In the phosphorus cycle, phosphorus can recycle through waste products, decomposition, and of terrestrial and aquatic ecosystems.
food chains
the atmosphere
assimilation

Answer is - Assimilation

Reasoning - photosynthetic organisms such as Algae or Plants assimilation phosphorus into their cell. Which they later transfer it to animals whom consume which then starts the cycle.

I answered the same question in another post but here I'll post what I wrote with reasoning why.