Phosphorus cycle

The phosphorus cycle is the biogeochemical cycle that involves the movement of phosphorus through the lithosphere, hydrosphere, and biosphere. Unlike many other biogeochemical cycles, the atmosphere does not play a significant role in the movement of phosphorus, because phosphorus and phosphorus-based materials do not enter the gaseous phase readily,[1] as the main source of gaseous phosphorus, phosphine, is only produced in isolated and specific conditions.[2] Therefore, the phosphorus cycle is primarily examined studying the movement of orthophosphate (PO4)3-, the form of phosphorus that is most commonly seen in the environment, through terrestrial and aquatic ecosystems.[3]

Living organisms require phosphorus, a vital component of DNA, RNA, ATP, etc., for their proper functioning.[4] Plants assimilate phosphorus as phosphate and incorporate it into organic compounds. In animals, phosphorus is a key component of bones, teeth, etc.[5] On the land, phosphorus gradually becomes less available to plants over thousands of years, since it is slowly lost in runoff. Low concentration of phosphorus in soils reduces plant growth and slows soil microbial growth, as shown in studies of soil microbial biomass. Soil microorganisms act as both sinks and sources of available phosphorus in the biogeochemical cycle. Short-term transformation of phosphorus is chemical, biological, or microbiological. In the long-term global cycle, however, the major transfer is driven by tectonic movement over geologic time and weathering of phosphate containing rock such as apatite.[6] Furthermore, phosphorus tends to be a limiting nutrient in aquatic ecosystems.[7] However, as phosphorus enters aquatic ecosystems, it has the possibility to lead to over-production in the form of eutrophication, which can happen in both freshwater and saltwater environments.[8][9][10]

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 limiting nutrient for aquatic organisms. Phosphorus forms parts of important life-sustaining molecules that are very common in the biosphere. Phosphorus does enter the atmosphere in very small amounts when dust containing phosphorus is dissolved in rainwater and sea spray, but the element mainly remains on land and in rock and soil minerals. Phosphates which are found in fertilizers, sewage and detergents, can cause pollution in lakes and streams. Over-enrichment of phosphate in both fresh and inshore marine waters can lead to massive algae blooms. In fresh water, the death and decay of these blooms leads to eutrophication. An example of this is the Canadian Experimental Lakes Area.

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 orthophosphate ion (PO4)3−, consisting of a P atom and 4 oxygen atoms. On land most phosphorus is found in rocks and minerals. Phosphorus-rich deposits have generally formed in the ocean or from guano, and over time, geologic processes bring ocean sediments to land. Weathering of rocks and minerals release phosphorus in a soluble form where it is taken up by plants, and it is transformed into organic compounds. The plants may then be consumed by herbivores and the phosphorus is either incorporated into their tissues or excreted. After death, the animal or plant decays, and phosphorus is returned to the soil where a large part of the phosphorus is transformed into insoluble compounds. Runoff may carry a small part of the phosphorus back to the ocean. Generally with time (thousands of years) soils become deficient in phosphorus leading to ecosystem retrogression.[13]

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 (ATP) or when linked together, form the nucleic acids DNA and RNA. The double helix of our DNA is only possible because of the phosphate ester bridge that binds the helix. Besides making biomolecules, phosphorus is also found in bone and the enamel of mammalian teeth, whose strength is derived from calcium phosphate in the form of hydroxyapatite. It is also found in the exoskeleton of insects, and phospholipids (found in all biological membranes).[15] It also functions as a buffering agent in maintaining acid base homeostasis in the human body.[16]

Phosphorus cycling
Part of a series on
Biogeochemical cycles
Water cycle
  • Water cycle
    • deep water cycle
Carbon cycle
Nutrient cycle
  • Hydrogen cycle
  • Nitrogen cycle
    • human impact
    • nitrification
    • nitrogen and lichens
    • fixation
    • assimilation
  • Oxygen cycle
  • Phosphorus cycle
    • assimilation
  • Sulfur cycle
    • assimilation
Rock cycle
Marine cycle
  • Biological pump
    • microbial loop
    • viral shunt
  • Calcareous ooze
  • Siliceous ooze
Methane cycle
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  • SOLAS
  •  Category

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 phosphonate and phosphite (P3+ oxidation state) as a P source by oxidizing it to orthophosphate.[29] Recently, rapid production and release of reduced phosphorus compounds has provided new clues about the role of reduced P as a missing link in oceanic phosphorus.[30]

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 oxalic acid by mycorrhizal fungi explain their importance in maintaining and supplying phosphorus to plants.[17][33]

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 phosphatases, nucleases and phytase involved for the degradation. Some of the abiotic pathways in the environment studied are hydrolytic reactions and photolytic reactions. Enzymatic hydrolysis of organic phosphorus is an essential step in the biogeochemical phosphorus cycle, including the phosphorus nutrition of plants and microorganisms and the transfer of organic phosphorus from soil to bodies of water.[34] Many organisms rely on the soil derived phosphorus for their phosphorus nutrition.[35]

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

Oceanic ecosystems gather phosphorus through many sources, but it is mainly derived from weathering of rocks containing phosphorus which are then transported to the oceans in a dissolved form by river runoff.[41] Due to a dramatic rise in mining for phosphorus, it is estimated that humans have increased the net storage of phosphorus in soil and ocean systems by 75%.[42] This increase in phosphorus has led to more eutrophication in ocean waters as phytoplankton blooms have caused a drastic shift in anoxic conditions seen in both the Gulf of Mexico[43] and the Baltic Sea.[44] Some research suggests that when anoxic conditions arise from eutrophication due to excess phosphorus, this creates a positive feedback loop that releases more phosphorus from oceanic reserves, exacerbating the issue.[45] This could possibly create a self-sustaining cycle of oceanic anoxia where the constant recovery of phosphorus keeps stabilizing the eutrophic growth.[45] Attempts to mitigate this problem using biological approaches are being investigated. One such approach involves using phosphorus accumulating organisms such as, Candidatus accumulibacter phosphatis, which are capable of effectively storing phosphorus in the form of phosphate in marine ecosystems.[46] Essentially, this would alter how the phosphorus cycle exists currently in marine ecosystems. Currently, there has been a major influx of phosphorus due to increased agricultural use and other industrial applications,[45] thus these organisms could theoretically store phosphorus and hold on to it until it could be recycled in terrestrial ecosystems which would have lost this excess phosphorus due to runoff.[46]

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 mining phosphate rock. For millennia, phosphorus was primarily brought into the environment through the weathering of phosphate containing rocks, which would replenish the phosphorus normally lost to the environment through processes such as runoff, albeit on a very slow and gradual time-scale.[47] Since the 1840s, when the technology to mine and extract phosphorus became more prevalent, approximately 110 teragrams of phosphorus has been added to the environment.[48] This trend appears to be continuing in the future as from 1900-2022, the amount of phosphorus mined globally has increased 72-fold,[49] with an expected annual increase of 4%.[48] Most of this mining is done in order to produce fertilizers which can be used on a global scale. However, at the rate humans are mining, the geological system can not restore what is lost quickly enough.[50] Thus, researchers are examining ways to better recycle phosphorus in the environment, with one promising application including the use of microorganisms.[46][51] Regardless, humans have had a profound impact on the phosphorus cycle with wide-reaching implications about food security, eutrophication, and the overall availability of the nutrient.[52]

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

References
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