Excessive addition of fertilizers to the soil causes the fertilizer to accumulate because it cannot be fully used by the plants. When this happens, rainfall can wash this fertilizer into bodies of water such as lakes.
The fertilizer apports a lot of nutrients (including nitrogen) to the water bodies stimulating the growth of algae. Then the algae consume the oxygen of the lake and also prevent the sunlight to pass impairing the survival of other organisms and plants, even, the decomposition of the dead algae can stimulate the growth of pathogenic microorganisms, affecting the health of the animals in the lake. This is the process called eutrophication.
Because nitrogen is the main nutrient in fertilizers that cause eutrophication, the cycle that is disrupted is the nitrogen cycle.