All Subjects



AP Enviro



Unit 1

1.6 The Phosphorous Cycle

2 min readseptember 18, 2020

Sumi Vora

Movement of Matter 

The earth is a closed system; matter can never be removed or created. Instead, matter moves across the earth in cycles called biogeochemical cycles. These basic cycles are essential for supporting life on Earth, but when they are disrupted, it can cause matter to get “stuck” or move too quickly between stages. 

Phosphorus Cycle 🗻

Phosphorus is really similar to nitrogen. We need phosphorus for DNA, RNA, and ATP, and it is also a limiting macronutrient. Phosphorus only cycles between land and water -- not the atmosphere. And, phosphorus doesn’t change form like nitrogen (bless up 🙌) -- it is almost always found as phosphate (PO4). 

Source: NASA Earth Observatory

Plants assimilate phosphate, and when organisms die, fungi and bacteria mineralize phosphate and it goes back into the soil. This is really similar to the nitrogen cycle, but without all of the chemical changes. 
The other portion of the phosphorus cycle involves the movement of phosphorus between land and water. Just like the carbon cycle, this is the slow part of the phosphorus cycle. Phosphorus enters the ocean through precipitation or runoff, but it doesn’t dissolve in the water. Instead, phosphorus is deposited in the sediments at the bottom of the ocean. 
Over time, a process called geologic uplift brings the ocean layers up to become mountains, and the phosphate rocks weather which brings the nutrient into the soil. Phosphate usually clings tightly onto soil, and like we’ve seen before, it doesn’t dissolve easily in water. Therefore, phosphate is a limiting nutrient for aquatic ecosystems. 

Human Impacts on the Phosphorus Cycle🙌

The two main anthropogenic sources of phosphorus are laundry detergents and synthetic fertilizers 
Like nitrogen, phosphorus plays a big role in plant growth, so humans often add it to synthetic fertilizers. The excess phosphorus often runs off into nearby bodies of water, leading to excess algae growth that prevents sunlight from penetrating the water. The existing aquatic plants die and eat up all the available oxygen, which leads to massive dead zones in the water. 
As we have seen with nitrogen, phosphorus can also alter plant communities especially in aquatic systems, causing smaller plants to die off as the larger ones grow larger. 

Was this guide helpful?

FREE AP enviro Survival Pack + Cram Chart PDF
Sign up now for instant access to 2 amazing downloads to help you get a 5
Join us on Discord
Thousands of students are studying with us for the AP Environmental Science exam.
join now
Browse Study Guides By Unit
Big Reviews: Finals & Exam Prep
Free Response Questions (FRQs)
Multiple Choice Questions (MCQs)
Unit 2: The Living World: Biodiversity
Unit 3: Populations
Unit 4: Earth Systems and Resources
Unit 5: Land and Water Use
Unit 6: Energy Resources and Consumption
Unit 7: Atmospheric Pollution
Unit 9: Global Change