One should know the following points regarding cycling of an element:
(i) The source of the element.
(ii) The form in which it is available.
(iii) The form in which it is converted during cycling.
(iv) The form in which it is released to the nutrient pool.

The following are some methods which can prevent or reduce soil erosion:
(i) Growing vegetation on the ground reduces soil erosion by wind and water.
(ii) Avoid overgrazing.
(iii) Afforestation, the roots of trees check the flow of running water. Trees also check speed of strong winds. Thus, prevent erosion of soil.

Yes, all organisms require water to stay alive because:
(i) all cellular processes take place in a water medium in the body.
(ii) all the substances are transported from one part of the to the other by water in dissolved or suspended form.

(i) Global warming 
(ii) Environmental pollution
(iii) Depletion of ozone layer.

Harmful effects of air pollution
(i) Respiratory problems
(ii) Acid rain or Global warming.

Write important points.
if decomposers are eliminated or they do not perform their work; following things may happen:
(i) Dead bodies of animals and plants and other dead organic matter will remain undecomposed.
(ii) After sometimes, the dead bodies of organisms will occupy the whole space available on this earth.
(iii) There will be no recycling of matter between biotic an abiotic components of an echo synthesis because all the matters will remain locked up in the dead bodies.
Thus the existence of life on the earth will become impossible.

It is very necessary to replenish forests for the following reasons:
(i) Rainfall: Trees give out enormous amount of water during transpiration. This water vapour helps in the formation of rain clouds. If trees are destroyed and not replenished the rainfall in the area will reduce. This may lead to less number of trees to grow.
(ii) Soil erosion: If trees are cut at a large rate the soil becomes barren. The top soil which is rich in organic matter will be washed away by water or carried away by wind. Thus reduction in rainfall and soil erosion may lead to the formation of desert.
(iii) Carbon dioxide-oxygen balance: Forests have very large number of trees. They give out \({ O }_{ 2 }\) and take in \({ CO }_{ 2 }\) in day time by the process of photosynthesis. Thus they help in maintaining carbon dioxide-oxygen balance in the atmosphere.

A. Affect of addition of undesirable substances
(i) This may cause poisoning of water such as addition of mercury salt, lead, etc. cause the death of aquatic life such fishes.
(ii) Addition of estables, sewage, fertiliser, etc. cause growth of enormous algae and microbes that cause oxygen deficiency and death of aquatic life.
(iii) The polluted water may cause serious diseases in human beings, such as diahorrea, minamata (due to mercury salts) and many other diseases.
B. Change in temperature
(i) Affects the balance between various organism and may affect their breeding.
(ii) The eggs and larvae of various animals susceptible to temperature changes. For example, trout eggs fail to hatch at higher temperature.

The term water pollution covers the following effects:
(i) The addition of undesirable substances to water bodies: For example, addition of industrial wastes containing poisonous salts like mercury salts, pesticides, insecticides, etc. Addition of disease causing organisms, like cholera causing bacteria through sewage. 
(ii) The removal of desirable substances from water bodies: For example, depletion of dissolved oxygen and nutrients. This adversely affects aquatic animals and plants.
(iii) A change in temperature of water in water bodies: The aquatic animals are adapted to live under certain range of temperature. A sudden change of temperature may affect breeding of aquatic animals, their eggs and larvae etc. For example, pouring of water at very high temperature from nuclear reactors sometimes may cause death of fishes and aquatic animals.

There are two types of sources of water pollution. These are:
(i) The sources which are located to close to water source and discharge pollutants directly into the particular water source. These may be factories, power plants, underground coal mines and oil wells situated near to water sources. It is possible to treat the pollutants before their release into the water body.
(ii) Some sources are scattered and do not have specific situation for discharging pollutants into a particular water body. These are run-off water from agricultural fields, gardens, lawns, roads, logging sites, construction sites etc. It is very difficult to monitor, regulate and treat non-point sources.

The messages that the children sent out to the public on EARTH DAYwere:

• How would you feel if somebody drew a nail into you?
• Even trees feel the pain
• Do not nail advertisements on trees
• Do not nail advertisements on trees
• Do not throw plastic bags in parks,gardens,etc They spoil the advertisement Always collect them and throw them in dustbine
• Join the stufdents of Delhi in saving the greens.
  (b) similar activities could be carried out in any town and city and one needn't wait for the Earth day or environment day top act that it was only through actions that we could bring about a change in peoples attitudes
  (c) Social responsibility
  (2) Keep the environment clean.

It is very necessary to replenish forests for the following reasons:
(i) Rainfall: Trees give out enormous amount of water during transpiration. This water vapour helps in the formation of rain clouds. If trees are destroyed and not replenished the rainfall in the area will reduce. This may lead to less number of trees to grow.
(ii) Soil erosion: If trees are cut at a large rate the soil becomes barren. The top soil which is rich in organic matter will be washed away by water or carried away by wind. Thus reduction in rainfall and soil erosion may lead to the formation of desert.
(iii) Carbon dioxide-oxygen balance: Forests have very large number of trees. They give out \({ O }_{ 2 }\) and take in \({ CO }_{ 2 }\) in day time by the process of photosynthesis. Thus they help in maintaining carbon dioxide-oxygen balance in the atmosphere.