Phosphorus cycle

We live in a world that is constantly recycling materials. All life is composed of the very same matter that exists in the non-living, or abiotic, world. The elements that are found in living things, like carbon , hydrogen , and calcium are also found in abioic compounds of the environment, like soil or rock. Because the quantities of usable sources of materials and elements that compose the living things on our planet are limited, life on Earth is dependent on recycling. The chemical constituents that make up a plant , for instance, might once have been the constituents of a former animal that died. The water we drink and is incorporated into our bodies, might once have been the same water that existed within dinosaurs, now long extinct. But matter is not simply recycled among living things. It is also recycled between the living and the non-living. The potassium in a banana , for instance, is recycled from potassium found in soil. This process of recycling, especially nutrients , between living and non-living components of the environment is called biogeochemical cycling. The phosphorus cycle is the biogeochemical cycling of phosphorus, a very important element of living things, between the living and nonliving parts of our world. Human activity can have effects on phosphorus cycling, which in turn has profound effects on ecological systems.

Biogeochemical cycles

Life is a complex interplay of matter and energy . All life on Earth ultimately derives its energy from the Sun . The Sun is a star that bombards our planet with solar radiation . It is a nearly inexhaustible source of light energy for the living organisms that inhabit the earth. As abundant as this energy source is, however, there is a finite quantity of matter, or chemical elements, available to make up living things. Therefore, life on Earth, because it depends both on energy and matter, must depend on the reclaiming of materials for use over and over again. Essential nutrient elements are recycled between living and abiotic components of ecosystems in biogeochemical cycles, or cycles involving living (bio-), geological (geo-), and chemical processes. When living things die, they return their chemical elements to the non-living components of ecosystems as they decompose. However, even while alive, organisms contribute to nutrient cycling as they consume matter and excrete waste products into the environment.

There are several major biogeochemical cycles rotating continuously within and among ecosystems. An ecosystem is an area including all of the living and nonliving things found within it. The most important cycles of ecosystems are the carbon cycle , the nitrogen cycle , the phosphorus cycle, and the water cycle. These interacting biogeochemical cycles involve travel of carbon, nitrogen , phosphorus, and water through living things, air, water, soil, and rock. For instance, the carbon cycle involves the gas carbon dioxide found in air. Plants use carbon dioxide during photosynthesis to make plant material, like cellulose . Here, carbon moves from an inorganic gaseous form, to living, or organic, form. Then, as plants die, they decompose and release organic molecules into water, which then runs into oceans. The organic material settles to the bottom where, over very long time periods, is incorporated into rock. Thus, the carbon existed as a gas in air, living material in plants, dissolved matter in water, and as solid form in rock. In much the same way, phosphorus is recycled in the environment. Not every cycle, however, includes each of these stages.

Phosphorus functions and recycling

All living things require phosphorus. In the environment, phosphorus is often in the form of phosphate molecules, composed of one phosphorus atom and four oxygen atoms . One important function of phosphate groups of organic molecules within living organisms is energy storage. Adenosine triphosphate , or ATP, is an example. ATP, the "energy currency" of cells is used to transfer stored chemical energy from one molecule to another to perform work. The energy is stored in the phosphate portion of the molecule. The energy we derive from food, for example, is stored in the form of ATP. Phosphorus is also required for the formation of phospholipids of cells. Phospholipids are the major component of cell membranes. Also, phosphate groups activate and deactivate enzymes within cells that catalyze major chemical reactions . Phosphate is a mineral salt component of bones and teeth in vertebrate animals. In addition, phosphate is an important structural component of DNA itself. So, recycling of limited phosphorus is vital.

Unlike the carbon cycle, the phosphorus cycle does not include transition of phosphorus through the atmosphere as a gas. Phosphorus-containing gases are not common. Also, phosphate has a limited number of inorganic forms outside of living organisms, making its recycling scheme relatively simple. Weathering of rocks containing phosphate minerals is accomplished by rain. The erosion moves inorganic phosphate into soil where it is rapidly absorbed by plants and incorporated into organic molecules (DNA, ATP, phospholipids). Plants containing phosphorus die or are consumed by animals. When consumed by animals, the phosphorus is incorporated into animal mass . When phosphorus containing animals die, along with plants, their decomposition returns phosphorus from their tissues back into soil for new use by plants (or by fungi ).

Not all of the phosphate eroded from rock is incorporated into plant and animal tissue directly. A portion of the run-off from phosphorus deposits in rock either enters streams and rivers that flow to the ocean , or leaches into the water table, gradually draining into the sea. Phosphates in the ocean very gradually build-up in sediments. Also, phosphorus in decaying aquatic organisms falls to the bottom to accompany the phosphorus built-up in inorganic sediment. Over extremely long periods of time, phosphorus-containing sediment is transformed into rock, buried deep in the ocean floor. Here, the phosphorus remains, not participating in the rest of the cycle. Most of the phosphorus on Earth is found here, at the bottom of the ocean as a part of the earth's crust. Periodically, violent geological shifts raise the once buried deposits. Now on land, exposed to wind and rain, the phosphorus minerals are free to participate in the rest of the cycle.

Phosphorus as a limiting nutrient in ecosystems

The measure of how quickly and to what extent sunlight is converted into organic material by plants during photosynthesis is called primary productivity. Some ecosystems have high primary productivity, while others have very low productivity. For example, the ocean is the world's most productive ecosystem because of its huge area. Oceanic algae create new plant biomass , or weight of living material, on a vast scale. However, plant primary productivity is not simply dependent on the availability of sunlight alone. In addition to water, other vital inorganic nutrients are required for growth and optimum primary productivity. Phosphorus is one such nutrient.

In ecosystems, rarely will all required nutrients be used up at the same rate . When one nutrient is used before other nutrients, it is called a limiting nutrient. Limiting nutrients prevent growth with their absence. When returned to the lacking environment, limiting nutrients jump-start productivity, which continues until the limiting nutrient again is depleted. Phosphorus is a limiting nutrient in many terrestrial and aquatic ecosystems. The productivity of the primary producers in these areas is limited, held in check, by the amount of available phosphorus that is so vital for life. This fact is why phosphorus is a main component of agricultural and household plant foods and fertilizers . The addition of phosphorus that is normally in limited supply allows for maximal plant growth.

Normally, because phosphorus availability is limited in the phosphorus cycle, plant growth in lakes is also limited. A major problem with the use of phosphorus in fertilizers is the process of artificial eutrophication . Eutrophication is a large increase in the primary productivity of a lake . Eutrophication can be harmful to the natural balance of a lake and result in massive death of fish and other animals as dissolved oxygen levels are depleted from the water. As the growth of algae and aquatic plants goes unchecked, the lake slowly stagnates, becoming fouled. Artificial eutrophication can occur when run-off rain water from agricultural fertilizers that are used in excess reaches lakes. Another human cause of artificial eutrophication is run-off from mines. Mining in areas where rock is rich in phosphorus minerals can create dust that is blown by wind into nearby water systems. Similarly, rain-water can wash from mining areas to nearby lakes. A third cause of artificial eutrophication is the introduction of phosphorus into phosphorus-limited lakes by man-made laundry detergents. Many detergents in the past contained phosphorus. Effluent from households eventually made its way to lakes where massive plant overgrowth occurred, spoiling the natural balance present. Today, considerable progress has been made in producing phosphate-free detergents, which do not cause artificial eutrophication and preserve the normal cycling of phosphorus.

Resources

books

Cunningham, W.P. Understanding Our Environment: An Introduction. W.C.Brown, 1994.

Ricklefs, R.E. The Economy of Nature. 3rd ed. New York: W. H.Freeman, 1993.

Walker, D. Energy, Plants, and Man. University Science Books, 1992.

Terry Watkins

KEY TERMS

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Abiotic

—A term used to describe the portion of an ecosystem that is not living, such as water or soil.

Artificial eutrophication

—A large increase in the primary productivity of a lake due to the actions of man, like fertilizer run-off from agricultural activities.

Biogeochemical cycle

—The process of recycling nutrients necessary for life among living non-living components of an ecosystem. The recycling can include geological, chemical, and living components.

Biomass

—Total weight, volume, or energy equivalent of all living organisms within a given area.

Ecosystem

—All of the organisms in a biological community interacting with the physical environment.

Limiting nutrient

—A chemical nutrient, such as phosphorus, which is necessary for growth but is found in limited quantities in a given ecosystem. Limiting nutrients limit the growth of dependent organisms.

Primary productivity

—The rate of conversion of sunlight energy into chemical energy within plants, called primary producers.