TOPIC ECOSYSTEM

Part 5: Carbon & Nitrogen Cycles

The nutrient cycles are important because the nutrients found within the soil or

the air is always dynamic and changing in terms of their concentration.

What is utilised must be replenished; otherwise the ecosystem will not be

maintained and balanced. Cycles will always recycle the nutrients to be reused.

Microorganisms play a key role in recycling.

Some microorganisms build up organic molecules and some

break down complex organic molecules in the bodies of dead

animals and plants into simpler substances, which they release

into the environment.

Hence, the next topic microbes will be indirectly related to that

of the ecosystem. In fact, all topics are inter-linked!

In any cycle, there are always two important points to remember – (i) the process and (ii) the storage.

Always relate and think logically using the processes you have used so far; most will be common sense.

CARBON CYCLE

Carbon is a major component found in all organisms. They form the basic skeleton structure of

different major molecules – carbohydrates, lipids, proteins, nucleic acids and vitamins.

Atmospheric carbon dioxide, CO2, consist in small traces approximately 1%.

Refer to Figure 1 to identify some of the important processes of how carbon is generated and recycled.

C1 Photosynthesis

» Plants (or autotrophs) 'fix' carbon atoms from carbon dioxide to form glucose (carbohydrates) and

other organic compounds such as proteins, nucleic acids and lipids.

» Such organic compounds are important for cellular processes and structures.

C2 Respiration

» Living organisms breathe out inorganic carbon dioxide from the breaking down of organic compounds

and releases heat energy.

C3 Feeding & assimilation

» Transfers carbon atoms already in organic compounds along food chains.

C4 Fossilisation & Decay

» Some living things do not decay fully when they die due to the soil being acidic; hence decaying is

delayed.

» As a result, fossil fuels (coal, oil, natural gas and peat) are formed.

C5 Combustion

» Burning of fossil fuels releases carbon dioxide into the atmosphere when fossil fuels are burned.

Figure 1: Plants play vital roles in both the

carbon cycle (directly) and nitrogen cycle

(indirectly).

Figure 2: The carbon cycles in general. The blue colour coded boxes refer to the processes while the grey boxes are the carbon stores.

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NITROGEN CYCLE

Nitrogen is a key component in organisms. It is one of basic elements found in protein molecules,

amino acids, DNA, RNA and ATP (adenosine triphosphate).

Proteins are the cellular machinery substances that carry out specific functions.

» Enzymes – regulate all cell chemical reactions;

» Transport proteins – regulate and facilitate the movement of substances across the cell membrane;

» Communication (hormones);

» Structural molecules.

Respiration and photosynthesis are not involved because the processes are involved in fixing the

carbon not the nitrogen.

There are two sides of this story cycle – (i) processes and (ii) the bacteria.

PROCESSES – Figure 4

P1 Lightening occurs during a thunder storm.

» The lightening provides enough energy for oxygen (O2) and nitrogen (N2) to combine in the atmosphere.

» The rain which falls will contain higher than normal levels of nitrate (NO3-) ions.

P2 Feeding, excretion & assimilation

» Nitrogen passes from one organism to the next as food, typically as protein.

» Animals are able to digest protein to amino acids and reorganise these amino acids into useful proteins

they require.

» Excretions in the form of urine (which contains urea).

P3 Decomposition

» Decomposers produce ammonia (NH3) from the nitrogen in decaying animals and plants like proteins,

DNA and vitamins.

» N2 → NH3

P4 Nitrate, NO3- ions

» Very soluble in soil water as they are not held to the soil clay particles very well. Hence:

Very easily be absorbed into a plant as the plant absorbs water (which the nitrate ions dissolved

into) from the soil.

Easily leaches deep into the soil and into the underground water table leading to excessive levels

of nitrates in lakes and rivers. This can lead to eutrophication.

P5 Other sources

» Gases produced when fossil fuels are burnt (nitrous oxide NO2);

» Gases released in volcanic activity (nitrous oxide NO2);

» Agricultural fertilisers – urea, ammonium nitrate (NH4NO3), potassium nitrate (KNO3).

BACTERIA – Figure 3

The atmosphere is approximately 79% nitrogen (N2); but this form cannot be used by plants as N2 is

very stable.

Plants need their nitrogen as either nitrate ions (NO3-) or as ammonium ions (NH4

+). Hence, the

bacteria found in the soil acts as an agent to convert N2 to either NO3- or NH4

+.

Some of these bacteria are able to form mutualism associations with specific species of plants.

The interactions between the bacteria and the nitrogen component affect amount of nitrate (NO3-) or

ammonium (NH4+) ions in soil available to plants.

B1 Nitrogen fixing bacteria (Rhizobium bacteria) (Nitrification)

» Found in the soil and within some plant root systems; used with agriculture plants.

» Forms a mutual symbiotic relationship with legume plants. Eg.: peas, beans, soya beans, lentils, faba

beans, lucerne, medic, colver, acacia (wattle).

» Convert nitrogen gas to ammonium ions (N2 → NH4+) or ammonia (N2 → NH3)

» Nitrogen fixation increases the amount of nitrate available to plants.

B2 Nitrifying bacteria (Nitrification)

» Capable of converting or oxidises ammonia to nitrate ions (NH4+ → NO3

-).

» Nitrate ions (NO3-) are then available for assimilation by plants.

B3 Denitrifying bacteria

» Act on proteins in dead organic material and release nitrogen back into the air.

» Reduces the amount of nitrate in soil.

Figure 3: The nitrogen cycle. This diagram is suitable to describe the bacteria (B1, B2, B3) involved in the

fixation of nitrogen.

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TTTHHHEEE NNNIIITTTRRROOOGGGEEENNN CCCYYYCCCLLLEEE

Figure 4: The nitrogen cycle. Click on the picture for an interactive. This figure is suitable to describe the processes (P1, P2, P3, P4, P5) involved in the fixation of nitrogen.

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