Nitrogen Cycle - Free MCQ Practice Test with solutions, UPSC Lucent

The main source of nitrogen for the nitrogen cycle is the atmosphere (Option A). The Earth's atmosphere is composed of approximately 78% nitrogen gas (N2), making it the largest reservoir of nitrogen.

The process that converts atmospheric nitrogen into a form usable by plants is nitrogen fixation (Option A). Nitrogen fixation is carried out by certain types of bacteria, which have the ability to convert atmospheric nitrogen into ammonia (NH₃) or ammonium (NH⁴⁺), which can be taken up by plants.

The bacteria responsible for nitrogen fixation is Rhizobium (Option C). Rhizobium forms a symbiotic relationship with leguminous plants, such as soybeans and clover, and resides in their root nodules. These bacteria have the enzyme nitrogenase, which enables them to convert atmospheric nitrogen into ammonia, benefiting both the bacteria and the plants.

Plants primarily absorb nitrogen in the form of nitrate (NO₃^-) (Option A). Nitrate is the most common form of nitrogen taken up by plant roots from the soil.

The process that returns nitrogen back to the atmosphere is denitrification (Option A). Denitrifying bacteria convert nitrate (NO₃^-) back into nitrogen gas (N2), completing the nitrogen cycle and releasing nitrogen back into the atmosphere.

Transpiration (Option D) is not a step in the nitrogen cycle. Transpiration refers to the process by which plants release water vapor from their leaves into the atmosphere.

The role of denitrifying bacteria in the nitrogen cycle is to convert nitrate to nitrogen gas (Option C). These bacteria carry out denitrification, breaking down nitrate and releasing nitrogen gas back into the atmosphere.

The human activity that has disrupted the nitrogen cycle by increasing the amount of nitrogen in the environment is agricultural practices (Option B). The use of synthetic fertilizers in agriculture, as well as the rearing of livestock, has led to increased nitrogen runoff into water bodies and nitrogen deposition in ecosystems, causing environmental issues such as eutrophication and water pollution.

The significance of the nitrogen cycle in agriculture is that it increases the yield of crops (Option B). Nitrogen is an essential nutrient for plant growth, and the availability of nitrogen through the nitrogen cycle supports healthy plant development, leading to increased agricultural productivity.

Combustion (Option C) is the process that releases nitrogen gas into the atmosphere. When organic matter, such as fossil fuels or biomass, is burned, nitrogen present in the material is released as nitrogen gas.

Excess nitrogen in aquatic ecosystems negatively impacts the environment by promoting excessive plant growth (Option A). This leads to algal blooms, which can deplete oxygen levels in the water when the algae die and decompose. The reduced oxygen levels can harm other aquatic organisms and disrupt the ecosystem balance.

Plants play a crucial role in the nitrogen cycle as they fix nitrogen from the atmosphere (Option A). Through symbiotic relationships with nitrogen-fixing bacteria or through their own root nodules, certain plants have the ability to convert atmospheric nitrogen into forms that can be utilized by other organisms in the ecosystem.

An example of a symbiotic relationship in the nitrogen cycle is Nitrogen fixation and assimilation (Option C). Nitrogen fixation is the process of converting atmospheric nitrogen into a usable form, while assimilation refers to the incorporation of nitrogen into organic compounds by plants and other organisms.

Animals obtain nitrogen in the food chain by consuming plants or other animals that contain nitrogen (Option B). Nitrogen is an essential component of proteins and other biomolecules, and animals obtain it through their diet.

Lightning contributes to the nitrogen cycle by converting atmospheric nitrogen into nitrates (Option A). During a lightning strike, the high temperature and energy cause the nitrogen gas in the atmosphere to react with oxygen, forming nitrogen oxides that eventually dissolve in rainwater to form nitrates.

Human-induced nitrogen fixation occurs through nitrogen fixation by nitrogen-fixing bacteria (Option C). While there are natural processes like lightning and nitrogen fixation by leguminous plants, human activities such as the use of synthetic fertilizers and industrial processes have significantly increased the contribution of nitrogen-fixing bacteria in fixing nitrogen for agricultural purposes.

The primary cause of nitrogen pollution is agricultural runoff (Option D). Excess nitrogen from fertilizers and animal waste used in agriculture can be washed into water bodies through runoff, leading to water pollution and ecosystem disruption.

The impact of nitrogen on soil pH varies depending on the nitrogen compound (Option D). Ammonium-based fertilizers can decrease soil pH, making it more acidic, while nitrates can have a neutral or slightly alkaline effect on soil pH.

The consequence of eutrophication caused by excessive nitrogen in water bodies is oxygen depletion and fish kills (Option C). When excess nitrogen enters water bodies, it promotes the growth of algae and other aquatic plants. As these organisms die and decompose, the process consumes oxygen, leading to hypoxia (low oxygen levels) or anoxia (no oxygen) in the water, which can result in fish and other aquatic organism deaths.

An example of a natural nitrogen-fixing plant is soybeans (Option C). Leguminous plants, including soybeans, have a symbiotic relationship with nitrogen-fixing bacteria, such as Rhizobium, which enables them to fix atmospheric nitrogen and convert it into forms that can be used by the plants.