What is the Carbon Cycle?

Carbon is the building block of life and accounts for approximately half of the total dry biomass of the earth (biotic pool). Carbon is also present in abiotic components of earth. Carbon in biotic and abiotic compartment together forms the earth’s carbon pool. The biotic carbon is known to work in a cycle without external intervention. The external intervention comprises of natural and anthropogenic process. These processes result in carbon flux or mass transfer across various pools. The carbon flux across the lithospheric and atmospheric compartment is responsible for global warming and the flux between lithospheric and hydrospheric compartment is the cause of water pollution. Thus the movement of carbon mass between compartments is the area has drawn significant research attention. Accordingly a research scope exists towards utilizing various pools in a sustainable manner through strategic planning of carbon flux. Keywords — Carbon flux, Carbon pool, Lignocellulosic biomass, Sustainability. Introduction The global carbon cycle refers to the transaction of carbon within and betwixt four major reservoirs: the atmosphere, the oceans, land, and fossil fuels. Relocation of carbon from one stockpile to another may take place within seconds (e.g., the fixation of atmospheric CO2 into sugar through photosynthesis) or over millennia (e.g., the accumulation of fossil carbon coal, oil, gas through deposition and diagenesis of organic matter) [1].

The carbon cycle holds significance for at least three reasons. First, carbon forms the architecture of all life forms on the planet, making up about 50% of the dry weight of living things. Second, the cycling of carbon approximates the movements of energy around the Earth, the metabolism of natural, human, and industrial systems. Plants transmute incoming solar radiations into chemical energy in the form of sugars, starches, and auxiliary forms of organic matter; aforementioned energy, whether in contemporary organisms or deceased organic matter, supports food chains in natural ecosystems as well as human ecosystems. The third reason for escalating interest in carbon cycle is the elevated usage of fossil fuels. Carbon dioxide (CO2) and methane (CH4) are two which are imperative carbon containing greenhouse gases. These gases contribute to a natural greenhouse effect that has kept the planet ample warm to emerge and foothold life (without the greenhouse effect the Earth’s average temperature would be – 33 °C). Augmentation of greenhouse gases to the atmosphere from industrial bustle, notwithstanding, are broadening the concentrations of these gases, intensifying the greenhouse effect, and starting to warm the Earth [2], [3]. The rate and amplitude of the warming bank on, in part, on the global carbon cycle. If the rate at which the oceans discharge CO2 from the atmosphere were agile, e.g., concentrations of CO2 would have increased less over the last century. If the processes eliminating carbon from the atmosphere and storing it on land were to curtail, concentrations of CO2 would ramp up more expeditiously than projected on the footings of recent history.

The processes culpable for enumerating carbon to, and withdrawing it from, the atmosphere is not well enough understood to anticipate forthcoming levels of CO2 with enormous certainty. These processes are a part of the global carbon cycle [4], [5]. Some of the processes that add carbon to the atmosphere or abolish it, such as the kindling of fossil fuels and the formulation of tree plantations, are under forthright human domination. Others, such as the accretion of carbon in the oceans or on land as a consequence of changes in global climate (i.e., feedbacks between the global carbon cycle and climate), are not within unequivocal human restraint except through dominating rates of greenhouse gas emissions and, hence, climatic change. Because CO2 has been more decisive than all of the other greenhouse gases under human control, combined, and is expected to continue so in the future, understanding the global carbon cycle is a significant chunk of admonishing global climate [6], [7]. One of the dynamic key players in this critical scenario of environmental deterioration may be considered as lignocellulosic biomass due to their carbon neutrality feature and wide abundance. Lignocellulosic biomass primarily comprised of lignin and cellulosic compartments, and can be predominantly indicated to the biodegradable organic chunk, which originates through different biological processes and are considered as a budding determinant of renewable energy. Predominantly they can be subcategorized into certain class namely woody biomass, marine algae, agricultural residues, energy crops, etc. [6], [7]. Rational utilization of biomass derived energy need to be bolstered at national as well as international horizon, prior to the consumption of the conventional fossil fuel inventory. European Union (EU) has adopted a goal to obtain 20% of the total energy from renewables by 2020. [8], [9]. Presently biomass subsidizes around 20% of the global energy prerequisites. Utilization of biomass based energy is chiefly observed in rural areas as they are economical and easily accessible. Developed countries have comparatively less dependence on the biomass based energy when compared to the undeveloped nations.  In US and EU only about 5% of the total energy requirement is accosted by biomass. Presently straightforward combustion of biomass in furnace is the most prevailing method to obtain energy in form of electricity and heat. [10] – [12]. This review deals with few major objectives, first on the reservoirs and natural movements of carbon on the earth. It then addresses the antecedents of carbon to the atmosphere from human uses of land and energy and the sinks of carbon on land and in the oceans that have kept the atmospheric accumulation of CO2 lower than it would otherwise have been.

The work portrays changes in the dissemination of carbon amidst the atmosphere, oceans, and terrestrial ecosystems over the past 150 years as a result of human-coaxed exudations of carbon. The processes culpable for sinks of carbon on land and in the sea are abstracted from the perspective of feedbacks, and the work consummates with some green prospects for the future. Global Carbon Cycle Carbon is present in the Earth\’s atmosphere, soils, oceans, and crust. When viewing the Earth as a system, these components can be referred to as carbon pools (sometimes also called stocks or reservoirs) because they act as stockpile houses for enormous chunks of carbon. Any dynamism of carbon between these reservoirs is called a flux. In any integrated system, fluxes hook up reservoirs together to conceive cycles and feedbacks. On a global basis, this process deports considerable chunks of carbon from one pool (the atmosphere) to another (plants). With passing time, these plants die and decay, are resultantly bring harvested by the humans, or are burned either for energy or in wildfires. All of these processes are fluxes that can cycle carbon among distinct pools within ecosystems and eventually releases it back to the atmosphere. Viewing the Earth as a whole, individual cycles like this are linked to others implicating oceans, rocks, etc. on a range of spatial and temporal scales to form a unified global carbon cycle (Figure 1) [13]. Figure 1. A simplified diagram of the global carbon cycle. Carbon Pools The way in which the atmospheric CO2 will change in future, scientists must carefully study the places, where carbon is stored (pools), its residence time and the processes which involve its transfer from one pool to another (fluxes).

Accordingly, all of the considerable pools and fluxes of carbon on Earth encompasses what is referred to as the global carbon cycle. Banking on the aspiration, the Earth’s carbon pools can be organized into any number of contrasting sections. On an expansive scale carbon pool has been divided into four categories [14]. The gigantic chunk of carbon on Earth is accumulated in sedimentary rocks within the planet’s crust. These are rocks produced either by the hardening of mud (containing organic matter) into shale over geological time, or by the assemblage of calcium carbonate flecks, from the shells and skeletons of marine organisms, into limestone and other carbon containing sedimentary rocks. In sync all sedimentary rocks on Earth stockpile 100,000,000 Petagrams of carbon (PgC) where one Petagram is equivalent to 1×1015 grams. Another 4,000 PgC is accumulated in the Earth’s crust as hydrocarbons assembled over millions of years from age-old living organisms under fierce temperature and pressure. These hydrocarbons are commonly known as fossil fuels [15]. Figure 2. Different carbon pools on earth (Source: Schlesinger Biogeochemistry 1997 and 2013 editions) The Earth’s oceans contain 38,000 PgC, most of which is in the form of dissolved inorganic carbon stored at great depths where it resides for long periods of time. A much petite chunk of carbon, approximately 1,000 Pg, is located near the ocean surface. This carbon is swapped spontaneously with the atmosphere through both physical processes, such as CO2 gas dissolving into the water, and biological processes, such as the growth, death and decay of plankton. Although most of this surface carbon cycles rapidly, some of it can also be relocated by sinking to the deep ocean pool where it can be stored for a much protracted period [16]. The atmosphere contains approximately 750 PgC, most of which is in the form of CO2, with much minuscule chunks of CH4 and diversified other compounds. Although the amount of carbon is considerable less than that contained in oceans or crust, carbon in the atmosphere is of fundamental importance because of its clout on the greenhouse effect and climate.

The proportionately small size of the atmospheric C pool also makes it more sensitive to severances caused by a boost in sources or sinks of C from the Earth’s other pools. In fact, the present-day value of 750 PgC is substantially surpassing than that which occurred prior to the inception of fossil fuel combustion and deforestation. Before these activities began, the atmosphere contained approximately 560 PgC and this value is believed to be the normal upper limit for the Earth under natural conditions. In reference to the  global pools and fluxes, the boost that has occurred in the past several centuries is the result of C fluxes to the atmosphere from the crust (fossil fuels) and terrestrial ecosystems (via deforestation and other forms of land clearing)[15].

Terrestrial ecosystems encompass carbon in the form of plants, animals, soils and microbes (bacteria and fungi). Plants and soils are by far the bulkiest among these, when dealing with the entire globe; the smaller pools are often scorned. Unlike the Earth’s crust and oceans, most of the carbon in terrestrial ecosystems prevails in organic forms. In this context, the term “organic” refers to compounds produced by living things, including leaves, wood, roots, dead plant material and the brown organic matter in soils (which is the decomposed remains of formerly living tissues). Plants exchange carbon with the atmosphere relatively rapidly through photosynthesis, in which CO2 is absorbed and converted into new plant tissues, and respiration, where some chunk of the previously captured CO2 is released back to the atmosphere as a product of metabolism. Out of the various kinds of tissues manufactured by plants, woody stems such as those produced by trees have the utmost competence to cache huge chunks of carbon, because wood is dense and trees can be gigantic. The Earth’s plants collectively store approximately 560 PgC, with the wood in trees being the largest fraction.

The total amount of carbon in the world’s soils is estimated to be 1500 PgC. Proper estimation of the soil carbon can be challenging, but a few elemental presumptions can make reckoning it much easier. Primarily, the most prevalent form of carbon in the soil is organic carbon derived from dead and decaying plant materials and microorganisms. Second, as soil depth increases the plethora of organic carbon decreases. Standard soil measurements are typically only taken to 1 meter in depth. In most of these cases, this captures the dominant fraction of carbon in soils, although some environments have very deep soils where this rule doesn’t apply. Most of the carbon in soils enters in the form of dead plant matter that is broken down by microorganisms during decay. The process of decaying helps in the effective releases of carbon back to the atmosphere because the metabolism of these microorganisms eventually breaks most of the organic matter all the way down to CO2 [17]. Carbon Flux The drive of any quantifiable from one residence to another is termed as a flux. Fluxes are usually expressed as a rate with units of an amount of some substance being transferred over a certain period of time (e.g. g cm-1 s-1 or kg km2 yr-1). A single carbon pool may display more than one flux together adding and eradicating carbon concurrently.

The size of different fluxes can vary extensively [14]. Plants obtain energy from sunlight to integrate CO2 from the atmosphere with water from the soil to create carbohydrates (notice that the two parts of the word, carbo- and –hydrate, signify carbon and water) during photosynthesis. In this way, CO2 is evacuated from the atmosphere and stored in the structure of plants. Fundamentally all of the organic material on Earth was primarily formed through this progression. The underlying reason behind that is, some plants are capable to  live tens, hundreds or sometimes even thousands of years old (in the case of the longest-living trees), carbon may be stockpiled, or sequestered, for relatively long periods of time. When plants die, their tissues hover for a wide range of time periods. Tissues such as leaves, which have a high aspect for decomposer organisms, tend to decay expeditiously, while more resistant structures, such as wood can persist much longer. Present estimates advocates that photosynthesis removes 120 PgC/year from the atmosphere and about 610 PgC is stored in plants at any given time [7]. Plants also discharge CO2 back into the atmosphere via the process of respiration (the equivalent for plants of exhaling). Respiration occurs within a plant as a result that plant cells harness carbohydrates, produced during photosynthesis, to acquire energy. Plant respiration represents approximately half (60 PgC/year) of the CO2 that is returned to the atmosphere in the terrestrial portion of the carbon cycle. In accumulation to the demise of entire plants, living plants also shed some chunk of their leaves, roots and branches each year. Because all parts of the plant are made up of carbon, the loss of these fragments into the ground is a transmission of carbon (a flux) from the plant to the soil. Dead plant material is often referred to as litter (leaf litter, branch litter, etc.) and once on the ground, all forms of litter will commence the action of decomposition [17]. Figure 3. Major annual carbon fluxes into and out of the atmosphere Not only the living organisms respires CO2, but respiration of the plants including the micro-organism also results in the release of CO2. When dead organic matter is collapsed down or decomposed (consumed by bacteria and fungi), CO2 is released into the atmosphere at an average rate of about 60 PgC/year globally. Because it can take years for a plant to decompose (or decades in the case of large trees), carbon is momentarily stored in the organic matter of soil [4]. Inorganic carbon is absorbed and discharged at the interface of the oceans’ surface and surrounding air, through the process of diffusion. It may not seem obvious that gasses can be dissolved into, or released from water, but this is what leads to the formation of bubbles that appear in a glass of water left to sit for a long enough period of time. The air contained in those bubbles includes CO2 and this same process is the first step in the uptake of carbon by oceans.

Once in a dissolved form, CO2 goes on to react with water in what are known as the carbonate reactions. These are relatively simple chemical reactions in which H2O and CO2 join to form H2CO3 (also known as carbonic acid, the anion of which is called carbonate). Materialization of carbonate in sea-water permits oceans to take up and store a much superior quantity of carbon than would be conceivable if dissolved CO2 persisted in that form. Carbonate is also significant to an enormous numeral of marine organisms those who uses this mineral form of carbon to build shells [9]. Carbon is also recycled through the ocean via the biological practice of photosynthesis, respiration, and decomposition of aquatic plants. In contrast with terrestrial vegetation is the speed at which marine organisms decompose. Since ocean plants don’t have huge, woody trunks which will take years to breakdown, the process happens much more quickly in oceans than on land—often in a matter of days. Due to this reason, very little amount of carbon is warehoused in the ocean through biological processes. The overall quantity of carbon acceptance (92 PgC) and carbon debt (90 PgC) from the ocean is reliant on the equilibrium of organic and inorganic processes [16]. The carbon fluxes discussed thus far encompass natural practices that have helped to regulate the carbon cycle and atmospheric CO2 levels since millions of years.

However, the modern-day carbon cycle also comprises several important fluxes that stem from human activities. The most imperative of these is combustion of fossil fuels: coal, oil and natural gas. These materials comprise carbon that was apprehended by living organisms over epochs of millions of years and has been stockpiled in various places within the Earth\’s. However, since the onset of the industrial revolution, these fuels have been mined and combusted at increasing rates and have served as a primary foundation of the energy that pushes modern industrial human civilization. Because the main byproduct of fossil fuel combustion is CO2, these happenings may be observed in geological terms as a firsthand and comparatively swift flux to the atmosphere of bulky amounts of carbon. At present, fossil fuel combustion represents a flux to the atmosphere of approximately 6-8 PgC/year. Another human activity that has caused a flux of carbon to the atmosphere is land cover change, largely in the form of deforestation. With the upsurge of the human populace and growth of human reimbursements, a substantial amount of the Earth\’s land surface has been indoctrinated from native ecosystems to farms and urban areas. Native forests in many areas have been cleared for timber or burned for conversion to farms and grasslands. Because forests and other native ecosystems generally contain more carbon (in both plant tissues and soils) than the cover types they have been replaced with, these changes have resulted in a net flux to the atmosphere of about 1.5 PgC/year. In certain areas, regrowth of forests from previous land clearance activities can epitomize a sink of carbon (as in the case of forest growth following farm abandonment in eastern North America), but the net consequence of all human-induced land cover conversions globally represents a source to the atmosphere [18]. Environmental Issues Acids formed by the combustion of fossil fuels (e.g. in smelters for non-ferrous ores, industrial boilers, and transportation vehicles) can be transported over great distances through the atmosphere and deposited via precipitation on the earth on ecosystems that are exceedingly vulnerable to damage from excessive acidity.

This acid precipitation was found to be mainly attributable to emissions of SO2 and NOx. These pollutants have caused only local concerns related to health in the past. However, as cognizance of their involvement of the regional and trans-boundary problem of acid precipitation has grown, concern is now also focusing on other substances such as volatile organic compounds (VOCs), chlorides, ozone and trace metals that may contribute in the multifaceted set of chemical transformations in the atmosphere consequential in acid precipitation and the materialization of other regional air pollutants. The well-known effects of acid precipitation include: acidification of lakes, streams and ground waters, resulting in damage to fish and aquatic life; damage to forests and agricultural crops; and deterioration of materials, e.g., buildings, metal structures and fabrics. Certain energy-related happenings are major source of acid precipitation. For instance, electric power stations, housing heating and manufacturing energy use account for 80% of SO2 releases, with coal use alone accounts for approximately 70% of SO2 emissions. Additional cradle of acid precipitation is sour gas treatment which produces H2S that then reacts to form SO2 when unmasked to air. Road conveyance is an imperative foundation of NOx discharges, accounting for 48% of the over-all discharges in OECD countries. Most of the residual NOx emissions are owing to fossil fuel combustion in immobile sources [19]. It is well known that the ozone present in the stratosphere, roughly between altitudes of 12 and 25 km, plays a natural, equilibrium-maintaining role such as absorption of ultraviolet (UV) radiation and absorption of infrared radiation.

A universal environmental delinquent is the distortion and regional depletion of the stratospheric ozone layer which was revealed to be instigated by chlorofluorocarbons (CFCs), halons (chlorinated and brominated organic compounds) and N2O emissions. Ozone diminution in the stratosphere can result in preeminent levels of damaging ultraviolet radiation reaching the ground, triggering heightened rates of skin cancer, eye impairment and other harm to many biological species. Energy-related activities are only partially (directly or indirectly) responsible for the emissions which lead to stratospheric ozone depletion. Though such energy activities such as fossil fuel and biomass combustion account for 65- 75% of anthropogenic N2O emissions, CFCs, which are used in air conditioning and refrigerating equipment as refrigerants and in foam insulation as blowing agents, play the most important role in ozone depletion. Though scientific debate on ozone depletion has occurred for over a decade, only in 1987 was an international landmark protocol signed in Montreal to reduce the production of CFCs and halons. Conclusive scientific evidence of the destruction of stratospheric ozone by CFCs and halons has recently been gathered, and commitments for more drastic reductions in their production were undertaken at the 1990 London Conference. Replacement products and technologies without CFCs are gradually coming to the fore and should help allow for a total ban of CFCs ultimately. A significant contemplation in such a CFC ban is the prerequisite to dispense equitably the budgetary burdens deriving from the ban, particularly with admiration to developing countries, some of which have financed profoundly in CFC-related technologies [20]. Potentially the most important environmental problem relating to energy utilization is global climate change, also known as the global warming or the greenhouse effect. Swelling absorptions of greenhouse gases such as CO2, CH4, CFCs, halons, N2O, ozone and peroxyacetylnitrate (PAN) in the atmosphere are escalating the manner in which these gases trap heat energy radiated from the earth\’s surface, thereby hovering the surface temperature of the earth. The earth\’s surface temperature has increased about 0.6°C over the last century, and, as a consequence, the sea level is estimated to have risen by perhaps 20 cm. Such changes can have wide-ranging effects on human activities all over the world. Presently it is projected that CO2 subsidizes about 50% to the anthropogenic greenhouse effect. Humankind is contributing through many of its economic and other activities to the increase in the atmospheric concentrations of various greenhouse gases. For example, CO2 releases from fossil fuel incineration, methane discharges from enlarged human activity, CFCs releases and deforestation all subsidize to the greenhouse effect. Energy-related happenings contribute both directly as well as indirectly to the generation of CO2 and other potent greenhouse gases. CO2 emissions from fossil-fuel combustion are projected to account for more than half of the radioactive balance changes caused by greenhouse gases.

Methane emissions, which partially arise from natural gas leaks and coal mines, account for a significant fraction as well. The U.S. Environmental Protection Agency (EPA) study, aimed at determining policy options for stabilizing climate change, concluded that, in a rapidly changing world, it is possible to restrain emissions of greenhouse gases considerably through efforts aimed specifically at changing patterns of energy production and use (e.g. the implementation of energy conservation measures, increased use of biomass and some fuel switching). With the implementation of such actions, the upsurge in realized warming – a significant pointer of global climate change – could be controlled to only 2°C between 2000 and 2100 as compared to 5°C in the most extreme case. The report emphasized, however, that if industrialized countries were to embark on a climate stabilizing strategy without the involvement of developing countries, global temperatures would rise by 3.6°C by 2100. Hence, crucial to the success of future emissions abatement efforts will be the participation of all countries-both industrialized and developing [21], [22]. Ways of Mitigation Disproportionate utilization of fossil fuels now-a-days is escalating the greenhouse gas content in the atmosphere, which eventually consequences in global warming. As a remedial strategy, minimization of the lithospheric carbon reserve and at the same time employing other forms of renewable energy will consequence in the minimization of the atmospheric carbon content resulting as a remedial measure for increasing global warming [23]. The process by which carbon sinks eradicate CO2 from the earth’s atmosphere is termed as carbon sequestration. It can be both a natural and artificial process by which CO2 is removed from the earth’s atmosphere and then stored in liquid or solid form for a long period of time. The preliminary tenacity of undertaking this is the postponement global warming and avoiding extreme climate change. It is significant to note that supplementary forms of carbon are also stockpiled for the period of this sequestration process. A more scientific explanation (and example) is the removal and storage of carbon from the atmosphere to sinks – oceans, soil, forest through physical means and the natural process best known as photosynthesis [24]. The changeover from a fossil fuel- dependent development paradigm towards a development path that takes advantage of bio-based resources and new innovations within biochemistry and life sciences is prompting the formulation of new strategies and policies. With amplified research and inventions on bio-based energy forms, chemicals and materials, the use of the terms bio-based economy has evolved [25]. Presently, around the globe, there is a significant interest in using biomass for power generation as power generation from coal continues to raise environmental concerns. Using just biomass for power generation can bring a lot of environmental benefits. The restrictions of using biomass alone can embrace high investment costs and also the security of the feedstock supply which can be accredited to seasonal supply and in most of the countries biomass is dispersed and the infrastructure for biomass supply is not well recognized (EUROPE, Europe Commission).

The methodological boundaries of utilizing just biomass can be relatively lower heating values, low bulk densities which make large units of biomass to be transported (IEA Clean Coal Center, 2005). To overcome these boundaries conjoining biomass and coal for power cohort can be a potentially viable substitute. Advantages of co-firing biomass along with coal includes a) Coal can reduce the issues related to biomass quality and buffer the system when there is insufficient feedstock quantity and b) costs of adapting the existing coal power plants will be lower than building new systems dedicated only to biomass (IEA Clean Coal Center, 2005). Type of biomass feedstock accessible for energy commitments comprise of agricultural residues, dedicated energy crops, forestry, industry, parks and gardens, waste and other. Table 1 shows the list of biomass feedstock material available for energy recovery, their chemical composition, and few common examples [25]-[29]. Table 1. Classification of biomass and their chemical composition. Supply sector Type Example Cellulose (%) Hemicellulose (%) Lignin (%) Agro residues Dry lignocellulosic Straw, residues etc. 20-50 15-40 20-40 Energy crops Oil and starch energy crops Miscanthus, sugar beet, flax etc. 30-55 20-35 25-35 Forestry Branches, twigs Bark, wood blocks etc. 20-45 25-40 30-55 Industry Wood chips, saw dust Vegetable peels, black liquor 25-50 15-35 25-45 Park & gardens Herbs, grasses Grass, pruning 30-45 25-45 10-30 Waste Contaminated waste Demolition wood, Landfill gas 10-40 5-25 15-35 Others Roadside hay Olive, cacao, almond 15-40 5-35 10-40 Conclusion Population outburst, amplified urbanization has led to mounting dependency on fossil fuel usage to meet the energy demands which in turn raised a gamut of concerns like global warming, acid rain etc.

Continuous upsurge in the CO2 emission has many other damaging effects like polar ice cap melting, increase in net sea level, microbial contamination, floods, droughts etc. The emission from the burning of fossil fuels has depreciated the indoor as well as the outdoor air quality. In order to minimize such unwanted scenarios, energy retrieval from lignocellulosic biomass as a substitute to fossil fuels can be an outstanding methodology. This will also help in the fortification of public well-being. Significant research attention is still required for utilization of biomass in context to energy recovery from them in terms of process simplicity and cost to benefit economics. Pretreatment of biomass for energy densification can be an outstanding methodology for energy retrieval from biomass as because biomass has moderately less heating values as equated to the conventional fossil fuels like coal, petroleum etc. Hence more advanced research methodologies must be developed such that lignocellulosic biomass can be utilized more efficiently which will result in the minimization of the environmental concerns. 

 

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