27.10
The Phosphorus Cycle
Unlike carbon, water, and nitrogen, phosphorus is not present in the atmosphere as a gas. Instead, most phosphorus in the ecosystem exists as compound…
In the phosphorous cycle, phosphorous atoms exist primarily in rock, not a gaseous state. When rock erodes, the phosphorous is released and becomes dissolved in streams, lakes and ground water.
Plants and algae use these free inorganic phosphates in the soil or water to produce organic molecules. Then by consuming the plants, heterotrophs tap into the phosphorous stores to build their own compounds. When organisms die, decomposers break up the phosphorous molecules and release inorganic phosphate to be used again by plants and algae, a process called phosphate mineralization.
While phosphates cycle quickly through organisms in the food chain, they have a longer residence time in water. Dissolved phosphate ions react to form insoluble compounds that precipitate in the ocean and become sediment, then rock, and as result of tectonic uplift, eventually return to the environment.
However, since the amount of phosphorous is limited in the environment, it must be provided to agricultural crop plants as fertilizer to get maximum yield. Excess phosphorous becomes run-off in aquatic ecosystems resulting in a variety of environmental problems such as algal blooms.
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Q1: Why is phosphorus not found in the atmosphere like carbon or nitrogen?
Phosphorus exists primarily in rock and mineral forms rather than as a gas. Unlike carbon and nitrogen, phosphorus lacks a gaseous phase in the atmosphere. Instead, it cycles through soil, water, sediment, and organisms as inorganic phosphate ions and organic compounds found in biological structures like DNA and cell membranes.
Q2: How does phosphorus move from rocks into soil and water?
Rock and sediment weathering releases inorganic phosphate over time. This phosphate gradually reaches soil and surface water through natural erosion processes. Once dissolved in streams, lakes, and groundwater, phosphate becomes available for plants and algae to absorb and incorporate into organic molecules for growth.
Q3: What role do decomposers play in the phosphorus cycle?
When organisms die or excrete waste, decomposers break down organic phosphate compounds in a process called phosphate mineralization. This converts organic phosphates back into inorganic forms that plants and algae can reuse. This recycling allows phosphorus to cycle quickly through food chains and support continued biological productivity.
Q4: How does phosphorus eventually return to land from the ocean?
Phosphates transported to oceans by natural runoff accumulate in sedimentary layers on the ocean floor. Over thousands of years, geological uplift returns phosphorus-containing rocks from the ocean to land, completing the geochemical cycle. This slow process replenishes terrestrial phosphorus reserves over extended timescales.
Q5: Why do farmers add phosphorus to agricultural fertilizers?
Phosphorus is often a limiting nutrient in natural environments, restricting plant growth. Adding phosphorus to fertilizers increases crop yield by providing plants with adequate phosphate for organic molecule synthesis. However, excess phosphorus from agricultural runoff can cause environmental problems such as algal blooms in aquatic ecosystems.
Q6: What environmental problems result from excess phosphorus in water?
Agricultural runoff containing excess phosphorus stimulates rapid growth of aquatic producers, causing algal blooms and other environmental problems. Unlike the nitrogen cycle, phosphorus has a longer residence time in water, allowing dissolved phosphate ions to accumulate and precipitate as insoluble compounds that disrupt aquatic ecosystems.
Q7: How do heterotrophs obtain phosphorus in food chains?
Heterotrophs acquire phosphorus by consuming plants and other animals that have already incorporated phosphate into their organic structures. This transfers phosphorus through trophic levels, allowing animals to build their own phosphorus-containing compounds for bones, teeth, cell membranes, and other biological molecules.