Great question! You are right on track in your thinking - decomposition does indeed have something to do with nitrogen and ecosystems!

If you have been studying ecosystems, you have probably learned about the water cycle, the carbon cycle, the nitrogen cycle and the phosphorous cycle (the last one being representative of the "mineral" cycles).

Nitrogen is a key component of many compounds in living organisms - especially in the proteins of plant and animal bodies. But most of the nitrogen is in the atmosphere (about 78% of the atmosphere is nitrogen) - how is it converted into a form that living organisms can use? It turns out that most of it is available because of some specific kinds of bacteria - which can "fix" the atmospheric nitrogen. The best "fixers" are symbiotic bacteria that live with the roots of legumes (plants like peas and beans). So, in a healthy ecosystem over many years, there is plenty of nitrogen available for plants and animals to use.

But more to your question - what about decomposition? In some ways, you could consider decomposition as a type of nitrogen recycling. When living things die, they decompose. But what does that mean? The proteins in the plant or animal that has died are made of amino acids - and there is a lot of nitrogen in those proteins. Bacteria and fungi have the ability to break down those proteins (that is the "decomposition" action) and use the amino acids to make their own, new proteins. In this process, they release any excess nitrogen in the form of ammonium (NH⁴⁺) . Other bacteria can convert the ammonium to soil nitrates and nitrites which can then be absorbed by plants. The plants get eaten by animals, which use the nitrites and nitrates to make their own proteins.... the animal dies and the cycle starts over.

However, as in most recycling operations, the savings is not 100%. A certain amount of nitrogen is steadily lost back to the atmosphere in a process called denitrification (again, specific kinds of bacteria convert the nitrates and nitrites to nitrogen gas or nitrous oxide). But the nitrogen fixers can recapture it and start it all over again by converting it to ammonia and nitrates.

Keep asking great questions! It's really great that you are thinking about how certain processes are connected - this is called a "systems approach" to a biological question and it allows you to think about how even small changes in one part of the system can have major effects on another.