Biogeochemical Cycles

Nitrogen fixation is the process where nitrogen gas from the air is turned into a form plants can use. Most plants cannot use nitrogen directly from the atmosphere. Specific bacteria like Rhizobium live in the roots of pulses and beans. They convert the gas into ammonia or nitrates. Lightning also fixes nitrogen by breaking the strong bonds of nitrogen gas in the sky. Example: Farmers grow alfalfa or peas to naturally improve soil nitrogen without using chemical fertilizers.

In which nutrient cycle is the weathering of rocks the main source of nutrient release?

The Phosphorus Cycle is the only major biogeochemical cycle where rock weathering is the primary source of nutrient input. Phosphate-bearing rocks (apatite minerals) slowly weather, releasing phosphate ions (PO4³⁻) into soil and water. Unlike carbon, nitrogen, and sulphur cycles, the phosphorus cycle has NO significant gaseous phase — it operates purely through terrestrial and aquatic reservoirs. UPSC 2021 directly tested this: the answer is the Phosphorus Cycle.

Which biogeochemical cycle has no gaseous phase?

The Phosphorus Cycle has no gaseous phase — phosphorus moves through soils, rocks, water, and living organisms but does not have a significant atmospheric component. Carbon cycles through CO2 and CH4 (gaseous). Nitrogen cycles through N2, NH3, N2O (gaseous). Sulphur cycles through SO2, H2S (gaseous). This makes the phosphorus cycle unique and often a rate-limiting nutrient in ecosystems because atmospheric replenishment is unavailable — terrestrial organisms must recycle phosphorus through decomposition and weathering.

In which nutrient cycle is the weathering of rocks the main source of nutrient release?

The Phosphorus Cycle is the only major biogeochemical cycle where rock weathering is the primary source of nutrient input. Phosphate-bearing rocks (apatite minerals) slowly weather, releasing phosphate ions (PO4³⁻) into soil and water. Unlike carbon, nitrogen, and sulphur cycles, the phosphorus cycle has NO significant gaseous phase — it operates purely through terrestrial and aquatic reservoirs. UPSC 2021 directly tested this: the answer is the Phosphorus Cycle.

Which biogeochemical cycle has no gaseous phase?

The Phosphorus Cycle has no gaseous phase — phosphorus moves through soils, rocks, water, and living organisms but does not have a significant atmospheric component. Carbon cycles through CO2 and CH4 (gaseous). Nitrogen cycles through N2, NH3, N2O (gaseous). Sulphur cycles through SO2, H2S (gaseous). This makes the phosphorus cycle unique and often a rate-limiting nutrient in ecosystems because atmospheric replenishment is unavailable — terrestrial organisms must recycle phosphorus through decomposition and weathering.

What are the processes that add nitrogen to the soil?

UPSC PYQ confirmed: (1) Excretion of urea by animals — correct (urea contains nitrogen; soil microbes convert it to ammonia then nitrates). (2) Death of vegetation — correct (decomposition releases organic nitrogen into soil). (3) Burning of coal — incorrect (coal burning releases NOx into the atmosphere, NOT into soil; it contributes to acid rain, not soil nitrogen enrichment). Nitrogen-fixing bacteria (Rhizobium in legume root nodules, free-living Azotobacter, cyanobacteria) also add nitrogen to soil.

What is the carbon cycle and what are the main anthropogenic disruptions to it?

The carbon cycle circulates carbon between atmosphere (CO2, CH4), biosphere (living matter), hydrosphere (dissolved CO2, carbonates), and lithosphere (fossil fuels, limestone). Carbon is fixed by photosynthesis and released by respiration, decomposition, and combustion. Anthropogenic disruptions: (1) burning fossil fuels adding geological carbon to atmosphere (~10 Gt C/year); (2) deforestation reducing carbon sink capacity; (3) cement production releasing CO2 from limestone calcination (~1.5 Gt C/year). Net result: atmospheric CO2 rose from 280 ppm (pre-industrial) to 422 ppm (2024).

Key Facts

• Phosphorus Cycle: rock weathering = primary source; NO gaseous phase — unique among major cycles [Source: UPSC 2021 PYQ]
• Nitrogen fixers: Rhizobium (legume root nodules), Azotobacter (free-living), Anabaena/Anabaena (aquatic) [Source: UPSC PYQ]
• Nitrification: Nitrosomonas (NH4+ → NO2-) then Nitrobacter (NO2- → NO3-) — two-step, two-bacteria process [Source: Microbiology]

In which nutrient cycle is the weathering of rocks the main source of nutrient release?

The Phosphorus Cycle is the only major biogeochemical cycle where rock weathering is the primary source of nutrient input. Phosphate-bearing rocks (apatite minerals) slowly weather, releasing phosphate ions (PO4³⁻) into soil and water. Unlike carbon, nitrogen, and sulphur cycles, the phosphorus cycle has NO significant gaseous phase — it operates purely through terrestrial and aquatic reservoirs. UPSC 2021 directly tested this: the answer is the Phosphorus Cycle.

Which biogeochemical cycle has no gaseous phase?

The Phosphorus Cycle has no gaseous phase — phosphorus moves through soils, rocks, water, and living organisms but does not have a significant atmospheric component. Carbon cycles through CO2 and CH4 (gaseous). Nitrogen cycles through N2, NH3, N2O (gaseous). Sulphur cycles through SO2, H2S (gaseous). This makes the phosphorus cycle unique and often a rate-limiting nutrient in ecosystems because atmospheric replenishment is unavailable — terrestrial organisms must recycle phosphorus through decomposition and weathering.

What are the processes that add nitrogen to the soil?

UPSC PYQ confirmed: (1) Excretion of urea by animals — correct (urea contains nitrogen; soil microbes convert it to ammonia then nitrates). (2) Death of vegetation — correct (decomposition releases organic nitrogen into soil). (3) Burning of coal — incorrect (coal burning releases NOx into the atmosphere, NOT into soil; it contributes to acid rain, not soil nitrogen enrichment). Nitrogen-fixing bacteria (Rhizobium in legume root nodules, free-living Azotobacter, cyanobacteria) also add nitrogen to soil.

What are the steps of the Nitrogen Cycle and which bacteria are involved at each step?

The Nitrogen Cycle steps: (1) Nitrogen Fixation: N2 → NH3 by Rhizobium (legumes), Azotobacter (free-living), Anabaena (cyanobacteria); (2) Ammonification: organic N → NH3/NH4+ by decomposer bacteria and fungi during decay; (3) Nitrification: NH4+ → NO2- by Nitrosomonas; NO2- → NO3- by Nitrobacter; (4) Assimilation: plants absorb NO3- from soil; (5) Denitrification: NO3- → N2 by Pseudomonas, Paracoccus — returns nitrogen to atmosphere. Denitrification is enhanced in waterlogged, anaerobic soils.

A carbon sink is anything that absorbs more carbon than it releases. Examples include thick forests and the vast oceans. A carbon source is anything that releases more carbon than it absorbs. Examples include burning coal, forest fires, and volcanic eruptions. In the UPSC exam, you must identify if a process helps the environment (sink) or increases warming (source). Forests are often called 'lungs' because they act as major sinks.

A carbon sink is any natural environment that absorbs more carbon dioxide than it releases. This helps in cooling the Earth. Forests are great carbon sinks because trees use carbon dioxide for photosynthesis. Oceans are the largest carbon sinks because they dissolve carbon gas into the water. Example: Protecting the Amazon rainforest is important because it acts as a massive global carbon sink, reducing the impact of climate change.

This is the final step that completes the cycle. Denitrifying bacteria like Pseudomonas convert nitrates back into nitrogen gas. This process happens mostly in waterlogged or swampy soils where oxygen is very low. It helps maintain the 78 percent nitrogen level in our atmosphere. Without this step, all the nitrogen would eventually get stuck in the soil and water.

Blue carbon is the carbon stored in coastal and marine ecosystems. This includes mangroves, salt marshes, and seagrass meadows. These ecosystems are much more efficient at burying carbon than land forests. They store carbon in the soil for thousands of years. Protecting these areas is a key strategy for fighting climate change. UPSC frequently asks about this in the context of marine conservation.

These cycles involve elements that do not stay in the atmosphere as a gas. The main reservoir for these elements is the Earth's crust. Elements like Phosphorus and Calcium move from rocks to soil through weathering. Plants absorb them from the soil, and they move up the food chain. Eventually, they return to the ground through waste or death. Example: The Phosphorus cycle is a slow process because it depends on the physical breakdown of mountain rocks.

This concept explains that higher levels of carbon dioxide in the air can speed up plant growth. Since plants use CO2 for photosynthesis, more CO2 acts like extra food. This leads to increased biomass or plant size. However, this does not solve global warming because plants also need water and nutrients to grow. It is a common scientific term used by examiners to test your understanding of plant biology and atmosphere interaction.

Nitrification is the process of converting ammonia into nitrates. This is done by soil bacteria in two stages. First, Nitrosomonas converts ammonia into nitrites. Second, Nitrobacter converts those nitrites into nitrates. Plants prefer nitrates because they are easier to absorb. An example is how compost turns into plant food in a garden. This process requires oxygen to be present in the soil.

This is the first step of the cycle where atmospheric nitrogen (N2) is converted into ammonia (NH3). This happens in three ways: Atmospheric fixation by lightning, Industrial fixation for fertilizers, and Biological fixation by bacteria. For example, Rhizobium bacteria live in a symbiotic relationship with pea plants to provide them with nitrogen. This step is crucial because it makes nitrogen enter the food chain.