Note on Primary and Secondary Productivity, Biogeochemical Cycle and Nitrogen Cycle

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Method to Study Primary Productivity:

There are various methods to measure the primary productivity. They are:

  • Harvested method: It can be used in situations in which herbivores animal are not important and in which steady state condition is never reached. It is usual in crop fields involving animal species. Since efforts are made to prevent insects and other animal from removing materials and the rate of production starts from zero at the time of plantation and reaches maximum at the time of harvest. Weighing the growth produced by cultivated crops and determining the calorific value gives the primary productivity.
  • Light and Dark Bottle Method (Oxygen Measurement Method): There exists a relationship between oxygen evolved and the food produces during photosynthesis. Dark and light bottles can be used to find productivity. The two sample bottles of same capacity are filled with water that drawn from the same depth and left to the same depth using stopper. The dark bottle is made light proofs only respiration occurs and in light bottles both respiration and photosynthesis takes place. A sample from the same depth is fixed to know the initial oxygen concentration. The suspended bottles are drawn out after 24 hours and change in oxygen concentration are measured by using Winkler’s method or by using electronic sensor.
  • Leaf Area Index Method (LAI): Since leaf is the primary organ in which primary production takes place, its area in relation to ground area is of greater significance. LAI is the total area of leaf per unit area of ground. The rate of production increases with increase in leaf area. LAI up to a certain extent provided other factor especially moisture are not limiting. Generally, beyond four or five LAI value, the increase in production values stops or falls depending upon the type of species and prevailing condition. In vertical leaves a high leaf area index is needed to intercept the same amount of light and thus a higher photosynthetic rate is achieved.
  • Chlorophyll Method: Chlorophyll content has certainly relationship with primary productivity. In an ecosystem like a desert or an oligotrophic lake the quantity of chlorophyll is low per unit area (i.e. m-2 ha-1) and hence productivity is also low. Similarly, multi-storied forests in tropics the quantity of chlorophyll is high and also the productivity.
  • If the assimilation ratio and the available light are known gross production and can be estimated by excretory pigments and then measuring chlorophyll concentration with the spectrophotometer.
  • CO2 measurement method (Enclosure method): A large transparent tent or glass box is placed over a plant community. Air is drawn through the enclosure and the CO2 concentration in incoming and outgoing air is measured by absorption of KOH column or by infrared gas analyzer. It is assumed that any decrease in CO2 in incoming air has been used in photosynthesis. If the enclosure is made light, respiration can be measured. By calculating change in oxygen concentration the productivity can be measured.
  • Aerodynamic method.
  • pH method.
  • Bomb calorimetric.
  • Oxygen diurnal curve method.
  • Extinction of raw materials.
  • Radio-isotope method.

Common methods for the measurement of secondary productivity:

Biomass method: To measure secondary productivity, a biomass method is commonly used. This method is used for invertebrate animals. Invertebrate animals are collected from a known area and dried in an oven at eighty degree celcius to hundred degree celcius for 12 hours. The weight of dried animal is taken and productivity is expressed in terms of biomass per unit area per unit of time.

Bio geo Chemical Cycle:

Biogeochemical cycle is the circulation of essential elements of protoplasm from non-living environment to living organisms. The movement of mineral is accomplished by the operation of different chemical cycle that keeps on passing the minerals back and forth between organisms and their environment. Because the flow involves biological, geological, chemical nature of process these cycles are called biogeochemical cycles.

Nitrogen Cycle in the Environment:

Nitrogen cycle is the most complex of the earth’s biogeochemical cycles. Although nitrogen present in atmosphere is seventy eight percent it can’t be directly utilized by organisms.

The first step in nitrogen cycle involves nitrogen fixation in which the gaseous nitrogen is converted into nitrogenous compounds by physiochemical and biological process. The lightening atmospheric reactions are physiochemical and the bacteria (Symbiotic Rhizobium and free living like Azobacter) fix atmospheric nitrogen biochemically to form ammonia.

N2 + H2 → ammonia.

Ammonia reacting with water is converted into ammonium ions and which are absorbed by plants in the form of nitrites as nutrients. The conversion of ammonia and other nitrogen compounds into nitrates is called nitrification.

Fig: Nitrogen Cycle
Fig: Nitrogen Cycle

During assimilation plants absorb ammonia, ammonium and nitrates which are incorporated in organic molecules like DNA, amino acids and proteins. The animal acquires nitrogen compounds by eating plants (producers) and by plant eating herbivores. Specialized decomposer bacteria and converted the nitrogen rich organic compounds, wastes, dead bodies of organisms into simpler nitrogen containing inorganic compounds such as ammonia and water soluble salts containing ammonium ions (NH4+). This is ammonification.

In a process called denitrification, the ammonium ion NH4 and mainly NH3 is converted into nitrogen oxide (NO), Nitrous oxide (N20) and ultimately N2. The denitrification product gets way into the atmosphere and they can be the same cycle.


E.p., Odum. Fundamentals of Ecology. USA: W.B Saunters Company, n.d.

Jr., Miller G.T. Living in the Environment. Balmot, Californea, USA: Wadsworth Publishing Company, 2003.

  • The nitrogen cycle is the most complex of the earth’s biogeochemical cycles. Although nitrogen present in the atmosphere is seventy-eight percent, it can’t be directly utilized by organisms.

                 N2 + H2 → ammonia.

  • A biogeochemical cycle is the circulation of essential elements of protoplasm from a non-living environment to living organisms.
  •  To measure secondary productivity, a biomass method is commonly used. 




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