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Abiogeochemical cycle, or more generally acycle of matter,[1] is the movement and transformation of chemical elements and compounds between living organisms, the atmosphere, and the Earth's crust. Major biogeochemical cycles include thecarbon cycle, thenitrogen cycle and thewater cycle. In each cycle, the chemical element or molecule is transformed and cycled by living organisms and through various geological forms and reservoirs, including the atmosphere, the soil and the oceans. It can be thought of as the pathway by which achemical substancecycles (is turned over or moves through) thebiotic compartment and theabiotic compartments ofEarth. The biotic compartment is thebiosphere and the abiotic compartments are theatmosphere,lithosphere andhydrosphere.
For example, in the carbon cycle, atmosphericcarbon dioxide is absorbed by plants throughphotosynthesis, which converts it intoorganic compounds that are used by organisms for energy and growth.Carbon is then released back into the atmosphere throughrespiration anddecomposition. Additionally, carbon is stored infossil fuels and is released into the atmosphere through human activities such as burningfossil fuels. In the nitrogen cycle, atmosphericnitrogen gas is converted by plants into usable forms such asammonia andnitrates through the process ofnitrogen fixation. These compounds can be used by other organisms, and nitrogen is returned to the atmosphere throughdenitrification and other processes. In the water cycle, theuniversal solvent water evaporates from land and oceans to form clouds in the atmosphere, and thenprecipitates back to different parts of the planet. Precipitation canseep into the ground and become part of groundwater systems used by plants and other organisms, or canrunoff the surface to form lakes and rivers. Subterranean water can then seep into the ocean along withriver discharges, rich withdissolved andparticulate organic matter and other nutrients.
There are biogeochemical cycles for many other elements, such as foroxygen,hydrogen,phosphorus,calcium,iron,sulfur,mercury andselenium. There are also cycles for molecules, such aswater andsilica. In addition there are macroscopic cycles such as therock cycle, and human-induced cycles for synthetic compounds such as forpolychlorinated biphenyls (PCBs). In some cycles there are geological reservoirs where substances can remain or besequestered for long periods of time.
Biogeochemical cycles involve the interaction of biological, geological, and chemical processes. Biological processes include the influence ofmicroorganisms, which are critical drivers of biogeochemical cycling. Microorganisms have the ability to carry out wide ranges ofmetabolic processes essential for the cycling of nutrients (macronutrients andmicronutrients) and chemicals throughout global ecosystems. Without microorganisms many of these processes would not occur, with significant impact on the functioning of land and ocean ecosystems and the planet's biogeochemical cycles as a whole. Changes to cycles can impact human health. The cycles are interconnected and play important roles regulating climate, supporting the growth ofplants,phytoplankton and other organisms, and maintaining the health of ecosystems generally. Human activities such as burning fossil fuels and using large amounts of fertilizer can disrupt cycles, contributing to climate change, pollution, and other environmental problems.
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Energy flows directionally through ecosystems, entering as sunlight (or inorganic molecules forchemoautotrophs) and leaving as heat during the many transfers betweentrophic levels. However, the matter that makes up living organisms is conserved and recycled. The six most common elements associated with organic molecules — carbon, nitrogen, hydrogen, oxygen, phosphorus, and sulfur — take a variety of chemical forms and may exist for long periods in the atmosphere, on land, in water, or beneath the Earth's surface. Geologic processes, such asweathering,erosion,water drainage, and thesubduction of thecontinental plates, all play a role in this recycling of materials. Becausegeology andchemistry have major roles in the study of this process, the recycling of inorganic matter between living organisms and their environment is called a biogeochemical cycle.[3]
The six aforementioned elements are used by organisms in a variety of ways. Hydrogen and oxygen are found in water andorganic molecules, both of which are essential to life. Carbon is found in all organic molecules, whereas nitrogen is an important component ofnucleic acids andproteins. Phosphorus is used to make nucleic acids and thephospholipids that comprisebiological membranes. Sulfur is critical to the three-dimensional shape of proteins. The cycling of these elements is interconnected. For example, the movement of water is critical for leaching sulfur and phosphorus into rivers which can then flow into oceans. Minerals cycle through the biosphere between the biotic and abiotic components and from one organism to another.[4]
Ecological systems (ecosystems) have many biogeochemical cycles operating as a part of the system, for example, the water cycle, the carbon cycle, the nitrogen cycle, etc. All chemical elements occurring in organisms are part of biogeochemical cycles. In addition to being a part of living organisms, these chemical elements also cycle through abiotic factors of ecosystems such as water (hydrosphere), land (lithosphere), and/or the air (atmosphere).[5]
The living factors of the planet can be referred to collectively as thebiosphere. All the nutrients — such ascarbon,nitrogen,oxygen,phosphorus, andsulfur — used in ecosystems by living organisms are a part of aclosed system; therefore, these chemicals are recycled instead of being lost and replenished constantly such as in an open system.[5]
The major parts of the biosphere are connected by the flow of chemical elements and compounds in biogeochemical cycles. In many of these cycles, thebiota plays an important role. Matter from the Earth's interior is released by volcanoes. The atmosphere exchanges some compounds and elements rapidly with the biota and oceans. Exchanges of materials between rocks, soils, and the oceans are generally slower by comparison.[2]
The flow of energy in an ecosystem is anopen system; the Sun constantly gives the planet energy in the form of light while it is eventually used and lost in the form of heat throughout thetrophic levels of a food web. Carbon is used to make carbohydrates, fats, and proteins, the major sources offood energy. These compounds are oxidized to release carbon dioxide, which can be captured by plants to make organic compounds. Thechemical reaction is powered by the light energy of sunshine.
Sunlight is required to combine carbon with hydrogen and oxygen into an energy source, but ecosystems in thedeep sea, where no sunlight can penetrate, obtain energy from sulfur.Hydrogen sulfide nearhydrothermal vents can be utilized by organisms such as thegiant tube worm. In thesulfur cycle, sulfur can be forever recycled as a source of energy. Energy can be released through theoxidation andreduction of sulfur compounds (e.g., oxidizing elemental sulfur tosulfite and then tosulfate).
![The implications of shifts in the global carbon cycle due to human activity are concerning scientists.[6]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aGlobal_carbon_cycle.webp)
Although the Earth constantly receives energy from the Sun, its chemical composition is essentially fixed, as the additional matter is only occasionally added by meteorites. Because this chemical composition is not replenished like energy, all processes that depend on these chemicals must be recycled. These cycles include both the living biosphere and the nonlivinglithosphere,atmosphere, andhydrosphere.
Biogeochemical cycles can be contrasted withgeochemical cycles. The latter deals only withcrustal and subcrustal reservoirs even though some process from both overlap.
Biogeochemical cycles operate by moving substances, which may also undergo chemical rearrangements, through pathways in thebiotic compartment and theabiotic compartments of Earth. The biotic compartment is thebiosphere and the abiotic compartments are theatmosphere,lithosphere andhydrosphere.
Microorganisms drive much of the biogeochemical cycling in the earth system.[7][8]
The global ocean covers more than 70% of the Earth's surface and is remarkably heterogeneous. Marine productive areas, andcoastal ecosystems comprise a minor fraction of the ocean in terms of surface area, yet have an enormous impact on global biogeochemical cycles carried out bymicrobial communities, which represent 90% of the ocean's biomass.[10] Work in recent years has largely focused on cycling of carbon and macronutrients such as nitrogen, phosphorus, and silicate: other important elements such as sulfur or trace elements have been less studied, reflecting associated technical and logistical issues.[11] Increasingly, these marine areas, and the taxa that form their ecosystems, are subject to significant anthropogenic pressure, impacting marine life and recycling of energy and nutrients.[12][13][14] A key example is that ofcultural eutrophication, whereagricultural runoff leads to nitrogen and phosphorus enrichment of coastal ecosystems, greatly increasing productivity resulting inalgal blooms,deoxygenation of the water column and seabed, and increased greenhouse gas emissions,[15] with direct local and global impacts onnitrogen andcarbon cycles. However, the runoff oforganic matter from the mainland tocoastal ecosystems is just one of a series of pressing threats stressing microbial communities due to global change. Climate change has also resulted in changes in thecryosphere, as glaciers and permafrost melt, resulting in intensifiedmarine stratification, while shifts of theredox-state in different biomes are rapidly reshapingmicrobial assemblages at an unprecedented rate.[16][17][18][19][11]
Global change is, therefore, affecting key processes includingprimary productivity, CO2 and N2 fixation, organic matter respiration/remineralization, and the sinking and burial deposition of fixed CO2.[19] In addition to this, oceans are experiencing anacidification process, with a change of ~0.1pH units between the pre-industrial period and today, affectingcarbonate/bicarbonatebuffer chemistry. In turn, acidification has been reported to impactplanktonic communities, principally through effects on calcifying taxa.[20] There is also evidence for shifts in the production of key intermediary volatile products, some of which have marked greenhouse effects (e.g., N2O and CH4, reviewed by Breitburg in 2018,[17] due to the increase in global temperature, ocean stratification and deoxygenation, driving as much as 25 to 50% of nitrogen loss from the ocean to the atmosphere in the so-calledoxygen minimum zones[21] oranoxic marine zones,[22] driven by microbial processes. Other products, that are typically toxic for the marinenekton, including reduced sulfur species such as H2S, have a negative impact for marine resources like fisheries and coastal aquaculture. While global change has accelerated, there has been a parallel increase in awareness of the complexity of marine ecosystems, and especially the fundamental role of microbes as drivers of ecosystem functioning.[18][11]
The chemicals are sometimes held for long periods of time in one place. This place is called areservoir, which, for example, includes such things ascoal deposits that are storingcarbon for a long period of time.[23] When chemicals are held for only short periods of time, they are being held inexchange pools. Examples of exchange pools include plants and animals.[23]
Plants and animals utilize carbon to produce carbohydrates, fats, and proteins, which can then be used to build their internal structures or to obtain energy. Plants and animals temporarily use carbon in their systems and then release it back into the air or surrounding medium. Generally, reservoirs are abiotic factors whereas exchange pools are biotic factors. Carbon is held for a relatively short time in plants and animals in comparison to coal deposits. The amount of time that a chemical is held in one place is called itsresidence time orturnover time (also called the renewal time or exit age).[23]
Box models are widely used to model biogeochemical systems.[24][25] Box models are simplified versions of complex systems, reducing them to boxes (or storagereservoirs) for chemical materials, linked by materialfluxes (flows). Simple box models have a small number of boxes with properties, such as volume, that do not change with time. The boxes are assumed to behave as if they were mixed homogeneously.[25] These models are often used to derive analytical formulas describing the dynamics and steady-state abundance of the chemical species involved.
The diagram at the right shows a basic one-box model. The reservoir contains the amount of materialM under consideration, as defined by chemical, physical or biological properties. The sourceQ is the flux of material into the reservoir, and the sinkS is the flux of material out of the reservoir. The budget is the check and balance of the sources and sinks affecting material turnover in a reservoir. The reservoir is in asteady state ifQ =S, that is, if the sources balance the sinks and there is no change over time.[25]
The residence or turnover time is the average time material spends resident in the reservoir. If the reservoir is in a steady state, this is the same as the time it takes to fill or drain the reservoir. Thus, if τ is the turnover time, then τ =M/S.[25] The equation describing the rate of change of content in a reservoir is
When two or more reservoirs are connected, the material can be regarded as cycling between the reservoirs, and there can be predictable patterns to the cyclic flow.[25] More complexmultibox models are usually solved using numerical techniques.
Global biogeochemical box models usually measure:
- reservoir masses inpetagrams (Pg)
- flow fluxes in petagrams per year(Pg yr−1)
The diagram on the left shows a simplified budget of ocean carbon flows. It is composed of three simple interconnected box models, one for theeuphotic zone, one for theocean interior or dark ocean, and one forocean sediments. In the euphotic zone, netphytoplankton production is about 50 Pg C each year. About 10 Pg is exported to the ocean interior while the other 40 Pg is respired. Organic carbon degradation occurs asparticles (marine snow) settle through the ocean interior. Only 2 Pg eventually arrives at the seafloor, while the other 8 Pg is respired in the dark ocean. In sediments, the time scale available for degradation increases by orders of magnitude with the result that 90% of the organic carbon delivered is degraded and only 0.2 Pg C yr−1 is eventually buried and transferred from the biosphere to the geosphere.[26]
The diagram on the right shows a more complex model with many interacting boxes. Reservoir masses here representscarbon stocks, measured in Pg C. Carbon exchange fluxes, measured in Pg C yr−1, occur between the atmosphere and its two major sinks, the land and the ocean. The black numbers and arrows indicate the reservoir mass and exchange fluxes estimated for the year 1750, just before theIndustrial Revolution. The red arrows (and associated numbers) indicate the annual flux changes due to anthropogenic activities, averaged over the 2000–2009 time period. They represent how the carbon cycle has changed since 1750. Red numbers in the reservoirs represent the cumulative changes in anthropogenic carbon since the start of the Industrial Period, 1750–2011.[28][29][27]
There are fast and slow biogeochemical cycles. Fast cycle operate in thebiosphere and slow cycles operate in thelithosphere inrocks. Fast or biological cycles can complete within years, moving substances from atmosphere to biosphere, then back to the atmosphere. Slow or geological cycles can take millions of years to complete, moving substances through the Earth'scrust between rocks, soil, ocean and atmosphere.[31]
As an example, the fast carbon cycle is illustrated in the diagram on the right. This cycle involves relatively short-termbiogeochemical processes between the environment and living organisms in the biosphere. It includes movements of carbon between the atmosphere and terrestrial and marine ecosystems, as well as soils andseafloor sediments. The fast cycle includes annual cycles involving photosynthesis and decadal cycles involving vegetative growth and decomposition. The reactions of the fast carbon cycle to human activities will determine many of the more immediate impacts of climate change.[32][33][34][35]
The slow cycle is illustrated in the other diagram. It involves medium to long-termgeochemical processes belonging to therock cycle. The exchange between the ocean and atmosphere can take centuries, and theweathering of rocks can take millions of years. Carbon in the ocean precipitates to the ocean floor where it can formsedimentary rock and besubducted into theEarth's mantle.Mountain building processes result in the return of this geologic carbon to the Earth's surface. There the rocks are weathered and carbon is returned to the atmosphere bydegassing and to the ocean by rivers. Other geologic carbon returns to the ocean through thehydrothermal emission of calcium ions. In a given year between 10 and 100 million tonnes of carbon moves around this slow cycle. This includes volcanoes returning geologic carbon directly to the atmosphere in the form of carbon dioxide. However, this is less than one percent of the carbon dioxide put into the atmosphere by burning fossil fuels.[31][32]
The terrestrial subsurface is the largest reservoir of carbon on earth, containing 14–135Pg of carbon[36] and 2–19% of all biomass.[37] Microorganisms drive organic and inorganic compound transformations in this environment and thereby control biogeochemical cycles. Current knowledge of the microbial ecology of the subsurface is primarily based on16S ribosomal RNA (rRNA) gene sequences. Recent estimates show that <8% of 16S rRNA sequences in public databases derive from subsurface organisms[38] and only a small fraction of those are represented by genomes or isolates. Thus, there is remarkably little reliable information about microbial metabolism in the subsurface. Further, little is known about how organisms in subsurface ecosystems are metabolically interconnected. Some cultivation-based studies ofsyntrophicconsortia[39][40][41] and small-scale metagenomic analyses of natural communities[42][43][44] suggest that organisms are linked via metabolic handoffs: the transfer of redox reaction products of one organism to another. However, no complex environments have been dissected completely enough to resolve the metabolic interaction networks that underpin them. This restricts the ability of biogeochemical models to capture key aspects of the carbon and other nutrient cycles.[45] New approaches such as genome-resolved metagenomics, an approach that can yield a comprehensive set of draft and even complete genomes for organisms without the requirement for laboratory isolation[42][46][47] have the potential to provide this critical level of understanding of biogeochemical processes.[48]
Some of the more well-known biogeochemical cycles are shown below:
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Many biogeochemical cycles are currently being studied for the first time.Climate change and human impacts are drastically changing the speed, intensity, and balance of these relatively unknown cycles, which include:
![Kerogen cycle[51][52]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aOrganic_carbon_cycle_including_the_flow_of_kerogen.png)
Biogeochemical cycles always involve active equilibrium states: a balance in the cycling of the element between compartments. However, overall balance may involve compartments distributed on a global scale.
As biogeochemical cycles describe the movements of substances on the entire globe, the study of these is inherently multidisciplinary. The carbon cycle may be related to research inecology andatmospheric sciences.[53] Biochemical dynamics would also be related to the fields ofgeology andpedology.[54]
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