Question

How does the use of fertilizer affect the nitrogen cycle?

Answer 1

Fertilizer puts nitrogen into the soil, so using fertilizer puts more nitrogen into the carbon cycle. 

When you use fertilizer, its because a certain plant you are planting uses all the nitrogen in the soil, so it won't grow next time. Too much fertilizer, though, would burn up your plant (not litterally). Some plants put their own nitrogen into the soil. I can't think of one off the top of my head, but I believe potatoes do. Anyways, this is where crop rotation came into play. 

Things like corn take a lot of nitrogen, so the next time you had to plant whatever plant potatoes, which again may be wrong, in order to get nitrogen for the corn to grow next season. Today we have fertilizer, so we can give them nitrogen while they grow, instead of putting in other plants, taking more time. 
So really, it doesn't do much, because your plant takes all the nitrogen that the Fertilizer adds.

Answer 2

Use of fertilizer adds extra nitrogen into the soil and end up disturbing the natural nitrogen fixation cycle.

Explanation:

• We human may not be skilled enough to fix the nitrogen manually but we can surely do it industrially.
• The fertilisers used by farmers in their crop fields contain heavy proportion of nitrogen.
• Excess amount of nitrogen (fixed naturally by bacteria and added manually through fertilisers) can increase the amount of biologically available nitrogen in ecosystem.
• This lead to severe alteration in the nitrogen cycle.
• Excess amount of nitrogen can penetrate into the soil and can destroy our water resources. Human activities have already disrupted this natural process.