Rakesh KumarAssistant Professor

PG Dept of GeographyMagadh University, Bodh Gaya

The Sun is the ultimate source of energy for our planet. This

energy starts life processes in the biosphere through the process of photosynthesis, the main source of food and energy for green plants. Photosynthesis requires a series of complex biochemical reactions using water and carbon dioxide and light energy. It is also referred to as carbon fixation as in the process gaseous carbon dioxide is fixed to a solid form in carbohydrate and oxygen molecules are released as by-products.

H2O + CO2 + light energy → –CHOH– + O2

The solar energy flows through ecosystems, moving from one part of the food chain to the next, and finally it is lost to the space from the biosphere in the form of radiated energy.

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Biodiversity and its survival involves the presence of cyclical flows of energy, water and nutrients. These flows vary regionally and also according to the seasonal changes and local conditions.

Studies have shown that for the last one billion years, the atmosphere and hydrosphere have been composed of almost the same balance of chemical components.

It is well established that for any system to continue, the movement of materials must be in cyclical manner.

Matter moves through ecosystems, but unlike energy it cannot be lost in the global ecosystem because of the gravity. The matter is conserved within an ecosystem and atoms and molecules are used, reused, or recycled, within ecosystems.

This chemical balance is maintained by a cyclical pathway through the tissues of plants and animals of the earth’s ecosystem. These cyclic movements of chemical elements of biosphere from organisms and the environments is termed as biogeochemical cycles sometimes referred to as a material cycle or nutrient cycle.

There are two types of biogeochemical cycles:

Gaseous Cycle: The element or compound can be converted into a gaseous

form. The diffused gas from the atmosphere arrives over land or sea and are

reused by the biosphere in a much shorter period of time and goes back to the

atmosphere. Carbon, oxygen, nitrogen and hydrogen, the primary constituents

of all living matter move through gaseous cycle.

Sedimentary Cycle: Weathering releases compound or element from the

rock which is carried to the sea following the movement of running water either

in solution or as sediments. These materials are converted to rocks through

precipitation and sedimentation. When these rocks are exposed for weathering,

the cycle is completed.

Pool: Any area

or location

where material

is concentrated

is called pool.

These are of

two types.

1. Active Pools: Pools where

materials are easily accessible

to life processes.

Pathways between active pools

are generally controlled by life

processes.

Active pools are much smaller.

Materials move more faster.

Photosynthesis and respiration

can cycle carbon dioxide from

the atmosphere(active pool)

through plants in about 10

years.

2. Storage Pools: Materials are more or less

inaccessible to life processes.

Pathways between storage pools are

generally controlled by physical

processes.

Storage pools are much larger.

Materials move comparatively much slowly

in or out of storage pools.

It may take millions of years for carbonate

sediments(storage pool) to form as rock

and get uplifted and decomposed to

release carbon dioxide.

Pool and its Types

We could identify cycles for all elements available at the earth’s surface. However, only a few of them are important to life processes.

There are 15 elements which are the most abundant in the global living matter. Out of these, the three main components of carbohydrate- hydrogen, carbon and oxygen account for 99.5 % of all living matter and are called micronutrients. These are required in substantive quantities by organisms to survive.

Remaining 0.5 % is divided among 12 elements. Six of them are also macronutrients: nitrogen, calcium, potassium, magnesium , sulphur and phosphorus. The three of these: calcium, potassium and magnesium are elements derived from silicate rocks through mineral weathering. Sulphur and phosphorous are derived through rock weathering.

Basic Carbohydrate

Percent Other Nutrients Percent

Hydrogen(M) 49.74 Nitrogen(M) 0.272

Carbon (M) 24.90 Calcium(M) 0.072

Oxygen ( M) 24.83 Potassium(M) 0.044

Subtotal 99.47 Silicon 0.033

Magnesium(M) 0.031

Sulphur 0.017

Aluminium 0.016

Phosphorous(M) 0.013

Chlorine 0.011

Sodium 0.006

Iron 0.005

Manganese 0.003

Source: E.S DEEVEY, Jr., Scientific American, Vol 223

The hydrologic cycle/Water Cycle describes the global flow of water to and from oceans, land, and atmosphere. Water moves by evaporation, precipitation, and runoff.

Water from the oceans and from land surfaces evaporates, changing state from liquid to vapor and enter the atmosphere. Total evaporation is about six times greater over oceans than land because oceans cover most of the planet and because land surfaces are not always wet enough to yield much water.

Water vapor in the atmosphere can condense or deposit to form clouds and precipitation, which falls to Earth as rain, snow, or hail. The precipitation is about four times more over oceans than over land.

1. It evaporates and returns to the atmosphere as water

vapor.

2. It can go into the soil and then into the surface rock

layers below this, the subsurface water emerges from

below to feed rivers, lakes, and even ocean margins.

3. It can run off the land, concentrating in streams and

rivers that eventually carry it to the ocean or to lakes.

This flow is known as runoff.

The carbon cycle is a biogeochemical cycle in which carbon flows among storage pools in the atmosphere, ocean, and on the land.

Carbon moves through the cycle as a gas, as a liquid, and as a solid. In the gaseous portion of the cycle, carbon moves largely as carbon dioxide (CO2), which is a free gas in the atmosphere and a dissolved gas in fresh and saltwater.

In the sedimentary portion of its cycle, we find carbon in carbohydrate molecules in organic matter, as hydrocarbon compounds in rock (petroleum, coal), and as mineral carbonate compounds such as calcium carbonate (CaCO3).

Human activity has affected the carbon cycle, causing carbon dioxide concentrations in the atmospheric storage pool to increase. Burning of fossil fuels, clearing forests or abandoning agricultural areas have led to more carbon dioxide emission which in turn is responsible for Global warming. The ramifications of global warming are very complex and will affect life on earth in many unknown ways.

The complete picture of oxygen cycling includes its movements and storages when combined with carbon as carbon dioxide and as organic and inorganic compounds which have been shown in the carbon cycle.

Oxygen enters the atmospheric free oxygen pool(active pool) through release in photosynthesis , both in oceans and over the land.

A small amount of oxygen comes out through volcanic eruptions each year.

Loss of oxygen takes place through respiration and mineral oxidation, industrial activities through combustion of wood and fossil fuels, forest fires etc.

Oceans have a small active pool of dissolved gaseous oxygen.

Some oxygen in storage pools as mineral carbonates in ocean floor sediments.

Human activities reduce the amount of oxygen in the following ways:

Burning fossil fuels;

Clearing and draining of land speeds up the oxidation of soil and soil organic matter;

Clearing forests for agriculture;

Rapid urbanisation etc.

Nitrogen makes up 78 percent of the atmosphere by volume, so the atmosphere is a vast storage pool in this cycle.

Nitrogen as N2 in the atmosphere can’t be assimilated directly by plants or animals. But certain microorganisms, including some soil bacteria and blue-green algae, can change N2 into useful forms in a process called nitrogen fixation.

Legumes—such as clover, alfalfa, soybeans, peas, beans, and peanuts—are also able to fix nitrogen, with help from bacteria. They have a symbiotic relationship with bacteria of the genus Rhizobium, which is associated with some 190 species of trees and shrubs.

Other soil bacteria convert nitrogen from usable forms back to N2, in a process called denitrification that returns the nitrogen to the atmosphere and completes the cycle.

Presently the rate of nitrogen fixation is far greater than the rate of denitrification.

Human activity fixes nitrogen during the industrial fixation of nitrogen and by oxidising nitrogen in the combustion of fossil fuels.

Widespread cultivation of legumes.

The global impact of such large quantity of nitrogen reaching lakes , rivers and oceans on the global ecosystem remain uncertain.

Many elements move in sedimentary cycles i.e., from the land to ocean in running water and come back as uplifted terrestrial rock after millions of years. Such elements are present in the atmosphere in a very miniscule amount.

The nutrient elements are also held in storage pools such as sea water(inaccessible to living organisms), sediments on the sea floor, and enormous accumulations of sedimentary rock beneath both lands and oceans. These are finally released into the soil by weathering. Soil particles are lifted into air by winds and brought down by precipitation or gravity. Examples are chlorine and sulphur.

Producers, consumers and decomposers of biosphere and soil also interact in recycling of sediments. The elements used in the biosphere get back to sea via ions dissolved in stream runoff and groundwater flow.

Q.1. What is a biogeochemical cycle? Identify and compare any two types of biogeochemical cycles.

Q.2. What are the flow pathways of macronutrients in sedimentary cycles?

Q.3. What are the main features and flow pathways of oxygen cycle? Discuss the impact of human activities on the oxygen cycle.

Q.4. What are the main features and flow pathways of carbon cycle? Discuss the impact of human activities on the carbon cycle.

Q.5. What are the main features and flow pathways of nitrogen cycle? Discuss the impact of human activities on the nitrogen cycle. What role do bacteria play in nitrogen fixation?

Strahler, A.N. and Strahler, A.H, 2002 : Physical Geography: Science and Systems of the Human Environment, 2nd ed., John Wiley, New York.

Strahler, A., and Merali, Z., 2007: Visualizing Physical Geography, Wiley, New York.

Strahler, A.H., , 2011: Introducing Physical Geography, John Wiley, New York.

Singh, L. , 2010: Environmental Geography, APH Publishing Corporation, New Delhi.

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