Final answer:
Phosphorus normally enters ecosystems through rock weathering, which releases phosphate into the soil, water, and air. This phosphate becomes available to terrestrial and marine food webs.
Step-by-step explanation:
In nature, phosphorus enters ecosystems through the process of rock weathering. Weathering of rocks and volcanic activity releases phosphate into the soil, water, and air, where it becomes available to terrestrial food webs. Phosphate enters the oceans via surface runoff, groundwater flow, and river flow. Phosphate dissolved in ocean water cycles into marine food webs.