Biofertilizers

1. Rhizobium is a

Rhizobium is a

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biopesticide
biofertilizer
bioherbicide
mycoherbicide
This question was previously asked in
UPSC CISF-AC-EXE – 2024
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Rhizobium is a genus of bacteria known for forming a symbiotic relationship with the roots of leguminous plants. Within root nodules, these bacteria fix atmospheric nitrogen (N₂) and convert it into a form usable by plants (ammonia/ammonium), thereby enriching the soil and providing essential nutrients for plant growth. This process makes Rhizobium a type of biofertilizer.
Rhizobium bacteria fix atmospheric nitrogen, acting as a natural fertilizer for plants, especially legumes.
Biofertilizers are substances containing living microorganisms which, when applied to seed, plant surfaces, or soil, colonize the rhizosphere or the interior of the plant and promote growth by increasing the supply or availability of primary nutrients to the host plant. Biopesticides, bioherbicides, and mycoherbicides are types of pesticides or herbicides derived from natural materials (e.g., bacteria, fungi, plants, minerals) used to control pests or weeds.

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2. With reference to the role of biofilters in Recirculating Aquaculture

With reference to the role of biofilters in Recirculating Aquaculture System, consider the following statements:

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  • 1. Biofilters provide waste treatment by removing uneaten fish feed.
  • 2. Biofilters convert ammonia present in fish waste to nitrate.
  • 3. Biofilters increase phosphorus as nutrient for fish in water.

How many of the statements given above are correct?

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Only one
Only two
All three
None
This question was previously asked in
UPSC IAS – 2023
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The correct option is A, indicating that only one of the given statements is correct.
Recirculating Aquaculture Systems (RAS) use biofilters as a key component for water treatment.
Statement 1 is incorrect. Biofilters primarily treat dissolved waste products, particularly nitrogenous compounds. Solid wastes, such as uneaten fish feed and feces, are typically removed by mechanical filters (like drum filters or settling tanks) *before* the water reaches the biofilter. While the decomposition of uneaten feed *produces* substances that the biofilter treats, the biofilter itself does not remove the solid feed.
Statement 2 is correct. The main function of a biofilter in RAS is nitrification, which is the biological process where beneficial bacteria convert toxic ammonia (excreted by fish and from decomposition) first into nitrite and then into much less toxic nitrate. This is essential for maintaining water quality suitable for fish.
Statement 3 is incorrect. Biofilters are designed to remove or convert waste products, not increase nutrients for fish. While phosphorus is present in fish feed and waste, biofilters are primarily focused on managing nitrogen. Systems for nutrient addition for fish involve providing high-quality feed, not manipulating waste products in the water circuit.
RAS significantly reduces water usage compared to traditional aquaculture by recycling water. Effective biofiltration is crucial for removing metabolic wastes and maintaining a healthy environment for the fish. Other components of RAS often include mechanical filtration, oxygenation/aeration, pH control, and temperature control.

3. Consider the following organisms : 1. Agaricus 2. Nostoc 3. Spirog

Consider the following organisms :

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  • 1. Agaricus
  • 2. Nostoc
  • 3. Spirogyra

Which of the above is/are used as biofertilizer/biofertilizers?

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1 and 2
2 only
2 and 3
3 only
This question was previously asked in
UPSC IAS – 2013
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Nostoc is used as a biofertilizer. Agaricus and Spirogyra are not typically used as biofertilizers.
1. Agaricus is a genus of fungi (mushrooms) and is not generally used as a biofertilizer. Some fungi like mycorrhizal fungi are used as biofertilizers as they enhance nutrient uptake, but Agaricus is not primarily used for this purpose.
2. Nostoc is a genus of cyanobacteria (blue-green algae) known for its ability to fix atmospheric nitrogen. It is widely used as a biofertilizer, especially in paddy fields.
3. Spirogyra is a genus of filamentous green algae. It is photosynthetic but does not fix nitrogen and is not used as a biofertilizer.
Biofertilizers are substances containing live microorganisms which, when applied to seed, plant surfaces, or soil, colonize the rhizosphere or the interior of the plant and promote growth by increasing the supply or availability of primary nutrients to the host plant. Examples include nitrogen-fixing bacteria (Rhizobium, Azotobacter), cyanobacteria (Anabaena, Nostoc), and phosphorus-solubilizing microorganisms.

4. Match List I with List II and select the correct answer using the code

Match List I with List II and select the correct answer using the code given below the lists :

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List I
(Plant)		List II
(Biofertilizer)
A. Soyabean1. Anabaena
B. Azolla2. Rhizobium
C. Pine3. Azospirillum
D. Maize4. Mycorrhiza
3 4 1 2
2 1 4 3
2 4 1 3
3 1 4 2
This question was previously asked in
UPSC CAPF – 2011
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The correct matching is Soyabean with Rhizobium, Azolla with Anabaena, Pine with Mycorrhiza, and Maize with Azospirillum. This corresponds to option B (2 1 4 3).
Different plants form symbiotic relationships with specific microorganisms that act as biofertilizers, enhancing nutrient uptake, most notably nitrogen fixation. Legumes like soybean associate with *Rhizobium*. The aquatic fern *Azolla* hosts the cyanobacterium *Anabaena azollae*. Many trees, including pine, form mutualistic associations with fungi known as *Mycorrhizae*. Cereals like maize can associate with bacteria like *Azospirillum* in their root zone.
Biofertilizers are substances containing living microorganisms which, when applied to seed, plant surfaces, or soil, colonize the rhizosphere or the interior of the plant and promote growth by increasing the supply or availability of primary nutrients to the host plant. Examples include bacteria (Rhizobium, Azospirillum, Azotobacter), fungi (Mycorrhiza), and cyanobacteria (Anabaena, Nostoc). The specific plant-microbe associations listed are well-documented examples used in agriculture and forestry to improve soil fertility and plant health naturally.

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5. Duckweed or Azolla is used as a/an

Duckweed or Azolla is used as a/an

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pesticide
biofertilizer
arsenic indicator
antiseptic
This question was previously asked in
UPSC Geoscientist – 2023
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Azolla is a small aquatic fern that lives in symbiotic association with the nitrogen-fixing cyanobacterium Anabaena azollae. This symbiosis allows Azolla to fix atmospheric nitrogen into a usable form for plants, making it an excellent natural biofertilizer, particularly for rice paddies. Duckweed (Lemna, etc.) is also an aquatic plant that can be used as green manure, contributing organic matter and nutrients, broadly fitting the category of biofertilizer in this context, though Azolla’s nitrogen-fixing capability is more prominent.
– Azolla is a symbiotic host for nitrogen-fixing cyanobacteria (Anabaena azollae).
– Nitrogen fixation makes Azolla a valuable biofertilizer, especially in agriculture.
– Duckweed can also be used as a source of organic matter and nutrients.
Biofertilizers are substances containing living microorganisms which, when applied to seed, plant surfaces, or soil, colonize the rhizosphere or the interior of the plant and promote growth by increasing the supply or availability of primary nutrients to the host plant. Azolla has been used for centuries as a biofertilizer in wetland rice cultivation.

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6. Azotobacter and Clostridium convert

Azotobacter and Clostridium convert

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gaseous molecular nitrogen into ammonia
ammonia present in soil to gaseous nitrogen
humus present in soil to mineralized constituents
normal healthy soil to toxic soil
This question was previously asked in
UPSC Geoscientist – 2022
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Azotobacter and Clostridium are genera of free-living bacteria known for their ability to fix atmospheric nitrogen. Nitrogen fixation is the process by which gaseous molecular nitrogen (N₂, which is abundant in the atmosphere but unusable by most organisms) is converted into ammonia (NH₃), which can then be incorporated into organic molecules usable by plants and other organisms. Azotobacter is aerobic, while Clostridium is anaerobic.
– Azotobacter and Clostridium are free-living nitrogen-fixing bacteria.
– Nitrogen fixation is the conversion of N₂ gas into ammonia (NH₃).
– This process is essential for making nitrogen available in ecosystems.
– Ammonia can be further converted into other nitrogen compounds like nitrates by other bacteria (nitrification).
Nitrogen fixation is a crucial part of the nitrogen cycle. Besides free-living bacteria like Azotobacter and Clostridium, symbiotic bacteria (like Rhizobium in legume root nodules) are also major nitrogen fixers. Denitrification, the conversion of nitrates back to N₂, is carried out by a different group of bacteria.

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