Phosphorus cycle
The phosphorus cycle is the
Living organisms require
Humans have caused major changes to the global phosphorus cycle primarily through the mining and subsequent shipping of phosphorus minerals for use in fertilizer and industrial products. Some phosphorus is also lost as effluent through the shipping process as well.
Phosphorus in the environment
Ecological function
Phosphorus is an essential nutrient for plants and animals. Phosphorus is a
Freshwater algal blooms are generally caused by excess phosphorus, while those that take place in saltwater tend to occur when excess nitrogen is added.[11] However, it is possible for eutrophication to be due to a spike in phosphorus content in both freshwater and saltwater environments.[11][12][10]
Phosphorus occurs most abundantly in nature as part of the
Major pools in aquatic systems
There are four major pools of phosphorus in freshwater ecosystems: dissolved inorganic phosphorus (DIP), dissolved organic phosphorus (DOP), particulate inorganic phosphorus (PIP) and particulate organic phosphorus (POP). Dissolved material is defined as substances that pass through a 0.45 μm filter.[14] DIP consists mainly of orthophosphate (PO43-) and polyphosphate, while DOP consists of DNA and phosphoproteins. Particulate matter are the substances that get caught on a 0.45 μm filter and do not pass through. POP consists of both living and dead organisms, while PIP mainly consists of hydroxyapatite, Ca5(PO4)3OH .[14] Inorganic phosphorus comes in the form of readily soluble orthophosphate. Particulate organic phosphorus occurs in suspension in living and dead protoplasm and is insoluble. Dissolved organic phosphorus is derived from the particulate organic phosphorus by excretion and decomposition and is soluble.
Biological function
The primary biological importance of phosphates is as a component of nucleotides, which serve as energy storage within cells (
Phosphorus cycling
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Biogeochemical cycles |
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Phosphates move quickly through plants and animals; however, the processes that move them through the soil or ocean are very slow, making the phosphorus cycle overall one of the slowest biogeochemical cycles.[17][18]
The global phosphorus cycle includes four major processes:
- (i) tectonic uplift and exposure of phosphorus-bearing rocks such as apatite to surface weathering;[19]
- (ii) physical erosion, and chemical and biological weathering of phosphorus-bearing rocks to provide dissolved and particulate phosphorus to soils,[20] lakes and rivers;
- (iii) riverine and subsurface transportation of phosphorus to various lakes and run-off to the ocean;
- (iv) sedimentation of particulate phosphorus (e.g., phosphorus associated with organic matter and oxide/carbonate minerals) and eventually burial in marine sediments (this process can also occur in lakes and rivers).[21]
In terrestrial systems, bioavailable P (‘reactive P’) mainly comes from weathering of phosphorus-containing rocks. The most abundant primary phosphorus-mineral in the crust is apatite, which can be dissolved by natural acids generated by soil microbes and fungi, or by other chemical weathering reactions and physical erosion.[22] The dissolved phosphorus is bioavailable to terrestrial organisms and plants and is returned to the soil after their decay. Phosphorus retention by soil minerals (e.g., adsorption onto iron and aluminum oxyhydroxides in acidic soils and precipitation onto calcite in neutral-to-calcareous soils) is usually viewed as the most important process in controlling terrestrial P-bioavailability in the mineral soil.[23] This process can lead to the low level of dissolved phosphorus concentrations in soil solution. Various physiological strategies are used by plants and microorganisms for obtaining phosphorus from this low level of phosphorus concentration.[24]
Soil phosphorus is usually transported to rivers and lakes and can then either be buried in lake sediments or transported to the ocean via river runoff. Atmospheric phosphorus deposition is another important marine phosphorus source to the ocean.[25] In surface seawater, dissolved inorganic phosphorus, mainly orthophosphate (PO43-), is assimilated by phytoplankton and transformed into organic phosphorus compounds.[21][25] Phytoplankton cell lysis releases cellular dissolved inorganic and organic phosphorus to the surrounding environment. Some of the organic phosphorus compounds can be hydrolyzed by enzymes synthesized by bacteria and phytoplankton and subsequently assimilated.[25] The vast majority of phosphorus is remineralized within the water column, and approximately 1% of associated phosphorus carried to the deep sea by the falling particles is removed from the ocean reservoir by burial in sediments.[25] A series of diagenetic processes act to enrich sediment pore water phosphorus concentrations, resulting in an appreciable benthic return flux of phosphorus to overlying bottom waters. These processes include
- (i) microbial respiration of organic matter in sediments,
- (ii) microbial reduction and dissolution of iron and manganese (oxyhydr)oxides with subsequent release of associated phosphorus, which connects the phosphorus cycle to the iron cycle,[26] and
- (iii) abiotic reduction of iron (oxyhydr)oxides by hydrogen sulfide and liberation of iron-associated phosphorus.[21]
Additionally,
- (iv) phosphate associated with calcium carbonate and
- (v) transformation of iron oxide-bound phosphorus to vivianite play critical roles in phosphorus burial in marine sediments.[27][28]
These processes are similar to phosphorus cycling in lakes and rivers.
Although orthophosphate (PO43-), the dominant inorganic P species in nature, is oxidation state (P5+), certain microorganisms can use
Phosphatic minerals
The availability of phosphorus in an ecosystem is restricted by its rate of release during weathering. The release of phosphorus from apatite dissolution is a key control on ecosystem productivity.[31] The primary mineral with significant phosphorus content, apatite [Ca5(PO4)3OH] undergoes carbonation.[17][32]
Little of this released phosphorus is taken up by biota, as it mainly reacts with other soil minerals. This leads to phosphorus becoming unavailable to organisms in the later stage of weathering and soil development as it will precipitate into rocks. Available phosphorus is found in a biogeochemical cycle in the upper soil profile, while phosphorus found at lower depths is primarily involved in geochemical reactions with secondary minerals. Plant growth depends on the rapid root uptake of phosphorus released from dead organic matter in the biochemical cycle. Phosphorus is limited in supply for plant growth. Phosphates move quickly through plants and animals; however, the processes that move them through the soil or ocean are very slow, making the phosphorus cycle overall one of the slowest biogeochemical cycles.[17][18]
Low-molecular-weight (LMW) organic acids are found in soils. They originate from the activities of various microorganisms in soils or may be exuded from the roots of living plants. Several of those organic acids are capable of forming stable organo-metal complexes with various metal ions found in soil solutions. As a result, these processes may lead to the release of inorganic phosphorus associated with aluminum, iron, and calcium in soil minerals. The production and release of
The availability of organic phosphorus to support microbial, plant and animal growth depends on the rate of their degradation to generate free phosphate. There are various enzymes such as
Eutrophication
Eutrophication is when waters are enriched by nutrients that lead to structural changes to the aquatic ecosystem such as algae bloom, deoxygenation, reduction of fish species. It does occur naturally, as when lakes age they become more productive due to increases in major limiting reagents such as nitrogen and phosphorus.[36] For example, phosphorus can enter into lakes where it will accumulate in the sediments and the biosphere. It can also be recycled from the sediments and the water system allowing it to stay in the environment.[37] Antrhopogenic effects can also cause phosphorus to flow into aquatic ecosystems as seen in drainage water and runoff from fertilized soils on agricultural land.[38] Additionally, eroded soils, which can be caused by deforestation and urbanization, can lead to more phosphorus and nitrogen being added to these aquatic ecosystems.[39] These all increase the amount of phosphorus that enters the cycle which has led to excessive nutrient intake in freshwater systems causing dramatic growth in algal populations. When these algae die, their putrefaction depletes the water of oxygen and can toxify the waters. Both these effects cause plant and animal death rates to increase as the plants take in and animals drink the poisonous water.[40]
Saltwater Phosphorus Eutrophication
Wetland
Wetlands are frequently applied to solve the issue of eutrophication. Nitrate is transformed in wetlands to free nitrogen and discharged to the air. Phosphorus is adsorbed by wetland soils which are taken up by the plants. Therefore, wetlands could help to reduce the concentration of nitrogen and phosphorus to remit eutrophication. However, wetland soils can only hold a limited amount of phosphorus. To remove phosphorus continually, it is necessary to add more new soils within the wetland from remnant plant stems, leaves, root debris, and undecomposable parts of dead algae, bacteria, fungi, and invertebrates.[38]
Human influences
Nutrients are important to the growth and survival of living organisms, and hence, are essential for development and maintenance of healthy ecosystems. Humans have greatly influenced the phosphorus cycle by
Other human processes can have detrimental effects on the phosphorus cycle, such as the repeated application of liquid hog manure in excess to crops. The application of biosolids may also increase available phosphorus in soil.[53] In poorly drained soils or in areas where snowmelt can cause periodic waterlogging, reducing conditions can be attained in 7–10 days. This causes a sharp increase in phosphorus concentration in solution and phosphorus can be leached. In addition, reduction of the soil causes a shift in phosphorus from resilient to more labile forms. This could eventually increase the potential for phosphorus loss. This is of particular concern for the environmentally sound management of such areas, where disposal of agricultural wastes has already become a problem. It is suggested that the water regime of soils that are to be used for organic wastes disposal is taken into account in the preparation of waste management regulations.[54]
See also
- Peak phosphorus
- Planetary boundaries
- Oceanic carbon cycle
References
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External links
- Holding B (2006). "Matter Cycles". Lenntech. Water treatment & air purification.
- "Phosphorus Cycle". Environmental Literacy Council. 10 July 2023.
- "section 5.6 Phosphorus". Monitoring and assessing water quality. U.S. Environmental Protection Agency.
- Miller KR, Levine J (2001). Biology. Prentice Hall. Archived from the original on 2008-08-12.
- Corbin K. "The Phosphorus Cycle". Biogeochemical Cycles – Soil Microbiology. Virginia Polytechnic Institute and State University. Archived from the original on 2008-09-14.