Consortium for Educational Communication
Frequently asked questions
Q1. What is diazotrophy? How is it related to nitrogen nutrition of
higher plants?
Answer. Diazotrophy refers to the ability of some prokaryotic species
to directly utilize atmospheric N2 to fulfil their N demand. The
prokaryotes with this ability are known as diazotrphs. Diazotrophs
play a significant role in the nitrogen nutrition of higher plants
as these from the gateway through which nitrogen from the
inorganic world enters into organic living world. Diazotrophs fix the
atmospheric nitrogen into plant usable form either at their own or
in combination with their eukaryotic symbiotic partners and enrich
the soil with plant usable nitrogen.
Q2. Write a short note on nitrification.
Answer. The process of microbial oxidation of NH4+ and organic N
into NO3- via NO2- is known as nitrification. Nitrification proceeds
rapidly in warm, moist and well aerated soils. Nitrification adds
nitrate to soil which is the chief source of plant nitrogen nutrition.
Theoretically, the oxidation of atmospheric nitrogen to nitrate is
favourable however, due to the occurrence of triple bond in N2
between two nitrogen atoms this conversion does not occur in
nature very readily. Lightening provides enough energy to favour
this transformation and contributes anything between 0.5 to 5.0
kg of nitrate nitrogen ha-1 year-1 to soil. Nitrification occurs in two
steps, first is the conversion of ammonium into nitrite and second,
the conversion of nitrite into nitrate. The microbes involved in the
first reaction i.e the conversion of ammonia into nitrite are prefixed
with “nitroso” and those catalysing the conversion of nitrite into
nitrate are prefixed with “nitro.”
NH4+
NO2-
NO3-
Q3. What is denitrification?
Answer. Denitrification is the process of loss of nitrogen from
the soil in the form of gases like N2, NO or N2O. Denitrification
leads to loss of nitrogen from the soil and decreases soil fertility.
Denitrification losses can be as high as 5% of the available nitrate
per day. Denitrification occurs when under anaerobic conditions
nitrogen is used as the terminal electron acceptor and is converted
to gaseous nitrogen.
Q4. What are the factors that influence nitrogen content in soil?
Answer. Soil nitrogen content is influenced by various environmental
Consortium for Educational Communication
factors which include soil moisture, soil pH, daily temperature,
incidence of rainfall, fire and soil aeration status. Besides,
populations of soil microbes which mediate nitrogen conversions
(nitrification, ammonification, immobilization, nitrogen fixation
etc.) in soil also influence the soil nitrogen content.
Q5. Define nitrogen immobilization. How does the C:N ratio in
litter affect the fate of nitrogen in it.
Answer. Nitrogen immobilization refers to the temporary
unavailability of soil nitrogen for plant uptake due to its uptake by
soil microbes which utilize it for building up their own cell mass.
Thus, immobilization means locking up of nutrients in microbial
biomass. C: N ratio of litter determines the fate of nitrogen present
in it as for example, net mineralization occurs when C: N ratio is
less than 20:1, organic remains with C: N ratio 30:1 favour net
immobilization. Most of the well decomposed organic matter has
C: N ratio of 10:1.
Q6. What do you understand by dissimilatory nitrate reduction to
ammonia (DNRA)?
Answer. Dissimilatroy nitrate reduction refers to reduction of nitrate
to ammonia. It is catalysed by the enzyme nitrate reductase an
inducible enzyme induced by nitrate in absence of oxygen. DNRA
occurs in long-term anaerobic environments like sediments and
estuaries. Nitrate reductase is a molybdenum containing enzyme.
DNRA is an energy generating process and under anaerobic
conditions is used to generate energy.
Q7. Write a note on biological nitrogen fixation.
Answer. Nitrogen fixation is the process whereby atmospheric
nitrogen is taken up and fixed (reduced to NH4+) by some prokaryotic
organisms either at their own or in combination with some
eukaryotic plants. Nitrogen fixing organisms convert atmospheric
nitrogen into plant usable form. The nitrogen fixing organisms
are known as diazotrophs. Nitrogen fixation is exergonic process
and consumes two ATP molecules for each electron transferred to
N. Overall ATP consumption for each molecule of N fixed is 16 as
two electrons are consumed in evolution of hydrogen during this
process. The overall reaction for nitrogen fixation is
N2 + 3H2
2 NH3 + H2
Q8. What is assimilatory nitrate reduction? Comment on the
pathways of incorporation of nitrogen into the web of life.
Consortium for Educational Communication
Answer. Assimilatory nitrate reduction refers to the reduction of
nitrate to ammonia to incorporate it into the carbon skeleton of amino
acids. Inorganic ammonium ion is integrated (assimilated) into the
organic amino acid molecules through glutamate dehydrogenase
pathway (GDH) when concentration of ammonium is high. GDH
has very high Km for its substrate. Another pathway known as
glutamine synthetase (GS) and glutamine α – oxoglutarate amino
transferase (GOGAT) commonly known as glutamate synthase
(GS/ GOGAT) is operative when ammonium occurs in relatively
low amounts. GS/ GOGAT thus involves two enzyme systems. The
first reaction of incorporating ammonium into the organic carbon
skeleton is performed by GS resulting in the formation of glutamine.
Glutamine is then used in subsequent transamination reactions to
serve as amine group donor for synthesis of other amino acids.
Q9. What is ammonification? What role does it play in the cycling
of nitrogen through the different atmospheric compartments?
Answer. Ammonification is the process of converting organic nitrogen
into ammonia. Ammonification is also known as mineralization.
Ammonification returns nitrogen to inorganic pool and therefore
makes available nitrogen locked in organic molecules for uptake by
plants and soil microbes. Ammonification is a significant process in
nitrogen cycling as it forms a link between organic and inorganic
forms of nitrogen. In absence of ammonification all the nitrogen
in the world will be locked up in the organic molecules and will not
be available for plant uptake.
Q10. How do you see the role of microbes in the cycling of nitrogen?
Answer. Soil microbes play important roles in the cycling of mineral
nutrients. However, the involvement of soil microbes in cycling
of nitrogen is far greater. At the outset nitrogen present in the
atmosphere cannot be directly used by plants and most of the
terrestrial ecosystems are underproductive on account of lack
availability of this nutrient in appropriate forms. Microbes make
nitrogen amenable to plant uptake after fixing it into to ammonia.
This process which is commonly known as nitrogen fixation is
the single most dominant process determining the availability of
nitrogen to plants. Soil microbes also interconvert various forms
of nitrogen in soil e.g. nitrifies convert ammonium into nitrite
and nitrate. Nitrate is the preferred form of nitrogen for plant
uptake. Mineralization of organically bound nitrogen is affected
by nitrogen mineralizing bacteria which again make it available
for plant uptake. Some soil organisms convert nitrate nitrogen
into gaseous forms like N2 and N2O etc. and cause loss of nitrogen
Consortium for Educational Communication
from the soil, these are known as denitrifies. Soil microbes also
use nitrogen for building their own cell mass therefore making
nitrogen temporarily unavailable for plant uptake this process
known as immobilization helps prevent losses of nitrogen from
the soil by leaching or volatilization.
Q11. Write a short note on cycling of nitrogen in the environment.
Answer. Nitrogen cycling is the process by which different
forms of nitrogen interconvert and move from one atmospheric
compartment to another and even through biological world.
Nitrogen cycle involves biological as well as abiological processes.
Atmosphere is the largest reservoir of nitrogen. It contains 78.09%
of nitrogen on volume basis. Nitrogen from atmosphere is fixed
by diazotrphs. Nitrogen that enters the living compartment of the
environment at the level of prokaryotes enters the successive
levels of hierarchy of life. Nitrogen assimilated by living organisms
into vital bio-molecules like amino acids, proteins and nucleic
acids is returned to the non-living world through the process of
ammonification. Different forms of nitrogen like nitrate, nitrite and
ammonia are inter-converted by different soil microorganisms.
Denitrification returns gaseous nitrogen to the atmosphere.
Q12. What do you know about the role of nitrogen in global
warming?
Answer. Nitrogen forms the dominant component of the
atmosphere. Various oxides of nitrogen have been listed as
greenhouse gases having global warming effect. Nitrous oxide
and nitric oxide are potent greenhouse gases. Nitrogen which
is added to soil in the from of fertilizers is not completely taken
up by plants but a part of it is converted into greenhouse gases.
Soil warming increases emission of N2O from the soil. Agricultural
soils are the main source of N2O. One nitrous oxide molecule,
traps heat about 200 times more effectively than each molecule
of carbon dioxide. Nitrous oxide also remains in the air for a long
time on the order of a century because it does not dissolve easily
in water and resists reacting with other chemicals. Consequently,
it eventually reaches the stratosphere where sunlight breaks it
into nitric oxide, a key link in the chain of reactions that damages
the Earth’s protective ozone layer. At the same time, other fixednitrogen gases released from fertilizers contribute to producing
ozone in the lower atmosphere, where it is a pollutant rather than
a protector. This reactive nitrogen can also lead to production of
aerosols that can induce serious respiratory illness, cancer, and
cardiac disease when in the air.
Consortium for Educational Communication
Q13. Define nitrate respiration.
Answer. Respiration in which nitrate rather than oxygen is used
as the terminal electron acceptor is known as nitrate respiration.
Nitrate respiration is thus an energy yielding process that occurs
under anaerobic conditions. The reduction of nitrate to nitrite is
catalysed by nitrate reductase. Nitrate respiration is regulated by
oxygen and it causes loss of nitrate nitrogen from the soil when it
proceeds to formation of gaseous nitrogen.
Q14. What do you know about the types of nitrogen fixation?
Name some of the important players in nitrogen fixation.
Answer. Nitrogen fixation is the process of fixing atmospheric N2
into ammonium. Nitrogen fixation converts non-usable form of
nitrogen (N2) into the usable one (NH4+). Nitrogen fixation is of
two types a. symbiotic and b. Non-symbiotic. In symbiotic nitrogen
fixation the nitrogen fixing prokaryote is housed in the eukaryotic
host. While the host provides the bacteria with carbon source
the bacteria provides fixed nitrogen to the host. Thus, both the
participants are benefited by this mutual association. Not only
bacteria, but even actinomycetes have been reported to fix nitrogen
in association with plants. The other type of nitrogen fixation is
non-symbiotic nitrogen fixation which is carried out by free living
bacteria without any association with the plant hosts. Among the
symbiotic nitrogen fixing organisms various species of Rhizobium
e.g., R. leguminosarum, and R. elti top the list. Rhizobia in general
are Gram negative, motile non-spore forming bacilli. Among
non-symbiotic nitrogen fixing organisms Clostridium is the best
characterised one. It is an obligate anaerobic, endospore forming
Gram positive bacillus that fixes nitrogen at an intermediate pH
range. Azotobacter which is a Gram negative, motile soil bacterium
fixes nitrogen asymbiotically under aerobic conditions. It grows
best in soils with neutral to alkaline pH and is not found in acidic
soils. Frankia that forms symbiotic association with actinorhizal
plants like alder belongs to the filamentous bacteria. Frankia also
forms root nodules in its host plants.
Q15. What is Haber-Bosch process?
Answer. The method of industrial fixation of nitrogen is known as
Haber-Bosch process after the name of its discoverers. During
this process nitrogen is fixed under the extreme heat (200 oC) and
pressure (200bars) conditions on account of existence of a strong
triple bond between two nitrogen atoms in the nitrogen molecule.
The reaction is:
Consortium for Educational Communication
N2 + 3H2
2NH3
Haber-Bosch process made the production of ammonia easy at
industrial scale. Haber-Bosch process is regarded as the most
significant invention of the 20th century. The discoverers were
honoured with Nobel Prize F. Haber in 1918 and C. Bosch in 1931.
Hydrogen and nitrogen are combined to from ammonia under very
high temperatures and pressure. 500 million tons of fertilizer is
produced world over annually through this process which consumes
1% of the world’s energy supply. World agriculture is highly
dependent upon nitrogen fertilizers which are made available to
the farmers due to Haber-Bosch process.