Nutrient cycle in an ecosystem (carbon cycle and nitrogen cycle)
 Carbon cycle – constant removal and addition of carbon compounds from
and into the soil.
 How? Through photosynthesis, feeding, respiration, decomposition and
combustion.
Diagram showing the carbon cycle
Importance of the carbon cycle
1. ensures that there’s a continuous supply of inorganic carbon dioxide for
the plants to carry out photosynthesis.
2. enables a linear flow of energy through the ecosystem. The carbon
compounds are the vehicles through which the trapped solar energy
passes from one organism to another in the food chains of an ecosystem.
Nitrogen cycle
 the process in which nitrogen is constantly removed from the soil and
eventually returned to it in the form of nitrates
Loss of nitrogen from the soil
 Absorption by the roots of green plants – nitrates in the soil are absorbed
by the root hair cells
 Loss in drainage water – can lead to eutrophication (algae bloom)
 Denitrification – denitrifying bacteria in the soil convert the nitrate into
atmospheric nitrogen
Return of Nitrogen to the soil
 Decay
 Nitrification – ammonium compounds may be formed from nitrogenous
waste products excreted by animals. The ammonium compounds in the soil
are first changed to nitrites by bacteria Nitrosomonas and then to
nitrates by bacteria Nitrobacter . The process by which ammonium
compounds are changed to nitrates is called nitrification.
 Nitrogen – fixing – it refers to the conversion of atmospheric nitrogen to
nitrogenous compounds. This can occur in 3 ways:
- By free nitrogen fixing bacteria Azotobacter
- By nitrogen fixing bacteria, e.g. Rhizobium in the root nodules of
leguminous plants.
- By lightning and thunderstorm