Phosphorus Cycle

Ecosystem of Class 12

Phosphorus is one of the most important ingredients of the protoplasmic materials (nucleotides, nucleic acids, ATP, GTP, NAD, NADP, FAD, Phospholipids, co-enzymes etc.) of autotrophic as well as heterotrophic organisms of the ecosystem.

Phosphorus is also added to soil from external source as chemical manure in crop fields (man’s engineered ecosystem).Weathering of rocks and decomposition of organic wastes provide phosphorus to soil.

Plants take phosphorus as orthophosphates from soil solution through their roots and incorporate it in their biomass.

Secondary producers obtain their phosphorus requirements from primary producers directly or indirectly in the form of organic phosphorus. After the death of organisms of various trophic levels, their dead organic remains are subjected to microbial decomposition.

Phosphorus cycle

It involves :

  • Mineralization Mineralization of organic phosphorus contained in the dead organic remains of organisms into insoluble inorganic phosphorus by enzyme phosphatase of microorganisms.
  • Solubilization Solubilization or conversion of insoluble inorganic phosphorus to soluble inorganic phosphorus. Microorganisms produce varieties of organic and inorganic acids which convert insoluble phosphorus to soluble phosphorus in soil usually near the root surface and then roots absorb it as its one of the important nutrients. The microorganisms, which grow in closed association of roots for these transformation form the rhizosphere microflora. They prefer rhizosphere region in attraction of the root exudate which is secreted by root surface in the form of sugars, amino acids, proteins, etc.

Phosphorus Cycle

Fig. : A phosphorous cycle in nature

Aspergillus, Penicillium, Fusarium, many Bacilli and Pseudomonas are commonly found associated with rhizosphere flora. It is a type of protocooperation between the microorganisms and roots of higher plants, where both the partners are obliged or benefited but they do not establish physical or physiological contact.

Phosphorus Cycle


Different parts of the world are inhabited by different species of plants and animals. The major terrestrial biotic communities of the world, each comprising a characteristic array of plant and animal life are called biomes. They are the largest ecological units. The biomes contain many ecosystems, but are defined by climate.

They are of two types :
  • terrestrial biomes
  • aquatic biomes.

Terrestrial biomes contain Forest Biomes, Grassland Biomes and Desert Biomes.


The characteristic vegetation of forest biomes is dominated by densely growing trees having a closed or nearly so, canopy cover. Depending upon the climate type, a wide variety of forest types occur in the world.

Major forest biomes in India are :

  • Tropical rain forest biome
  • Tropical deciduous forest biome
  • Temperate broad-leaf forest biome
  • Temperate needle-leaf or coniferous forest biome

The tropical rain (evergreen) forest biome occupies low-lying areas near the equator with heavy rainfall. It is found in central America, the Amazon and Orinoco river basins of South America, the Congo river basin of Africa, Malagasy Republic and South-East Asia. In India, the tropical rain forests occur along the West coast and in and around Assam. This biome covers about 1/12th of the earth’s surface but has over half of the world’s flora and fauna. The tropical rain forests are very thick and nearly impenetrable, hence called jungles.

The climate is warm and moist, and there are almost no seasonal variations. There are, thus, excellent conditions for plant growth throughout the year.

The rainfall is high (about 200 cm or more annually) and uniform, there being no dry periods, hence the name rain forests. The sunlight is strong, but very little light reaches the ground through the dense foliage. Temperature is high and fairly constant. The soil is very rich in minerals and humus as the high temperature and moisture are favourable for the decomposition of fallen leaves and dead organic matter by the decomposers.

  • Flora : Dense evergreen trees with broad leaves form the dominant vegetation. The trees are of various heights, and distinct stratification (storeys, layers) can be recognized. The tallest trees (50-100 m. high) form a nearly continuous canopy at the top.The taller, tall and medium trees form lower canopies in the order named. Many types of epiphytes and woody climbers (lianas) grow on the trees to reach light for food making. The epiphytes are also called air plants. They have aerial roots to absorb water, and trap falling leaves and other organic debris as a source of nutrients. Epiphytes includes orchids, bromeliads, and ferns. There is sparse herbaceous vegetation on the floor, and actual bare spots occur in certain areas. In India, the tropical rain forests include rubber trees, figs, timber trees, such as rosewood, ebony and mahogany; spice plants, such as cloves, cinnamons and nutmegs; and palms, plantains, bamboos and cycads.
  • Fauna : Each story of the tropical rain forests is inhabited by different animals. More animals live on the richer higher levels than in the lower ones. Brilliantly coloured birds–parakeets, toucans, birds of paradise, etc., form the dominant vertebrates of the upper storeys. Other characteristic animals are monkeys, lemurs, bats, sloths, anteaters, arboreal reptiles and amphibians, insects, centipedes, scorpions, snails and leeches. Leopards, jaguars, deer, antelope, forest goat, elephants and tapirs also occur in these forests. A variety of fish inhabit rivers. About 200 species of trees, 71 to 80% of the world’s insect species, and 80 to 85% of all bird species are found in the tropical rain forest biome alone. This rich life diversity is associated with animal-plant interactions for pollination and dispersal. As most animals live on the food-rich higher levels, we know so little about them. It is difficult to reach the canopy.

The tropical deciduous forest biome is found in the West Indies, the Eastern region of Brazil, the central plateau of India, Indo-China and the North-Eastern projection of Australia. The climate is warm, and there is a marked alternation of wet and dry seasons. The annual rainfall varies from 100 to 200 cm. The sunlight is strong, and enough of it reaches the ground through the foliage to support vegetation. The soil is rich in minerals and humus as the climate is favourable for the reducers to decompose the fallen leaves. Life is rich in the tropical deciduous forest biome also.

  • Flora : The tropical deciduous forests consists of trees with broad leaves. The trees shed their leaves during the dryseason, hence the name tropical deciduous. These trees are less dense and form fewer stories (levels) than those of the tropical evergreen forests. A very dense undergrowth of shrubs and herbs makes a jungle as the sunlight is available at the bottom. The epiphytes and climbers are fewer. The trees in tropical deciduous forests of India include sal, teak, mahua, semul, jamun, amla, and palas.
  • Fauna : Abundant vegetation supports rich animal community. The common animals include deer, elephant, pigs, bear, tiger, lion, variety of birds, reptiles (lizards, snakes, tortoises), insects and worms.

Temperate broad-leaf forests mainly occur between 1500 and 2400 m altitude in the western Himalayas. Several species of oak (Quercus) predominate in the temperature broad-leaf forests. These include banj oak (Quercus leucotrichophora), and rianj oak (Q. lanuginose). All oak species in the Himalayan region are evergreen. The evergreen oaks in Himalaya show peak leaf fall during summer, but never become leafless. These four strata forests have 25-30 m height. The tree canopy is dense, herbaceous layer is least developed, and grasses are generally lacking. The oak forests are often rich in epiphytic flora.


The areas have warm summer and moderately cool winter. Rainfall is 100–250 cm. Dominant trees are Oak, Elm, Birch, Maple Ash, Chestnut, Hickory, Beech, Poplar and Magnolia. Conifers occur at places. Shrubs, herbs, ferns, mosses, lichens, grasses, vines and epiphytes are abundant. Most trees shed their leaves during autumn or fall making the soil brown. A coniferous-broad leaved forest occurs in western Himalayas at an altitude of 1500-2400 m. Tree height is 25-30 m. Upto 4 strata occur in forests. Tree canopy is dense. Herbaceous layer is poorly developed. Grasses are generally absent.Indian temperate broad leaves forests are dominated by Oak like Quercus semecarpifolia (Brown Oak of Himalayas, Khrsu Oak), Q. floribunda (Tilinaj Oak), Q. lanuginose (Rianj Oak) and Q. leucotrichophora (Banj Oak). Unlike Oaks of other places, Indian Oaks are almost evergreen as they never become leafless. Peak leaf fall occur during summer. Fauna of latitudinal temperate broad leaf forests consists of Deer, Fox, Beaver, Wild Cat, Racoon, Opossum, Squirrel, Rabbit, Hare, Snakes, Lizards, Salamanders, Thrushes, Owls, Sparrows and song birds.


Grassland ecosystems have treeless herbaceous plant cover dominated by wide variety of grass species (family, Poaceae). These occupy about 19% of the total land mass. These biomes are characterized by the presence of grasses of varying sizes.

Tropical grasslands : These grasslands have scattered trees and savannah. This is meadow like plain biome mainly found in South America, Africa and Australia. These are also found in India, Burma and adjoining areas.

  • The annual rainfall is 100-150cm and is seasonal. The climate is warm and fires are common. The name savannah is derived from occurrence of certain dominant trees like Acacia savannah (Babul), Phoenix savannah, and Eucalyptus savannah. Trees are less in number. Grass like species of Panicum, Pennisetum, Andropogon are dominant.
  • The biome is characterized by presence of hoofed herbivorous animals like antelope, zebra, giraffe and elephant.

Other common animals are mice, rabbit, fox, wolf, tiger and lion. Kangaroo occurs in savannah of Australia. This biome is ideal for hunting.

Temperate grassland : The temperate grasslands are present in North America (Canada and U.S.A.), South America, Eastern Europe, Central Asia, South Africa and Australia.

  • These are of different types depending upon the constituent flora in different countries, such as prairiesc(North America), pampas (South America), steppes (Europe and Asia–Russia), veldts (Africa), tussocks (Australia).
  • The annual rainfall varies between 25-75 cm. There are distinct cold winter and hot summer. The winter is characterized by stormy winds. Dry periods extend over longer span.
  • The dominant vegetation of the biome includes grasses of varying heights, legumes and composits. The biome is characterized by the presence of grazing and burrowing animals. Bison, elk, zebra, bighorn, prong horned antelope; deer and sheep are prominent grazing animals.

It is a warm climate plain with coarse grasses, scattered shrubs and trees, seasonal rain (wet and dry periods) and frequent fire. Height of woody species is 1-8 m. Savanna occurs in North Australia, India, Central and Southern Africa including east and central South Africa.

They are natural as well as anthropogenic. Indian savannas are largely anthropogenic being derived from tropical forests and maintained by grazing as well as fire. Availability of soil moisture determines composition and productivity. Root system of grasses are present in upper 30 cm of soil while that of woody species penetrate to deeper horizons.

Savannah is named after dominant tree like Acacia, Phoenix, Eucalyptus. Common trees and shrubs of Indian savannahs are Acacia, Butea, Prosopis, Zizyphus and Capparis. Common grasses are Dichanthium, Sehima, Cenchrus, Lasiurus, Imperiata and Saccharum.

Many of them perform C4 photosynthesis that is helpful in maintaining high productivity even under conditions of low soil moisture. Hoofed herbivores are quite common. Animals include Antelope, Zebra, Giraffe, Goat, Gazelle, Rhino, Elephant, Fox, Wolf, Lion, Tiger, Kangaroo (in Australia).


Desert biome experiences prolonged moisture scarcity. The desert biome covers about one-fifth of the earth’s surface. Deserts are generally located in rain shadow areas, around latitudes 20 to 30° north and south, where dry air from the equator falls from the upper atmosphere, warming as it is compressed near the earth.

More deserts occur in the northern hemisphere than in the southern hemisphere. The deserts of the northern hemisphere are the Great Western Desert (Death Valley) of the United States, the Sahara Desert of North Africa, and the Tibet, Gobi, Arabian and Thar Deserts of Asia. The deserts of the southern hemisphere occur in coastal areas of Chile and Peru in South America and in central-western Australia. Deserts have been variously classified as true deserts, having less than 120 mm annual rainfall, or extreme deserts showing less than 70 mm yr–1 rainfall.

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