List four points which one should know regarding cycling of materials.

The points one should know regarding the cycling of material are: 

(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 into the nutrient pool.

Nitrogen exists as free nitrogen in the atmosphere. In air N₂ is about 78%. This free nitrogen is fixed into compounds of ammonia and nitrates. Most of the organisms cannot utilize nitrogen as molecular nitrogen.

Fixation of Nitrogen. Fixation of free nitrogen into compounds takes place by following means :

(a) Certain blue green algae and bacteria can fix atmospheric nitrogen.

(b) Nitrogen—fixing bacteria found in the nodules of roots of legumes such as gram, bean, pulses etc., fix atmospheric nitrogen into nitrogen containing compounds.

(c) Lightening also helps in the formation of nitrogen containing compounds.

Nitrogen containing fertilizers produced artificially in factories are the fixed form of nitrogen.

Plants take compounds containing nitrogen from the soil. Form plants nitrogen passes into food web. Decay of dead plants, animals and excreta like urine, faeces, causes return of nitrogen compounds to the soil. Denitrifying bacteria and fire cause liberation of free nitrogen in the atmosphere.

Fig. 14.2. Nitrogen cycle.

Importance of Nitrogen Cycle. Nitrogen is an important constituent of tissues, proteins, enzymes, nucleic acids, amino acids. Atmosphere contains about 78 per cent nitrogen but plants and animals cannot use nitrogen in this form. Plants take nitrogen in the form of nitrates—the usable form. From plants, through food nitrogen travels to animals. If nitrogen in the form of proteins, amino acids, enzymes etc. remains locked up in the bodies of organisms, there will be acute shortage of usable form of nitrogen. Therefore, circulation of nitrogen in nature is very essential.

The imporatnat steps involved in nitrogen cycle :

(i) Nitrogen fixation - and assimilation of free atmospheric nitrogen by Rhizobium or blue-green algae. 

(ii) Nitrogen is converted to its oxides during lightning and in the prsence of high temperature and pressure. These oxides dissolve to give nitric and nitrous acid and fall on land along with rain. 

(ii) Ammonification -  conversion of ammonia to nitrites by Nitrosomonas (nitrifying bacteria).

(iii) Nitrification - conversion of nitrites to nitrates by Nitrobactor.

(iv) Denitrification - break down of nitrates to free nitrogen by Pseudomonas (denitrifying bacteria).

Cycling of materials and flow of energy in biosphere differ mainly in following ways :

(i) Flow of energy is unidirectional, i.e., from non-living to living things.

Unlike energy cycle the material cycle through living and non-living components of the biosphere.

(ii) Energy once trapped get lost as heat during its passage through trophic levels of food chain. Therefore, constant supply of energy from the sun is necessary.

The amount of various elements/minerals cycling through living and non-living components more or less remains constant.

It is very necessary to replenish forests as forest control the following things: 

(i) Rainfall. Trees give out enormous amount of water during transpiration. This water vapour helps in the formation of rain clouds. If trees are killed and not replenished , it will reduce the rainfall in the area 

(ii) Soil erosion. If trees are cut at a large rate the soil becomes uncovered. The top soil which is rich in organic matter will be washed away by water or carried away by wind. It may lead to the formation of a desert.erosion may lead to the formation of desert.

(iii) Carbon dioxide-oxygen balance. Forests have very large number of trees. They give out O₂ and take in CO₂ in day time during photosynthesis. Thus they help in maintaining carbon dioxide-oxygen balance in the atmosphere.

(iv) Natural growth of trees. Forests cannot be grown in a few days or months. Trees take years together to grow their heights.

It is, therefore, necessary to replenish forests at proper time, i.e., we must plan and grow trees and avoid indiscriminate cutting down of forests.