AP Syllabus focus:
‘Weathering releases phosphate taken up by producers, transferred through food webs, and returned via excretion and decomposition.’
Phosphorus is an essential nutrient whose movement depends largely on rocks, soils, organisms, and sediments. Understanding how phosphate enters food webs and returns to the environment explains nutrient limitation and ecosystem productivity.
Core idea: how phosphorus moves
Unlike many other nutrients, phosphorus has no major atmospheric reservoir, so its availability is often controlled by slow geological processes and local recycling.
Conceptual diagram of the phosphorus cycle showing movement of phosphate among rocks/soils, organisms (producers → consumers), and aquatic systems, with recycling via decomposition and longer-term storage in sediments. Use it to visually connect the “no atmospheric reservoir” idea to the key fluxes that control phosphate availability in ecosystems. Source
Key chemical form: phosphate
Phosphate: Inorganic phosphorus, usually present as (and related ions), that organisms absorb and use to build biomolecules.
Interactive 3D visualization of the phosphate ion, , highlighting its tetrahedral arrangement (one P atom bonded to four O atoms). This helps link the ecological term “phosphate” to the actual chemical species that dissolves in water and is transported and taken up by organisms. Source
Phosphate is required for ATP, nucleic acids (DNA/RNA), and phospholipids, so shortages can directly limit growth and reproduction.
Main reservoirs and pathways
Long-term reservoirs (slow movement)
Phosphate-containing rocks (sedimentary and igneous sources)
Marine sediments formed from accumulated phosphate over time
Because these pools are geologically “locked,” ecosystems often rely on relatively small, active pools in soils and water.
Short-term, biologically active pools (faster movement)
Soil phosphate (dissolved in soil water or adsorbed to particles)
Dissolved phosphate in freshwater and coastal systems
Biomass phosphate inside living organisms and detritus
Step-by-step nutrient movement (syllabus-aligned)
1) Weathering releases phosphate
Weathering of rock (physical breakdown and chemical alteration) slowly releases phosphate into soils and surface waters.
Released phosphate may:
Dissolve in soil water and become available to roots
Bind to soil minerals (reducing immediate availability)
Run off into streams, lakes, and oceans
This step sets the baseline input of “new” phosphorus to many terrestrial ecosystems.
2) Producers take up phosphate (assimilation)
Primary producers incorporate phosphate through uptake across membranes:
Plants absorb phosphate via roots (often aided by root surface area and transport proteins).
Algae and aquatic plants absorb dissolved phosphate directly from water.
Once inside producers, phosphate is converted into organic phosphorus in cellular structures and molecules.
3) Transfer through food webs
Phosphorus moves through trophic levels by feeding:
Herbivores obtain phosphorus by consuming producer tissues.
Predators obtain phosphorus by consuming prey.
At each step, phosphorus is partitioned into:
Biomass (growth and storage)
Waste (unassimilated material and metabolic by-products)
Phosphorus does not “flow” like energy; it cycles because atoms are reused.
Labeled phosphorus-cycle diagram that summarizes how phosphate moves from rock weathering into soils and producers, transfers through consumers, and returns through waste and decomposition. It also visually reinforces the idea of cycling (matter is reused) rather than one-way flow like energy. Source
4) Return via excretion and decomposition
Organisms return phosphorus to the environment through:
Excretion (e.g., phosphate in urine and other wastes)
Egestion (undigested material in faeces)
Decomposition after death
Decomposers (bacteria and fungi) perform mineralisation, converting organic phosphorus back into inorganic phosphate that can re-enter soil water or aquatic systems and be taken up again by producers.
Sedimentation and long-term recycling
In aquatic environments, phosphate can:
Be taken up quickly by producers near the surface, or
Attach to particles and sink
Over time, sinking organic matter and mineral-bound phosphate form sediments. Burial can sequester phosphorus for long periods until geological uplift and renewed weathering expose it again. This slow “return loop” helps explain why many ecosystems depend heavily on efficient local recycling (excretion + decomposition) rather than constant external inputs.
What controls phosphate availability (within this cycle)
Rate of weathering: determines long-term input of new phosphate.
Soil binding and solubility: phosphate can be present yet poorly available if tightly bound.
Recycling efficiency: rapid excretion and decomposition increase immediate availability to producers.
Loss pathways: runoff and sedimentation can remove phosphate from local, biologically active pools.
FAQ
Phosphorus is not typically converted into a stable gas under Earth-surface conditions. It mostly remains in solid or dissolved forms, so movement depends on rock, water, and organisms rather than the air.
Phosphate can adsorb to mineral surfaces or form insoluble compounds. This lowers dissolved phosphate concentration, limiting what roots can absorb even when total soil phosphorus is high.
Dissolved phosphate can bind to particles, and organic matter containing phosphorus can sink after organisms die. Burial converts short-term pools into long-term sediment stores.
Key requirements include:
Phosphodiester bonds in DNA/RNA
Phosphate groups in ATP/ADP
Phospholipids in membranes
They secrete enzymes that break down phosphorus-containing molecules. This converts organic phosphorus into inorganic phosphate ions that can dissolve into soil water or aquatic systems.
Practice Questions
State two processes that return phosphorus to the environment after it has moved through a food web. (2 marks)
Excretion releases phosphate to soil/water (1)
Decomposition of dead organisms/detritus returns phosphate (1)
Describe how phosphate moves from rocks into producers, through a food web, and back to the abiotic environment. Include the role of decomposers. (5 marks)
Weathering of phosphate-containing rocks releases phosphate into soil/water (1)
Producers take up inorganic phosphate and incorporate it into organic molecules/biomass (1)
Consumers obtain phosphorus by feeding; transfer occurs along trophic levels (1)
Phosphate is returned in waste via excretion/egestion (1)
Decomposers break down dead material and mineralise organic phosphorus to inorganic phosphate available again (1)
