bosch process and biological fixation by Rhizobium bacteria. This quiz delves into the chemical reactions, conditions, and significance of ammonia production for agriculture.
