There are three main ways that nitrogen is cycled in : Biological Chemical/Physical and Human.
Biological
– As nitrogen is taken from the atmosphere it goes to bacteria in the soil. These bacteria can make it into ammonia also known as nitrogen fixation and that allows for other bacteria to make it into another form of nitrogen called nitrate. This now-called nitrate can be used by plants.
The plants provide food for animals and the animals then excrete the extra protein decomposed by decomposers. Excess nitrogen is joined back together known as denitrification. Now the bacteria can start the process all over again. This video might help you:
Human
– When nitrogen is taken from the atmosphere by humans they usually make it into fertilizer. They use this fertilizer to allow plants to grow quicker better and more healthy. When there is too much fertilizer it might harm an aquatic ecosystem. When there is too much of a nutrient known as a limiting nutrient algal bloom occurs. This is when algae float to the top of the water and prevent sunlight from coming through causing organisms in the water to lose much of their food source. There is not really a way that humans give nitrogen back into the atmosphere naturally.
Physical/Chemical
– Lightning can actually strike between two nitrogen molecules and cause them to be fixated and usable by plants/bacteria in the soil.
Ocean
– The same process that occurs biologically occurs in the ocean. Bacteria in the ocean take the nitrogen make it into ammonium then into nitrate. Now it is used by primary producers eaten by consumers and excreted out. The decomposers can now decompose the waste. The bacteria perform denitrification and release nitrogen into the atmosphere.
Sources: Biology class biology textbook and this video that could help you further: Nitrogen Cycle