What can happen if over-application of fertilizers occurs?

Over-application of fertilizers can lead to water quality degradation, which is the correct answer. When excessive fertilizers are used, particularly those high in nitrogen and phosphorus, they can leach into nearby waterways through rainfall or irrigation runoff. This runoff can cause nutrient pollution, leading to harmful algal blooms that degrade water quality. These blooms deplete oxygen in the water, harming aquatic life and disrupting ecosystems. This phenomenon is known as eutrophication and can result in significant ecological and economic impacts.

Other options, while related to soil and crop health, do not accurately describe the direct consequence of over-fertilization. Improved crop yield may seem like a potential outcome, but over-fertilization doesn't always lead to better yields and can actually harm plants by causing nutrient imbalances. Soil nutrient depletion is more likely to occur with nutrient mismanagement over time, rather than from over-application alone. Enhanced microbial activity can sometimes occur with proper fertilization but excessive fertilizers can actually inhibit microbial processes and biodiversity in the soil. Thus, the primary and most direct concern with over-application is the degradation of water quality.