Nutrient Cycling - UPSC Notes - Environment - Thought Chakra

Nutrient Cycling – UPSC Notes – Environment

A Nutrient cycle is a cyclic mechanism through which nutrients travel to be recycled and used. Cells, organisms, communities, and ecosystems are all included in the pathway of a nutrient cycle. This article will explain Nutrient Cycling, which is crucial for the Environment syllabus of the UPSC Civil Service exam.

Nutrient Cycling – Concept

  • Nutrients undergo absorption, transportation, release, and reabsorption during this process.
  • It’s a natural mineral-nutrient recycling mechanism.
  • After death and decomposition, nutrients consumed by plants and animals are returned to the environment, perpetuating the cycle.
  • Soil microbes play a crucial role in nutrient recycling, breaking down organic matter to release nutrients.
  • They are also essential for trapping and transforming nutrients into the soil for plant root absorption.
  • The pace of nutrient cycling is influenced by biotic, physical, and chemical variables.
  • Examples of nutrient cycles include the carbon cycle, nitrogen cycle, water cycle, and oxygen cycle.
Nutrient Cycling

Nutrient Cycling- Types

Based on the Replacement Period

  • Perfect Cycle: Nutrients are replaced at the same rate as they are used in a perfect nutrient cycle. Most Gaseous cycles are considered perfect cycles.
  • Imperfect Cycle: Sedimentary cycles are imperfect because certain nutrients are lost from the cycle and trapped in sediments, rendering them unavailable for immediate cycling.

Based on the Nature of the Reservoir

  • Gaseous Cycle: The atmosphere or the hydrosphere serves as the reservoir in the gaseous cycle.
  • Sedimentary Cycle: The reservoir in the sedimentary cycle is the Earth’s crust.

Gaseous Cycles

Water, carbon, and nitrogen are the three most significant gaseous cycles.

Water Cycle

  • The water (hydrological) cycle is the solar-driven continuous circulation of water in the Earth-atmosphere system.
  • Key reservoirs for water include the atmosphere, oceans, lakes, rivers, soils, glaciers, snowfields, and groundwater.
  • Processes involved in the cycle include evaporation, transpiration, condensation, precipitation, deposition, runoff, infiltration, and groundwater flow.
  • It entails the continuous flow of water between land surface, oceans, subsoil, and among species.
  • The evaporation of water from the ocean’s surface initiates the hydrologic cycle.
Water Cycle

Carbon Cycle

  • Carbon, mainly as carbon dioxide (CO2), exists in the atmosphere.
  • The continual exchange of carbon between the atmosphere and organisms is known as the carbon cycle.
  • Photosynthesis transports carbon from the atmosphere to green plants and eventually to organisms.
  • Carbon returns to the atmosphere through respiration and decomposition of dead organic matter.
  • It is predominantly a short-term cycle.
Carbon Cycle

Nitrogen Cycle

Nitrogen serves as a basic building unit of every living tissue and is a vital component of protein, constituting approximately 16 percent of all proteins by weight.

The nitrogen cycle consists of three primary phases: nitrogen fixation, nitrification, and denitrification.

This cycle involves the atmosphere, hydrosphere, and lithosphere.

  • Nitrogen fixation is an anaerobic process converting atmospheric nitrogen (N2) to NH3 by nitrogen-fixing bacteria.
  • Nitrification is a two-step process where NH4+ is first reduced to NO2 and then further oxidized to NO3. Soil bacteria facilitate both steps.
  • Denitrification involves the conversion of nitrates to nitrogen gas by denitrifying bacteria, akin to the process of nitrogen fixation.
Nitrogen Cycle

Sedimentary Cycle

  • Sedimentary cycles are biogeochemical cycles with the Earth’s crust as their reservoir.
  • Elements such as iron, calcium, phosphorus, sulphur, and others are part of the sedimentary cycle.
  • These cycles encompass both solution (water-related) and rock (sediment) phases.
  • They have an extended timeframe for completion.
  • Phosphorus cycle and Sulphur cycle are the most significant sedimentary cycles.

Phosphorus Cycle

  • Phosphorus circulates through rocks, water, soil, sediments, and organisms.
  • Rain and weathering liberate phosphate ions and other minerals from rocks over time.
  • Inorganic phosphate is dispersed throughout the soil and water.
  • Plants uptake inorganic phosphate from the soil, which can be consumed by animals.
  • Phosphate becomes part of organic molecules like DNA once inside plants or animals.
  • Upon death, organic phosphate is released into the soil during decomposition.
  • Bacteria break down organic materials, converting them into inorganic forms of phosphorus through mineralization.
  • Phosphorus can eventually reach streams and oceans, where it may be absorbed into sediments over time.
Phosphorus Cycle

Sulfur Cycle

The majority of the world’s sulphur is bound up in rocks and salts, or buried deep within oceanic sediments. Sulphur is also present in the air we breathe, with both natural and human sources contributing to its atmospheric presence.

  • Natural sources include volcanic eruptions, microbiological activities, water evaporation, and decomposing organisms.
  • Human activities, particularly industrial processes, generate significant amounts of sulphur dioxide (SO2) and hydrogen sulphide (H2S) gases, contributing to sulphur in the atmosphere.

Sulphur dioxide can react with oxygen to form sulphur trioxide gas (SO3) or with other chemicals in the atmosphere, leading to the production of sulphur salts.

  • In the presence of water, sulphur dioxide can form sulphuric acid (H2SO4).
  • Dimethyl sulphide, exhaled by plankton species, can also contribute to the formation of sulphuric acid.

These particles may fall back to earth or react with rain, resulting in acid deposits. Plants absorb these particles, releasing them back into the atmosphere, thus restarting the sulphur cycle.

Sulfur Cycle

Calcium Cycle

Calcium is primarily found in the form of rock, minerals, or structural calcium embedded in the mineral crystal lattices of soil particles, and it is not easily available.

  • The majority of calcium in the soil is insoluble unless it is ‘weathered off’ of minerals or broken down by bacteria from organic materials into soluble calcium.
  • However, some calcium is held loosely or securely in the soil or soil solution and is available to plants and microbes.

Animals, microbes, and plants decompose, releasing mineralized calcium back into the soil. Roots also regularly return minerals, carbohydrates, and other chemicals to the soil, including calcium.

  • Calcium is adsorbed to the surface of clay and negatively charged organic particles in the soil because it is a positively charged ion. These are referred to as “exchangeable ions” as they can be exchanged with other ions in the soil solution.
  • When taken by plants or microbes, calcium enters an organic phase, constantly exchanged between plant roots, microbes, and soil.
  • Decomposers break down plant, animal, or soil fauna after death, releasing calcium back into the soil in a soluble form.
  • Thus, calcium alternates between soluble (available) and insoluble (unavailable) phases.
Calcium Cycle

Importance of Nutrient Cycling

Nutrient Cycling is essential for the conversion of nutrients from one form to another, enabling their utilization by various organisms.

  • For instance, plants cannot directly utilize atmospheric nitrogen and must fix and convert it into ammonium and nitrate before absorption.

Nutrient cycles play a crucial role in maintaining ecosystem balance by storing nutrients for future use.

  • The transfer of nutrients between locations, such as from air to soil or water to soil, occurs through nutrient cycling.

Living creatures interact with abiotic components of their surroundings through nutrient cycling, facilitating the flow of nutrients for sustenance and growth.


Carbon, hydrogen, oxygen, nitrogen, sulphur, and phosphorus constitute the foundation of all living beings, biomolecules, and cells, serving as essential nutrients for life.

  • Recycling and replenishing these nutrients in the environment are crucial for the existence of life.

Nutrient cycles are vital for maintaining ecosystem balance by storing these nutrients for future utilization.

FAQs on Nutrient Cycling

Question: How are the nutrient cycles connected?

Answer: Nutrient cycles are interconnected through various processes. For example, in the carbon cycle, plants absorb carbon dioxide from the atmosphere during photosynthesis, and carbon is subsequently transferred to animals through consumption. When organisms die and decompose, carbon returns to the soil, completing the cycle. Similarly, other nutrient cycles, such as the nitrogen and phosphorus cycles, involve processes of absorption, transformation, and release, creating interconnected pathways for nutrient flow within ecosystems.

Question: Which nutrient cycle is the most important?

Answer: The importance of nutrient cycles can vary depending on the ecosystem and the organisms present. However, the carbon cycle is often considered one of the most vital nutrient cycles because carbon is a fundamental element of life and is involved in essential processes such as photosynthesis, respiration, and the formation of organic compounds. Without the carbon cycle, life as we know it would not be sustainable.

Question: What would happen without nutrient cycling?

Answer: Without nutrient cycling, ecosystems would struggle to support life. Nutrients would become depleted, leading to nutrient deficiencies in plants and subsequent disruptions in food chains and ecosystem dynamics. Dead organic matter would accumulate without decomposition, leading to nutrient imbalances and reduced soil fertility. Additionally, without nutrient cycling, essential processes such as photosynthesis and cellular respiration would be disrupted, ultimately affecting the overall health and functioning of ecosystems.

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