Nitrogen Cycle Questions in English

(C) $Bacillus \ rimosus$ is a soil bacterium known for its role in the nitrogen cycle.
Specifically,it is classified as an ammonifying bacterium.
Ammonifying bacteria break down organic nitrogenous compounds (like proteins and amino acids) from dead plants and animals into ammonia $(NH_3)$ or ammonium ions $(NH_4^+)$,which can then be utilized by other organisms or further processed in the nitrogen cycle.

(B) The process of converting ammonia $(NH_3)$ into nitrite $(NO_2^-)$ and then into nitrate $(NO_3^-)$ by soil bacteria is known as Nitrification.
This process is carried out by chemoautotrophic bacteria such as $Nitrosomonas$ and $Nitrobacter$.
Ammonification is the conversion of organic nitrogen into ammonia.
Denitrification is the reduction of nitrate back into gaseous nitrogen $(N_2)$.
Nitrogen fixation is the conversion of atmospheric nitrogen $(N_2)$ into ammonia.

(A) Nitrifying bacteria are chemoautotrophs that play a crucial role in the nitrogen cycle.
$Nitrosomonas$ oxidizes ammonia $(NH_3)$ into nitrite $(NO_2^-)$.
$Nitrobacter$ further oxidizes nitrite $(NO_2^-)$ into nitrate $(NO_3^-)$.
Therefore,these bacteria collectively convert ammonia or ammonium compounds into nitrates.

(D) Denitrification is a biological process in which nitrate $(NO_3^-)$ or nitrite $(NO_2^-)$ is reduced to gaseous nitrogen $(N_2)$ or nitrous oxide $(N_2O)$.
This process is carried out by denitrifying bacteria such as $Pseudomonas$ and $Thiobacillus$ in the soil under anaerobic conditions.
It is a crucial step in the nitrogen cycle that returns nitrogen to the atmosphere.

(C) Denitrification is the process by which certain bacteria reduce nitrates $(NO_3^-)$ back into gaseous nitrogen $(N_2)$.
These bacteria are known as denitrifying bacteria.
The sequence of reduction is: $NO_3^- \rightarrow NO_2^- \rightarrow NO \rightarrow N_2O \rightarrow N_2$.
Examples include $Pseudomonas$ and $Thiobacillus$.

(D) Nitrifying bacteria are chemoautotrophic organisms that oxidize ammonia to nitrites and then to nitrates.
$1$. Ammonia $(NH_3)$ is oxidized to nitrite $(NO_2^-)$ by bacteria such as $Nitrosomonas$ and $Nitrococcus$.
$2$. Nitrite $(NO_2^-)$ is further oxidized to nitrate $(NO_3^-)$ by bacteria such as $Nitrobacter$.
Since both steps are part of the nitrification process,the correct answer is that they convert ammonia into nitrites and nitrites into nitrates.

(A) The process of breaking down complex organic nitrogenous compounds like proteins into ammonia is known as ammonification.
This process is carried out by various soil microorganisms,including bacteria such as $Bacillus$ species (e.g.,$Bacillus$ $mycoides$,$Bacillus$ $vulgaris$) and fungi.
$Nitrosomonas$ is involved in the process of nitrification (converting ammonia to nitrite),not ammonification.
$Pseudomonas$ is primarily involved in denitrification.
Therefore,$Bacillus$ $vulgaris$ is a known ammonifying bacterium.

(B) The process of nitrification occurs in two steps:
$1$. Ammonium salts $(NH_4^+)$ are oxidized to nitrite $(NO_2^-)$ by bacteria such as $Nitrosomonas$ or $Nitrococcus$.
$2$. Nitrite $(NO_2^-)$ is further oxidized to nitrate $(NO_3^-)$ by the bacterium $Nitrobacter$.
Therefore,the bacteria that convert ammonium salts into nitrite is $Nitrosomonas$.

(B) The process of converting nitrates $(NO_3^-)$ present in the soil into gaseous nitrogen $(N_2)$ is known as denitrification.
This process is carried out by denitrifying bacteria.
$Pseudomonas$ and $Thiobacillus$ are the primary examples of bacteria that perform denitrification in the soil.
Therefore,the correct option is $B$.

(D) Plants generally absorb nitrogen from the soil primarily in the form of nitrates $(NO_3^-)$.
Although some plants can also absorb nitrogen as nitrites $(NO_2^-)$ or ammonium ions $(NH_4^+)$,nitrates are the most common and preferred form for uptake.
These ions are derived from the nitrogen cycle processes occurring in the soil,where atmospheric nitrogen is converted into usable forms by nitrogen-fixing bacteria and other soil microorganisms.

(B) Atmospheric nitrogen is chemically inert and cannot be directly utilized by plants.
One of the natural ways to fix atmospheric nitrogen into nitrogenous compounds (like nitrates) is through physical processes.
During lightning (electric discharge),the high temperature and pressure cause nitrogen $(N_2)$ and oxygen $(O_2)$ in the atmosphere to react,forming nitrogen oxides.
These oxides subsequently dissolve in rainwater to form nitric acid,which reaches the soil and is converted into nitrates,which are then absorbed by plants.
Therefore,$10\%$ of natural nitrogen fixation occurs due to lightning.

(C) The process of converting nitrates $(NO_3^-)$ back into atmospheric nitrogen $(N_2)$ is known as denitrification.
This process is carried out by specific soil bacteria known as denitrifying bacteria.
Examples of such bacteria include $Thiobacillus \ denitrificans$,$Pseudomonas \ aeruginosa$,and $Micrococcus \ denitrificans$.
These bacteria reduce nitrates present in the soil into gaseous nitrogen,which then returns to the atmosphere.

(B) The process of converting ammonia $(NH_3)$ into nitrites $(NO_2^-)$ and subsequently into nitrates $(NO_3^-)$ by soil bacteria is known as Nitrification.
$1.$ Ammonia is first oxidized to nitrite by bacteria such as $Nitrosomonas$ or $Nitrococcus$.
$2.$ The nitrite is then further oxidized to nitrate by the bacterium $Nitrobacter$.
These steps are crucial for the nitrogen cycle as they make nitrogen available for plant uptake.

(B) In water-logged soils and other anaerobic areas,some bacteria utilize the oxygen of nitrates for meeting their metabolic requirements.
This process reduces nitrate into gaseous forms of nitrogen,such as nitrous oxide $(N_2O)$ and nitrogen gas $(N_2)$,which escape into the atmosphere.
This biological phenomenon is known as denitrification.

(B) $Nitrobacter$ plays a crucial role in the nitrogen cycle. It is a chemoautotrophic bacterium that oxidizes nitrites $(NO_2^-)$ into nitrates $(NO_3^-)$,which are the forms of nitrogen most easily absorbed by plants.
This process is a vital part of nitrification,ensuring that nitrogen is available in the soil for plant uptake.
Once plants absorb these nitrates,nitrogen enters the food chain and moves from producers to consumers (herbivores and carnivores),thus maintaining the nitrogen cycle.

(D) The process of converting organic nitrogen (such as proteins and amino acids from dead plants and animals) into ammonia $(NH_3)$ or ammonium ions $(NH_4^+)$ is known as ammonification.
Bacteria that perform this conversion are called ammonifying bacteria.
These bacteria break down complex organic nitrogenous compounds into simpler inorganic forms,which can then be utilized by other soil microbes or plants.
Therefore,the correct term for these bacteria is ammonifying bacteria.

(C) The correct answer is $C$. Bacteria play the most active role in the nitrogen cycle through various processes:
$1$. Nitrification: Ammonium compounds are oxidized into nitrates in two stages. First,nitrite bacteria (e.g.,$Nitrosomonas$) oxidize ammonia into $NO_2^-$. Second,nitrate bacteria (e.g.,$Nitrobacter$) oxidize $NO_2^-$ into $NO_3^-$,which plants can absorb.
$2$. Nitrogen Fixation: Bacteria like $Rhizobium$ (in root nodules of legumes),$Azotobacter$,and cyanobacteria fix atmospheric nitrogen into organic forms.
$3$. Denitrification: Bacteria like $Pseudomonas$ convert nitrates back into atmospheric nitrogen $(N_2)$,completing the cycle.

(C) Non-biological nitrogen fixation occurs through physical processes in the atmosphere.
$1$. Electrochemical fixation: During lightning,the high temperature and electrical discharge cause atmospheric nitrogen $(N_2)$ to react with oxygen $(O_2)$ to form nitrogen oxides.
$2$. Photochemical fixation: High-energy solar radiation can also facilitate the reaction between nitrogen and oxygen in the atmosphere.
These nitrogen oxides dissolve in rainwater to form nitric acid and nitrous acid,which reach the soil and are converted into nitrates,thus becoming available to plants. Therefore,both electrochemical and photochemical processes contribute to non-biological nitrogen fixation.

(D) Plants and animals contain nitrogen in their body proteins. After death,the proteins of dead bodies are decomposed by microorganisms into amino acids and ammonia. This process is a crucial step in the nitrogen cycle,where organic nitrogen is converted back into inorganic forms like ammonia,which can then be converted into nitrates or released as free nitrogen gas into the atmosphere.

(A) Nitrogen fixation occurs through biological,industrial,and atmospheric processes.
Atmospheric nitrogen fixation occurs via electrochemical and photochemical reactions during lightning and ultraviolet radiation.
The estimated amount of nitrogen fixed annually through these atmospheric (electrochemical and photochemical) processes is approximately $35 \ mg/m^2/yr$.

(A) Nitrification is a biological process carried out by chemoautotrophic bacteria such as $Nitrosomonas$ and $Nitrobacter$.
These bacteria are highly sensitive to environmental conditions.
The process of nitrification is significantly influenced by the $pH$ of the soil.
Nitrification occurs most efficiently in neutral to slightly alkaline soils ($pH$ $7.0$ to $8.5$).
In acidic soils,the activity of nitrifying bacteria is severely inhibited,making $pH$ the primary limiting factor for this process.

(B) The nitrogen content of the biosphere remains constant due to the $Nitrogen \ cycle$.
This cycle involves a series of biological and physical processes such as nitrogen fixation,nitrification,assimilation,ammonification,and denitrification.
These processes ensure that nitrogen is continuously recycled between the atmosphere,soil,and living organisms,maintaining a dynamic equilibrium in the biosphere.

(B) The atmosphere is composed of various gases. Nitrogen $(N_2)$ is the most abundant gas,making up approximately $78.03\%$ of the Earth's atmosphere by volume.
Oxygen $(O_2)$ accounts for about $21\%$,while other gases like Argon,Carbon dioxide $(CO_2)$,and trace gases make up the remainder.
Nitrogen is continuously cycled through the atmosphere via processes such as nitrogen fixation,volcanic eruptions,erosion of sedimentary rocks,and denitrification.

(C) The correct answer is $(c)$.
Denitrifying bacteria,such as $Pseudomonas$ $aeruginosa$ and $Thiobacillus$ $denitrificans$,are responsible for the process of denitrification.
In this process,these bacteria reduce nitrates $(NO_3^-)$ present in the soil into gaseous nitrogen $(N_2)$,which then escapes into the atmosphere,completing the nitrogen cycle.

(B) Guano is the excrement of seabirds,cave-dwelling bats,and seals. It is rich in phosphorus and nitrogen. Fishing and guano birds play a significant role in the phosphorus cycle by transporting phosphorus from the ocean back to the land through their droppings (guano),which are then used as fertilizer.

(A) The concentration of nitrogen in the atmosphere remains constant primarily due to the nitrogen cycle.
The nitrogen cycle involves various processes such as nitrogen fixation,nitrification,assimilation,ammonification,and denitrification,which balance the nitrogen levels.
While thundering and lightning contribute to nitrogen fixation by converting atmospheric nitrogen into nitrogen oxides,the overall stability of nitrogen concentration is maintained by the complete nitrogen cycle.

(A) The process of converting complex organic nitrogenous compounds into inorganic forms like ammonia $(NH_3)$ is known as ammonification. This process is carried out by various saprophytic bacteria and fungi,such as $Pseudomonas$ species,which decompose organic matter in the soil. $Rhizobium$ is involved in nitrogen fixation,while $Nitrobacter$ and $Nitrosomonas$ are involved in the process of nitrification (converting ammonia to nitrites and nitrates). Therefore,$Pseudomonas$ is the correct answer.

(A) The process of reducing nitrate $(NO_3^-)$ to gaseous nitrogen $(N_2)$ is called denitrification.
This process is carried out by bacteria such as $Pseudomonas$ and $Thiobacillus$.
It is a key step in the nitrogen cycle that returns nitrogen to the atmosphere.

(A) The process of nitrification involves two steps carried out by specific chemoautotrophic bacteria:
$1$. Ammonia $(NH_3)$ is oxidized to nitrite $(NO_2^-)$ by bacteria such as $Nitrosomonas$ and $Nitrosococcus$.
$2$. Nitrite $(NO_2^-)$ is further oxidized to nitrate $(NO_3^-)$ by the bacterium $Nitrobacter$.
Therefore,the correct sequence is $Nitrosomonas$ for the first step and $Nitrobacter$ for the second step.

(B) The conversion of ammonia $(NH_3)$ into nitrate $(NO_3^-)$ by soil bacteria is known as nitrification.
This process occurs in two steps:
$1$. Ammonia is first oxidized to nitrite $(NO_2^-)$ by bacteria such as $Nitrosomonas$ and $Nitrococcus$.
$2$. The nitrite is then further oxidized to nitrate $(NO_3^-)$ by the bacterium $Nitrobacter$.
These nitrifying bacteria are chemoautotrophs.

(D) Denitrification is a biological process carried out by denitrifying bacteria such as $Pseudomonas$ and $Thiobacillus$.
In this process,nitrates $(NO_3^-)$ present in the soil are reduced to gaseous nitrogen $(N_2)$ or nitrous oxide $(N_2O)$.
This gaseous nitrogen is then released back into the atmosphere,completing the nitrogen cycle.
Therefore,the correct function is the reduction of nitrates to gaseous nitrogen.

(A) Nitrifying bacteria are chemoautotrophic organisms that play a crucial role in the nitrogen cycle.
They oxidize ammonia $(NH_3)$ to nitrite $(NO_2^-)$ and then to nitrate $(NO_3^-)$.
Examples include $Nitrosomonas$ (which oxidizes ammonia to nitrite) and $Nitrobacter$ (which oxidizes nitrite to nitrate).
Therefore,the correct statement is that they oxidize ammonia to nitrate.

(A) In nature,the nitrogen cycle is primarily driven by various microorganisms,especially bacteria.
$1$. Nitrogen-fixing bacteria (e.g.,$Rhizobium$,$Azotobacter$) convert atmospheric nitrogen into ammonia.
$2$. Nitrifying bacteria (e.g.,$Nitrosomonas$,$Nitrobacter$) convert ammonia into nitrites and nitrates.
$3$. Denitrifying bacteria (e.g.,$Pseudomonas$,$Thiobacillus$) convert nitrates back into atmospheric nitrogen.
Therefore,bacteria are the most active participants in the nitrogen cycle.

(A) Nitrifying bacteria,such as $Nitrosomonas$ and $Nitrobacter$,are chemoautotrophic organisms that play a crucial role in the nitrogen cycle.
They convert ammonia $(NH_3)$ into nitrites $(NO_2^-)$ and subsequently into nitrates $(NO_3^-)$ through the process of oxidation.
This process is known as nitrification,which makes nitrogen available in a form that plants can easily absorb from the soil.
Therefore,the correct process performed by nitrifying bacteria is the oxidation of ammonia to nitrates.

(D) Biological nitrogen fixation is the process by which atmospheric nitrogen $(N_2)$ is converted into ammonia $(NH_3)$ by living organisms,primarily bacteria like $Rhizobium$,$Azotobacter$,etc.
Some bacteria also convert nitrogen into nitrates $(NO_3^-)$ through the process of nitrification,which is a part of the nitrogen cycle.
Therefore,both the conversion of nitrogen to ammonia (fixation) and the conversion to nitrates (nitrification) are essential steps in the nitrogen cycle related to nitrogen availability for plants.
Thus,the correct option is $D$.

(D) The correct answer is $(D)$.
Denitrification is a biological process in which nitrates $(NO_3^-)$ in the soil are reduced to molecular nitrogen $(N_2)$ gas,which is then released into the atmosphere. This process is carried out by denitrifying bacteria such as $Pseudomonas$ $denitrificans$ and $Thiobacillus$ $denitrificans$.
Anaerobic ammonium oxidation (often referred to as $ANAMMOX$) is another microbial process where ammonium $(NH_4^+)$ is oxidized to dinitrogen gas $(N_2)$ under anaerobic conditions,contributing to the nitrogen cycle by returning $N_2$ to the atmosphere.

(A) $Nitrifying$ bacteria perform the process of nitrification,which involves the oxidation of ammonia to nitrates.
This process occurs in two steps:
$1$. Nitrite bacteria (e.g.,$Nitrosomonas$ and $Nitrococcus$) convert ammonia $(NH_3)$ into nitrites $(NO_2^-)$.
$2$. Nitrate bacteria (e.g.,$Nitrobacter$ and $Nitrocystis$) convert nitrites $(NO_2^-)$ into nitrates $(NO_3^-)$.
Therefore,the correct function is the oxidation of ammonia to nitrates.

(B) The nitrogen cycle involves several key processes:
$1$. $A$ represents Denitrification,where nitrates in the soil are converted back into atmospheric nitrogen $(N_2)$ by bacteria like $Pseudomonas$.
$2$. $B$ represents Ammonification,where organic nitrogen from dead plants and animals is converted into ammonia $(NH_3)$ by decomposers.
$3$. $C$ represents Animals,which consume organic nitrogen from plants.
$4$. $D$ represents Plants,which absorb nitrogenous compounds from the soil.
Therefore,option $B$ is the correct sequence.

(D) : Denitrification is a biological process in which nitrates $(NO_3^-)$ in the soil are reduced to molecular nitrogen $(N_2)$ gas, which is then released into the atmosphere. This process is carried out by denitrifying bacteria such as $Pseudomonas \text{ } denitrificans$ and $Thiobacillus \text{ } denitrificans$. Additionally, the anaerobic oxidation of ammonium $(NH_4^+)$, also known as the $Anammox$ process, involves the conversion of ammonium and nitrite directly into dinitrogen gas $(N_2)$ under anaerobic conditions.

(C) $1$. Nitrogen is a vital component of amino acids,proteins,nucleic acids,and vitamins,not carbohydrates.
$2$. Ammonification is the process of decomposition of organic nitrogen (proteins/amino acids) into ammonia $(NH_3)$.
$3$. Nitrification involves the oxidation of ammonia to nitrite $(NO_2^-)$ by $Nitrosomonas$ or $Nitrococcus$,and then nitrite to nitrate $(NO_3^-)$ by $Nitrobacter$.
$4$. Denitrification is the process where nitrate is reduced to nitrogen gas $(N_2)$ by bacteria like $Pseudomonas$ and $Thiobacillus$.
$5$. Therefore,the pair 'Nitrification - Nitrobacter' is correct because $Nitrobacter$ is involved in the conversion of nitrite to nitrate during the nitrification process.

(B) The process of decomposition of organic nitrogenous compounds (found in dead plants and animals) into ammonia is known as $Ammonification$.
$1$. $Ammonification$: Bacteria like $Bacillus$ and $Actinomycetes$ break down organic nitrogen into ammonia $(NH_3)$.
$2$. $Nitrification$: Ammonia is further oxidized to nitrite $(NO_2^-)$ and then to nitrate $(NO_3^-)$ by bacteria like $Nitrosomonas$ and $Nitrobacter$.
$3$. $Mineralisation$: This is a broader term referring to the release of inorganic nutrients from organic matter.
$4$. $Humification$: This is the process of formation of dark-colored amorphous substance called humus from organic matter.

(C) Denitrification is the process of reduction of nitrates present in the soil back into gaseous nitrogen $(N_2)$.
This process is carried out by specific denitrifying bacteria.
The most common examples of bacteria involved in denitrification are $Pseudomonas$ and $Thiobacillus$.
Therefore,the correct option is $C$.

(C) The conversion of nitrite $(NO_2^-)$ to nitrate $(NO_3^-)$ is a key step in the process of nitrification.
This specific oxidation reaction is carried out by the chemoautotrophic bacterium $Nitrobacter$.
$Nitrosomonas$ is responsible for the oxidation of ammonia $(NH_3)$ to nitrite $(NO_2^-)$.
Therefore,the correct organism for the given reaction is $Nitrobacter$.

(C) Let's analyze each pair:
$1$. $I$. Nitrosomonas: This bacterium converts ammonia to nitrite $(NH_3 \rightarrow NO_2^-)$,not nitrite to nitrate. Thus,this is incorrect.
$2$. $II$. Thiobacillus: This bacterium is involved in denitrification,where it converts nitrates back into nitrogen gas $(N_2)$. This is correct.
$3$. $III$. Nostoc: This is a cyanobacterium that acts as a free-living nitrogen fixer. This is correct.
$4$. $IV$. Azotobacter: This is an aerobic,free-living nitrogen-fixing bacterium,not anaerobic. Thus,this is incorrect.
Therefore,the correct pairs are $II$ and $III$.

(C) The correct matching is as follows:
$(i)$ Nitrosomonas: Converts Ammonia $(NH_3)$ to Nitrite $(NO_2^-)$. Thus, $(i-d)$.
$(ii)$ Agrobacterium: Some species are involved in denitrification, converting Nitrate $(NO_3^-)$ to Nitrogen gas $(N_2)$. Thus, $(ii-c)$.
$(iii)$ Rhizobium: $A$ well-known symbiotic nitrogen-fixing bacterium found in the root nodules of legumes. Thus, $(iii-b)$.
$(iv)$ Nitrobacter: Converts Nitrite $(NO_2^-)$ to Nitrate $(NO_3^-)$. Thus, $(iv-a)$.
Therefore, the correct sequence is $(i-d), (ii-c), (iii-b), (iv-a)$.

(D) The $N_2$ cycle involves several processes carried out by specific bacteria.
$Pseudomonas$ and $Thiobacillus$ are well-known denitrifying bacteria.
These bacteria reduce nitrates $(NO_3^-)$ present in the soil back into gaseous nitrogen $(N_2)$,which is released into the atmosphere.
This process is known as denitrification.
Therefore,the correct option is $D$.

(A) In the nitrogen cycle,the process of converting ammonia into nitrates is called nitrification. The process of converting organic nitrogen (from dead plants and animals) into ammonia is called ammonification. Based on the standard nitrogen cycle diagram:
$1$. $X$ represents the conversion of nitrates back into atmospheric nitrogen,which is known as denitrification.
$2$. $Y$ represents the conversion of organic nitrogenous compounds into ammonia,which is known as ammonification.
Therefore,$X$ is Denitrification and $Y$ is Ammonification.

(C) The process of nitrification involves the oxidation of ammonia to nitrite and then to nitrate.
$1$. Ammonia $(NH_3)$ is oxidized to nitrite $(NO_2^-)$ by bacteria such as $Nitrosomonas$ and $Nitrococcus$.
$2$. Nitrite $(NO_2^-)$ is then oxidized to nitrate $(NO_3^-)$ by the bacterium $Nitrobacter$.
Therefore,the conversion of nitrite to nitrate is carried out by $Nitrobacter$.

(C) Nitrification is a two-step biological process where ammonia is oxidized to nitrate.
In the first step,ammonia $(NH_3)$ is oxidized to nitrite $(NO_2^-)$ by bacteria such as $Nitrosomonas$ or $Nitrococcus$.
The balanced chemical equation for this step is:
$2NH_3 + 3O_2 \rightarrow 2NO_2^- + 2H^+ + 2H_2O$.
Therefore,the missing reactants are $2NH_3$ and $3O_2$.

(B) The correct matches are as follows:
$(a)$ Free living aerobic nitrogen fixer: $Beijerinckia$ is a free-living aerobic bacterium that fixes nitrogen.
$(b)$ Denitrification: $Thiobacillus$ is a denitrifying bacterium that converts nitrates back into nitrogen gas.
$(c)$ Free living anaerobic nitrogen fixer: $Anabaena$ (specifically in its heterocysts) can fix nitrogen under anaerobic conditions while living freely.
$(d)$ Symbionts: $Frankia$ is a symbiotic nitrogen-fixing bacterium that forms root nodules in non-leguminous plants like $Alnus$.
Therefore,the correct sequence is $(a-4), (b-3), (c-2), (d-1)$.