Nitrogen Cycle Questions in English

(A) Statement $A$ is correct: During lightning,the high temperature and electrical discharge provide sufficient energy to break the strong triple bond of atmospheric nitrogen $(N_2)$,allowing it to react with oxygen to form nitrogen oxides $(NO_x)$. Ultraviolet radiation also contributes to this process.
Statement $R$ is correct: Human activities such as industrial combustion,forest fires,automobile exhausts,and power-generating stations release significant amounts of nitrogen oxides into the atmosphere,acting as anthropogenic sources of nitrogen fixation and atmospheric nitrogen compounds.
Therefore,both statements are correct.

(A) Nitrifying bacteria,such as $Nitrosomonas$ and $Nitrobacter$,play a crucial role in the nitrogen cycle.
They oxidize ammonia $(NH_3)$ into nitrites $(NO_2^-)$ and subsequently into nitrates $(NO_3^-)$.
This process is essential for plants as they absorb nitrogen primarily in the form of nitrates from the soil.
Therefore,the correct function is the oxidation of ammonia to form nitrates.

(D) Thiobacillus is a genus of bacteria known for its role in the nitrogen cycle,specifically in the process of denitrification.
Denitrification is the process by which nitrate $(NO_3^-)$ is reduced to nitrogen gas $(N_2)$ or nitrous oxide $(N_2O)$.
This process is carried out by denitrifying bacteria such as Pseudomonas and Thiobacillus in soil,which helps in returning nitrogen to the atmosphere.

(C) The process of converting nitrate $(NO_3^-)$ present in the soil into gaseous nitrogen $(N_2)$ is known as denitrification.
This process is carried out by denitrifying bacteria.
Among the given options,$Thiobacillus$ and $Pseudomonas$ are well-known examples of denitrifying bacteria.
$Nitrobacter$ and $Nitrococcus$ are involved in the process of nitrification (converting nitrite to nitrate).
$Nitrosomonas$ is involved in the conversion of ammonia to nitrite.
Therefore,the correct answer is $Thiobacillus$.

(N/A) $\Rightarrow$ Apart from carbon,hydrogen,and oxygen,nitrogen is the most prevalent element in living organisms.
$\Rightarrow$ Nitrogen is a constituent of amino acids,proteins,hormones,chlorophylls,and many vitamins.
$\Rightarrow$ Plants compete with microbes for the limited nitrogen available in the soil.
$\Rightarrow$ Thus,nitrogen is a limiting nutrient for both natural and agricultural ecosystems.
$\Rightarrow$ Nitrogen exists as two nitrogen atoms joined by a very strong triple covalent bond $(N \equiv N)$.
$\Rightarrow$ The process of conversion of nitrogen $(N_2)$ to ammonia is termed as nitrogen fixation.
$\Rightarrow$ In nature,lightning and ultraviolet radiation provide enough energy to convert nitrogen to nitrogen oxides $(NO, NO_2, N_2O)$. Industrial combustions,forest fires,automobile exhausts,and power-generating stations are also sources of atmospheric nitrogen oxides.
$\Rightarrow$ Decomposition of organic nitrogen of dead plants and animals into ammonia is called ammonification.
$\Rightarrow$ Some of this ammonia volatilises and re-enters the atmosphere.
$\Rightarrow$ Most of it is converted into nitrate by soil bacteria in the following steps:
$\Rightarrow$ Ammonia is first oxidised to nitrite by the bacteria Nitrosomonas and/or Nitrococcus.
$\Rightarrow$ The nitrite is further oxidised to nitrate with the help of the bacterium Nitrobacter.
$\Rightarrow$ These steps are called nitrification.
$\Rightarrow$ Ammonia $\xrightarrow{\text{Oxidation}}$ Nitrite
$\Rightarrow$ Nitrite $\xrightarrow[\text{Oxidation}]{\text{Nitrobacter}}$ Nitrate
$\Rightarrow$ These nitrifying bacteria are chemoautotrophs.
$\Rightarrow$ The nitrate thus formed is absorbed by plants and transported to the leaves.
$\Rightarrow$ In leaves,it is reduced to form ammonia.
$\Rightarrow$ Nitrate $\rightarrow$ leaves $\xrightarrow{\text{Reduction}}$ Ammonia $\rightarrow$ Amino acid
$\Rightarrow$ It finally forms the amine group of amino acids. Nitrate present in the soil is also reduced to nitrogen by the process of denitrification.
$\Rightarrow$ Nitrate $\xrightarrow{\text{Denitrification}}$ Nitrogen
$\Rightarrow$ This process is carried out by bacteria like Pseudomonas and Thiobacillus.

(N/A) The nitrogen cycle is a biogeochemical process by which nitrogen is converted into various chemical forms as it circulates among the atmosphere,terrestrial,and marine ecosystems. The main phases are:
$1$. Nitrogen Fixation: The process of converting atmospheric nitrogen $(N_2)$ into ammonia $(NH_3)$ or other nitrogenous compounds. This can occur through biological fixation (by microbes like $Rhizobium$,$Azotobacter$),industrial fixation,or atmospheric fixation (lightning).
$2$. Nitrification: The process where ammonia is oxidized to nitrite $(NO_2^-)$ by bacteria like $Nitrosomonas$ and $Nitrococcus$,and then further oxidized to nitrate $(NO_3^-)$ by $Nitrobacter$.
$3$. Assimilation: Plants absorb nitrate or ammonia from the soil and incorporate them into organic molecules like amino acids and proteins.
$4$. Ammonification: The decomposition of organic nitrogenous compounds (from dead plants and animals) back into ammonia by decomposers like bacteria and fungi.
$5$. Denitrification: The process where nitrate in the soil is reduced back to gaseous nitrogen $(N_2)$ by denitrifying bacteria such as $Pseudomonas$ and $Thiobacillus$,returning it to the atmosphere.

(N/A) $(1)$ Ammonification: The process of decomposition of organic nitrogenous compounds from dead plants and animals into ammonia is known as ammonification.
$(2)$ Nitrogen Fixation: The process of conversion of atmospheric nitrogen $(N_2)$ into ammonia $(NH_3)$ is termed as nitrogen fixation.

Organisms like Pseudomonas and Thiobacillus are significant because they perform the process of denitrification.
In this process,these bacteria reduce nitrates $(NO_{3}^{-})$ and nitrites $(NO_{2}^{-})$ present in the soil back into gaseous nitrogen $(N_{2})$.
This nitrogen is then released back into the atmosphere,thereby completing the nitrogen cycle.

$(C)$. Thiobacillus performs denitrification, which is the process of converting nitrates back into nitrogen gas $(2)$.
$B$. Nitrosomonas is a nitrifying bacterium that oxidizes $NH_{3}$ to nitrite $(4)$.
$C$. Nostoc is a free-living nitrogen-fixing cyanobacterium $(1)$.
$D$. Azotobacter is a free-living aerobic nitrogen-fixing bacterium $(3)$.
Therefore, the correct matching is $A-2, B-4, C-1, D-3$.

(B) Denitrification is the reduction of nitrate $(NO_3^-)$ to gaseous nitrogen $(N_2)$ or other gaseous nitrogen compounds.
Under anaerobic conditions, certain microorganisms use nitrate and other oxidized ions as a source of oxygen for respiration.
Examples of such bacteria include $Pseudomonas \, denitrificans$, $Thiobacillus \, denitrificans$, and $Micrococcus \, denitrificans$.

(D) Nitrifying bacteria are chemosynthetic autotrophs that obtain energy by oxidizing inorganic nitrogen compounds.
They perform the process in two steps:
$1$. 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 bacteria such as $Nitrobacter$ and $Nitrocystis$.
Thus,the overall process is the oxidation of ammonia to nitrates.

(A) Non-biological nitrogen fixation occurs through physical processes in the atmosphere.
Lightning and cosmic radiation provide the high energy required to break the strong triple covalent bonds of atmospheric nitrogen $(N_2)$.
This energy allows nitrogen to react with oxygen to form nitrogen oxides,which eventually dissolve in rainwater to form nitrates,contributing to the nitrogen cycle.

(B) Nitrification is the biological process of converting ammonia into nitrates.
In the first step,ammonium ions $(NH_4^+)$ are oxidized to nitrites $(NO_2^-)$ by bacteria such as $Nitrosomonas$ and $Nitrosococcus$.
In the second step,nitrites $(NO_2^-)$ are oxidized to nitrates $(NO_3^-)$ by bacteria such as $Nitrobacter$ and $Nitrocystis$.

(A) The atmosphere contains approximately $78\%$ nitrogen gas $(N_2)$. Although plants cannot directly utilize this atmospheric nitrogen, it serves as the primary reservoir from which nitrogen is fixed by biological and physical processes into forms that plants can absorb. Therefore, the atmosphere is considered the ultimate source of nitrogen.

(B) Nitrogen is a critical element required by plants in large amounts. It is often the most limiting nutrient in both natural and agricultural ecosystems because,although it is abundant in the atmosphere as $N_2$ gas,plants cannot directly utilize it. It must be converted into usable forms like $NH_4^+$ or $NO_3^-$ through biological or industrial nitrogen fixation.

(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 denitrifying bacteria such as $Pseudomonas$ and $Thiobacillus$.
$Nitrococcus$,$Nitrosomonas$,and $Nitrobacter$ are involved in the process of nitrification,where ammonia is converted into nitrites and then into nitrates.

(C) The ultimate source of nitrogen is the atmosphere.
It is fixed into usable forms by several biological and non-biological processes.
Plants primarily absorb nitrogen from the soil in the form of nitrates $(NO_3^-)$ and ammonium salts $(NH_4^+)$.
Therefore,nitrogen is present in the soil in the form of nitrates and ammonium salts.

(C) $1$. $I$ Nitrosomonas: This bacterium oxidizes $NH_3$ to nitrite,not nitrite to nitrate. Nitrite to nitrate conversion is performed by Nitrobacter.
$2$. $II$ Thiobacillus: This bacterium is involved in the process of denitrification,where nitrate is reduced to nitrogen gas. This is a correct match.
$3$. $III$ Nostoc: This is a cyanobacterium that acts as a free-living nitrogen-fixer. This is a correct match.
$4$. $IV$ Azotobacter: This is a free-living nitrogen-fixing aerobic microbe,not anaerobic. Therefore,this is an incorrect match.
Thus,$II$ and $III$ are the correct matches.

(A) The nitrogen cycle is a biogeochemical process by which nitrogen is converted into various chemical forms as it circulates among the atmosphere,terrestrial,and marine ecosystems.
$1$. Nitrogen fixation converts atmospheric nitrogen $(N_{2})$ into ammonia $(NH_{3})$.
$2$. Nitrification is the process where ammonia is converted into nitrites $(NO_{2}^{-})$ and then into nitrates $(NO_{3}^{-})$ by nitrifying bacteria.
$3$. Assimilation involves plants taking up these nitrates to form organic nitrogen compounds.
$4$. Ammonification is the process where organic nitrogen from dead organisms is converted back into ammonia by decomposers.
$5$. Denitrification is the process where nitrates are reduced back into atmospheric nitrogen $(N_{2})$ by denitrifying bacteria.
Looking at the options,option $A$ correctly represents the sequence: Atmospheric Nitrogen $\rightarrow$ Ammonia (via fixation) $\rightarrow$ Nitrites $\rightarrow$ Nitrates $\rightarrow$ Atmospheric Nitrogen (via denitrification).

(A) The process of nitrification involves the oxidation of ammonia to nitrites and then to nitrates.
$Nitrosomonas$ and $Nitrococcus$ are chemoautotrophic bacteria that oxidize ammonia to nitrites $(NO_{2}^{-})$.
$Nitrobacter$ is responsible for the further oxidation of nitrites to nitrates $(NO_{3}^{-})$.
The chemical reaction for the conversion of ammonia to nitrites is:
$2NH_{3} + 3O_{2} \xrightarrow{\text{Nitrosomonas}} 2NO_{2}^{-} + 2H^{+} + 2H_{2}O + \text{Energy}$

(B) The process of converting ammonia $(NH_{3})$ into nitrite $(NO_{2}^{-})$ and subsequently into nitrate $(NO_{3}^{-})$ is known as nitrification.
This process is carried out by soil bacteria such as $Nitrosomonas$ and $Nitrobacter$.

(D) The reactions provided represent the process of nitrification,which is the conversion of ammonia into nitrates.
Step $I$ $(2 NH_{3} + 3 O_{2} \rightarrow 2 NO_{2}^{-} + 2 H^{+} + 2 H_{2} O)$ is performed by bacteria such as $Nitrosomonas$ or $Nitrococcus$.
Step $II$ $(2 NO_{2}^{-} + O_{2} \rightarrow 2 NO_{3}^{-})$ is performed by $Nitrobacter$.
Both steps together are collectively known as nitrification.
The bacteria involved in these processes are chemoautotrophs,not photoautotrophs,as they obtain energy by oxidizing inorganic substances rather than using light energy. Therefore,the statement that they are photoautotrophs is incorrect.

(B) The process of converting nitrate $(NO_{3}^{-})$ into gaseous nitrogen $(N_{2})$ is known as denitrification.
This process is carried out by denitrifying bacteria such as $Pseudomonas$ and $Thiobacillus$.
Therefore,the correct option is $B$.

(D) The Assertion is incorrect because the process of nitrification involves the oxidation of ammonia $(NH_3)$ to nitrite $(NO_2^-)$ by $Nitrosomonas$ and $Nitrococcus$,and then nitrite is oxidized to nitrate $(NO_3^-)$ by $Nitrobacter$. The statement in the Assertion reverses the order of products.
The Reason is also incorrect because plants primarily absorb nitrogen in the form of nitrate $(NO_3^-)$ or ammonium $(NH_4^+)$ through their roots,not through their leaves. Once absorbed,nitrate is transported to the leaves where it is reduced back to ammonia to be incorporated into amino acids.

$(A)$ The correct matching is as follows:
$(a)$ Nitrococcus: It is a nitrifying bacterium that converts ammonia to nitrite. So, $(a)-(ii)$.
$(b)$ Rhizobium: It is a symbiotic nitrogen-fixing bacterium that converts atmospheric nitrogen to ammonia. So, $(b)-(iv)$.
$(c)$ Thiobacillus: It is a denitrifying bacterium that converts nitrates back into atmospheric nitrogen (denitrification). So, $(c)-(i)$.
$(d)$ Nitrobacter: It is a nitrifying bacterium that converts nitrite to nitrate. So, $(d)-(iii)$.
Therefore, the correct sequence is $(a)-(ii), (b)-(iv), (c)-(i), (d)-(iii)$.

(N/A) $\Rightarrow$ Nitrifying bacteria are chemoautotrophs.
$\Rightarrow$ They obtain energy by oxidizing inorganic substances like ammonia $(NH_3)$ or nitrite $(NO_2^-)$ into nitrate $(NO_3^-)$.
$\Rightarrow$ This energy is used to fix carbon dioxide $(CO_2)$ into organic compounds.
$\Rightarrow$ The nitrate formed is absorbed by plants and transported to the leaves.
$\Rightarrow$ In the leaves,it is reduced to form ammonia,which is then incorporated into amino acids.
$\Rightarrow$ The pathway is: $\text{Nitrate} \rightarrow \text{Leaves} \xrightarrow{\text{Reduction}} \text{Ammonia} \rightarrow \text{Amino acid}$.

(A) The conversion of atmospheric nitrogen $(N_2)$ into ammonia $(NH_3)$ is known as Nitrogen fixation $(P)$.
Decomposition of organic nitrogen (from dead plants and animals) into ammonia is known as Ammonification $(Q)$.
Therefore,$P$ is Nitrogen fixation and $Q$ is Ammonification.

(D) The given reaction represents the oxidation of ammonia $(NH_3)$ to nitrite $(NO_2^-)$. This process is known as nitrification. The bacteria responsible for this specific step are Nitrosomonas and Nitrococcus. Nitrobacter is responsible for the subsequent oxidation of nitrite to nitrate $(NO_3^-)$. Therefore,both Nitrosomonas and Nitrococcus perform this reaction.

(C) The process of nitrification involves the oxidation of ammonia to nitrite and then nitrite to nitrate.
$1$. Ammonia is oxidized to nitrite $(NO_2^{-})$ by bacteria such as $Nitrosomonas$ and $Nitrococcus$.
$2$. Nitrite is further oxidized to nitrate $(NO_3^{-})$ by the bacterium $Nitrobacter$.
Therefore,the reaction $2 NO_2^{-} + O_2 \rightarrow 2 NO_3^{-}$ is specifically carried out by $Nitrobacter$.

(D) In the nitrogen cycle,denitrification is the process by which nitrates $(NO_3^-)$ are reduced back into atmospheric nitrogen $(N_2)$.
Looking at the provided diagram,the arrow labeled $S$ represents the conversion of soil nitrates $(NO_3^-)$ back into atmospheric nitrogen $(N_2)$.
Therefore,$S$ represents the process of denitrification.

(D) Denitrification is the process of reducing nitrate $(NO_3^-)$ to gaseous nitrogen $(N_2)$.
This process is carried out by denitrifying bacteria such as $Pseudomonas$ and $Thiobacillus$.
$Nitrobacter$ is involved in the process of nitrification,where it converts nitrite $(NO_2^-)$ to nitrate $(NO_3^-)$.
Therefore,both $Pseudomonas$ and $Thiobacillus$ are responsible for denitrification.

(D) Atmospheric nitrogen $(N_2)$ consists of two nitrogen atoms held together by a strong triple covalent bond $(N \equiv N)$.
This bond requires a significant amount of energy to break, making the molecule chemically inert or non-reactive under normal biological conditions.
Therefore, plants cannot directly utilize atmospheric nitrogen and must absorb it in the form of nitrates $(NO_3^-)$ or ammonium ions $(NH_4^+)$ from the soil, which are more reactive and biologically available.

(D) By $sedimentation$, nitrates of the soil are washed away to the sea or leached deep into the earth along with percolating water.