What primary biogeochemical process is disrupted by the application of synthetic fertilizers, leading to eutrophication?

The application of synthetic fertilizers primarily disrupts the nitrogen cycle, leading to eutrophication. Synthetic fertilizers often contain high concentrations of nitrogen and phosphorus, which are essential nutrients for plant growth. However, when these fertilizers are applied in excess or improperly, they can run off into nearby water bodies, such as rivers, lakes, and coastal waters. This runoff introduces large amounts of nitrogen and phosphorus into these aquatic ecosystems. Eutrophication is the process where excessive nutrient enrichment, particularly by nitrogen and phosphorus, stimulates excessive plant growth, primarily algae. The rapid growth of algae, known as an algal bloom, blocks sunlight from reaching submerged plants, which then die. As the algae die, they are decomposed by bacteria, which consume large amounts of dissolved oxygen in the water. This depletion of dissolved oxygen creates hypoxic or anoxic conditions, meaning there is very little or no oxygen available. These low-oxygen conditions can kill aquatic organisms, such as fish, shellfish, and other invertebrates, leading to significant ecological damage. The disruption of the nitrogen cycle occurs because the excess nitrogen from fertilizers overwhelms the natural processes that regulate nitrogen levels in aquatic ecosystems, such as denitrification (the conversion of nitrate to nitrogen gas) and assimilation by plants and microorganisms. The overload of nitrogen leads to an imbalance and the cascading effects of eutrophication.