2. how is nitrogen from the atmosphere, the abiotic part of the ecosystem, changed into a form the biotic part of the ecosystem can use?

Nitrogen is essential to the engird cycle of plants and bacteria. without it they would die. The nitrogen in the air is not in it's biological form, its N2 (abiotic). So the diatomic nitrogen goes through the nitrogen cycle and converted into NH3 or NH4+ its biological form (biotic)

There is nitrogen in the atmosphere when organsims die they go to the plants and ocean because their bodies decay. But yes what you have down already is correct.

What happens to the influenza virus every 20-40 years and why?