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Nutrient Cycles

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At the heart of both the carbon and nitrogen cycles are microorganisms such as bacteria. These key nutrients pass from plants to animals through ingestion and digestion (eating!) in a very straightforward way, but what happens after those organisms die is a far more interesting tale.


Plants take up atmospheric CO2 to photosynthesise: the carbon in CO2 is transferred to the carbon in glucose.

This carbon in glucose is either respired by the plant itself and released as CO2 back into the atmosphere, or is passed on to animals eating the plants. Here, it either gets released back as CO2 in respiration or becomes manure.

But eventually, all the carbon not quickly released back into the atmosphere as CO2 will end up in the ground, for microorganisms to decompose.

Saprobiotic* bacteria can assimilate organic waste (all of which is made of carbon) and use it for their own respiratory needs. This in turn produces CO2 or methane gas, depending on species. Both gases get released into the atmosphere from where plants take CO2 up again for photosynthesis, restarting the cycle.

*Externally digesting… kinda like someone chewing your food for you.


Nitrogen is an important element in organisms, taking part in one of their most important building blocks – amino acids. That’s not to mention DNA itself…

In the nitrogen cycle there are two stages of N presence: the atmosphere and the ground. Whenever N is in the atmosphere it’s in the form of nitrogen gas, N2 which of course is what most of the air is made of. In the ground, N is found in ammonia (NH3), nitrite (NO2-) and nitrate (NO3-).

Find it hard to distinguish the formulae for nitrite and nitrate? Needn’t be! A is large (3-) and i is little (2-), former’s nitrate, latter nitrite.

Both nitrogen-fixing bacteria and lightning can take the nitrogen gas in the air and fix it into the soil, where plants take it up (nitrate assimilation) and pass it on through the trophic levels to other organisms.

Upon their death, saprobiotic bacteria decompose the remains and produce ammonia which then undergoes nitrification to NO2- and NO3- by nitrifying bacteria.

Denitrifying bacteria turn the N in nitrates into nitrogen gas again, so the cycle may begin once more!

Summary: nitrogen fixation –> assimilation –> ammonification –> nitrification –> denitrification





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