Final answer:
Human activity can alter the nitrogen cycle by burning fossil fuels and using artificial fertilizers, causing problems like acid rain and eutrophication.
Step-by-step explanation:
Human activity can alter the nitrogen cycle in various ways. One way is through the burning of fossil fuels, which releases nitrogen oxide gases into the atmosphere, leading to problems like acid rain. Another way is by using artificial fertilizers in agriculture, which contain nitrogen and phosphorus compounds that are washed into lakes, streams, and rivers by surface runoff. This nutrient runoff causes the overgrowth of algae, depletion of oxygen, and death of aquatic fauna, a process known as eutrophication.