Question
I am just curious about some of the things you have said in your other answers. Is there both a carbon cycle and a nitrogen cycle and what is the difference? Are they both important?  

Answer
A chemistry question!

The Carbon Cycle and the Nitrogen Cycle are two different things.  Both are part of the "Biogeochemical Cycle" -- the recycling of molecules through an ecosystem.  There are other cycles, too.  The Hydrogen Cycle.  Water Cycle.  An Oxygen Cycle.  A Motor Cycle.

Just kidding.

Do I have your attention?

Good!

The CARBON CYCLE begins with the element Carbon.  All living things are made of Carbon.

Plants turn Carbon into energy to live.  But instead of using pure Carbon (C), they take it from Carbon Dioxide (CO2) when they breathe.  They put this together with water (H2O) and turn the results into organic matter, using the energy from sunlight.

The result: Carbohydrates, made of Carbon, Hydrogen, and Oxygen.

The Carbon is still there when they die, and as they decay, it returns to the earth and the atmosphere.  Animals eat the plants, and ingest the fats, proteins and sugars from the plants into their bodies.

Then there is the complicated NITROGEN CYCLE.

The basics: The earth's atmosphere is 78% Nitrogen (N2).

In addition, there are Nitrogen atoms in different forms everywhere on the ground - in Protein, in Amino Acids, in DNA, in Chlorophyll.

Nitrogen is necessary for many biological functions.  We all think we need Oxygen to live, because we need it to breathe.  But only 21% of the air is Oxygen.  We also depend on Nitrogen.

My first encounter with the Nitrogen Cycle was a saltwater aquarium.  You can't run a marine tank with beautifully colored saltwater fish without understanding the Nitrogen Cycle.  A successful fish tank depends on how good you are at setting up a Nitrogen Cycle inside the fishtank.
The plants are interesting.  The fish are awesome.  So I was highly motivated.  It's a great system because you don't have to clean it all the time.  But you need an intimate understanding of the Nitrogen Cycle to do it, and if you mess it up, the fish start dieing.  It's much more immediate and obvious than gardening in the soil.

So I had to learn the Nitrogen Cycle for my fishtank.  And it works this way:  Fish in the tank eat Food and use up Oxygen (O2) in the water.  They create waste -- Ammonia (NH3) and Carbon Dioxide (CO2) -- when they do that.   When the NH3 Ammonia hits the saltwater, there is an instant chemical reaction.  They mix, and they form something called Ammonium (NH4).

Assuming that things in the tank are running smoothly, there is a large population of friendly bacteria that lives off the Ammonium.  Another population of bacteria use the NO2 and turn it into Nitrates (NO3).  The green plants in the tank need the Nitrates and the Carbon Dioxide for Photosynthesis.  To do that, they turn those into Oxygen -- and the cycle starts all over again.

If everybody had to run a salt water aquarium at home, they would realize that putting fake fertilizer in the soil is damaging.

Nitrogen is needed for Photosynthesis.  It is there, naturally, in the form of Ammonium and Nitrates.

So yes, both are important.  But not as important as a Motor Cycle.