Question

A human impact on the nitrogen cycle, such as fertilizer

Answer 1

When farmers harvest crops, a lot of the plant material is also removed. Little is left to decay and return nitrogen to the soil. If the soil stays this way, it will start producing fewer crops. Fertilizer contains the nitrogen compounds that are essential for plant growth. It can be used to replace the nitrogen in the soil to ensure the growth of healthy crops.

Answer 2

When a human dies, his corpses gets decomposed by decomposing bacteria. What's produced by the decomposing bacteria is the "fertilizer" for plants.