Do plants use carbon dioxide

K-12 Soil Science Teacher Resources

Carbon Cycle

Why is carbon such a big deal, and what does it have to do with soils?

Did you know that over 2,700 Gigatonnes (Gt) of carbon is stored in soils worldwide?!? This is more than the carbon in all of the plants in the world, AND in the atmosphere! Because Carbon is so important to live on earth, the soils play an important role in the global carbon cycle.

All living things are made of carbon. When animals, bacteria, and other living organisms breathe out, their breath is filled with carbon dioxide. The carbon dioxide is taken out of the atmosphere by plant photosynthesis. This helps plants to grow.

When these plants grow, they create new leaves, roots, and shoots. At the end of the season, leaves fall to the ground, and turn into different types of soil organic matter (see the soil biology page). This dead organic matter creates food for microbes, which respire and create carbon dioxide back to the atmosphere. When plants or the soil are burned, this also releases carbon dioxide into the atmosphere.

In the soil, there are two major types of soil carbon. Biomass, which is the living bacteria and fungi, and non-biomass carbon, which is the cellulose, starch, and lignin in dead plants. Some of these bind soil particles together into soil structure.

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Do Plants Use Carbon: Learn About The Role Of Carbon In Plants

Before we tackle the question of, “How do plants take in carbon?” we must first learn what carbon is and what the source of carbon in plants is. Keep reading to learn more.

What is Carbon?

All living things are carbon based. Carbon atoms bond with other atoms to form chains such as proteins, fats and carbohydrates, which in turn provides other living things with nourishment. The role then of carbon in plants is called the carbon cycle.

How Do Plants Use Carbon?

Plants use carbon dioxide during photosynthesis, the process whereby the plant converts the energy from the sun into a chemical carbohydrate molecule. Plants use this carbon chemical to grow. Once the plant’s life cycle is over and it decomposes, carbon dioxide is formed again to return to the atmosphere and begin the cycle anew.

Carbon and Plant Growth

As mentioned, plants take in carbon dioxide and convert it to energy for growth. When the plant dies, carbon dioxide is given off from the decomposition of the plant. The role of carbon in plants is to foster healthier and more productive growth of the plants.

Adding organic matter, such as manure or decomposing plant parts (rich in carbon – or the browns in compost), to the soil surrounding growing plants basically fertilizes them, feeding and nourishing the plants and making them vigorous and lush. Carbon and plant growth are then intrinsically linked.

What is the Source of Carbon in Plants?

Some of this source of carbon in plants is used to create healthier specimens and some is converted into carbon dioxide and released into the atmosphere, but some of the carbon is locked into the soil. This stored carbon helps to combat global warming by binding to minerals or remaining in organic forms that will slowly break down over time, aiding in the reduction of atmospheric carbon. Global warming is the result of the carbon cycle being out of sync due to the burning of coal, oil and natural gas in large quantities and the resulting vast amounts of gas released from the ancient carbon stored in the ground for millennia.

Amending soil with organic carbon not only facilitates healthier plant life, but it also drains well, prevents water pollution, is beneficial to useful microbes and insects and eliminates the need for using synthetic fertilizers, which are derived from fossil fuels. Our dependency upon those very fossil fuels is what got us into this mess in the first place and utilizing organic gardening techniques is one way to combat the global warming debacle.

Whether carbon dioxide from the air or organic carbon in soil, the role of carbon and plant growth is extremely valuable; in point of fact, without this process, life as we know it would not exist.

How Plants Take Elements from the Soil

By Rene Fester Kratz

Plants get all the carbon, hydrogen, and oxygen they need from carbon dioxide and water, which they use to build carbohydrates during photosynthesis. To build other kinds of molecules they also need elements like nitrogen, phosphorous, and sulfur. Plants get these as well as other elements from the soil.

Just like you do, plants build their cells from carbohydrates, proteins, lipids, and nucleic acids. The difference between you and a plant is that you get all these molecules from your food, but plants need to build them for themselves.

Many elements exist as dissolved minerals in the soil. When plants absorb water through their roots, they also absorb minerals that act as both macronutrients and micronutrients. Macronutrients help with molecule construction, and micronutrients act as partners for enzymes and other proteins to help them function. Plants generally require large amounts of macronutrients and smaller amounts of micronutrients. The table lists the specific macronutrients and micronutrients plants absorb from soil.

You can remember the most important elements for plants with the phrase C. Hopkins Café, Mighty Good. The CHOPKNS CaFe Mg stands for carbon, hydrogen, oxygen, phosphorous, potassium, nitrogen, sulfur, calcium, iron, and magnesium. All these elements are macronutrients for plants, with the exception of iron, which is considered a micronutrient.

If plants don’t get enough of one of these important elements, they can’t function correctly. Without carbon, hydrogen, and oxygen (from carbon dioxide and water), plants can’t grow at all. And even though plants need smaller amounts of other elements, each missing element causes a specific problem.

Test your understanding of plant nutrition with the following questions.

For numbers 1-5, use the following terms to identify whether the nutrient is a macronutrient or a micronutrient.

a. Macronutrient

b. Micronutrient

  1. A plant gets sulfur from sulfate (SO4–) in the soil.

  2. A plant takes in calcium as calcium salts from the soil.

  3. A plant takes in nitrogen as nitrates (NO32–) from the soil.

  4. A plant takes in magnesium (Mg) from the soil.

  5. A plant absorbs copper (Cu) from the soil.

  6. True or false: Plants get carbon from the soil.

  7. True or false: A plant’s weight comes mostly from the minerals it takes from the soil.

The following are the answers to the practice questions:

  1. The answer is a. Macronutrient.

  2. The answer is a. Macronutrient.

  3. The answer is a. Macronutrient.

  4. The answer is a. Macronutrient.

  5. The answer is b. Micronutrient.

  6. The answer is false.

    Plants get carbon from the air as carbon dioxide.

  7. The answer is false.

    Although plants take minerals from the soil, the amount of these minerals is very small compared to the proteins, carbohydrates, lipids, and nucleic acids that make up the plant’s body. All these big molecules have a carbon backbone, so carbon atoms make up the majority of a plant’s weight. Plants get carbon from carbon dioxide in the atmosphere.

Soil Carbon Storage

Organic matter is a key component of soil that affects its physical, chemical, and biological properties, contributing greatly to its proper functioning on which human societies depend. Benefits of soil organic matter (SOM) include improvement of soil quality through increased retention of water and nutrients, resulting in greater productivity of plants in natural environments and agricultural settings. SOM improves soil structure and reduces erosion, leading to improved water quality in groundwater and surface waters, and ultimately to increased food security and decreased negative impacts to ecosystems. Since the beginnings of recorded history, societies have understood that human activities can deplete soil productivity and the ability to produce food (McNeill and Winiwarter 2004). Only in recent history has the understanding of soil productivity been tied to SOM levels, with the depletion of SOM stocks often leading to large-scale impacts on whole ecosystems as well as the entire planet. For example, destruction of rainforests that hold a significant amount of the carbon stored in terrestrial ecosystems contributes significantly to rising atmospheric carbon dioxide (CO2) levels linked to climate change, while reductions in SOM levels from soil disturbance from mining can impact infiltration of rainfall and the storage of soil moisture important for flood mitigation. Soil disturbance also leads to increased erosion and nutrient leaching from soils, which have led to eutrophication and resultant algal blooms within inland aquatic and coastal ecosystems, ultimately resulting in dead zones in the ocean (Fig. 1). Restoration of organic matter levels in soil requires an understanding of the ecological processes important for SOM storage. Proper restoration techniques can help restore terrestrial ecosystem functions.

Figure 1: Summer algal conditions along the US Gulf Coast. Red indicates high concentrations of algae due to nutrients flowing into the Gulf of Mexico, primarily from the Mississippi River basin. © 2012 Nature Education Photo courtesy of NASA/Goddard Space Flight Center Scientific Visualization Studio. All rights reserved.

Fundamentals of Soil Organic Carbon

Soil organic matter is composed of soil microbes including bacteria and fungi, decaying material from once-living organisms such as plant and animal tissues, fecal material, and products formed from their decomposition. SOM is a heterogeneous mixture of materials that range in stage of decomposition from fresh plant residues to highly decomposed material known as humus. SOM is made of organic compounds that are highly enriched in carbon. Soil organic carbon (SOC) levels are directly related to the amount of organic matter contained in soil and SOC is often how organic matter is measured in soils.

SOC levels result from the interactions of several ecosystem processes, of which photosynthesis, respiration, and decomposition are key. Photosynthesis is the fixation of atmospheric CO2 into plant biomass. SOC input rates are primarily determined by the root biomass of a plant, but also include litter deposited from plant shoots. Soil C results both directly from growth and death of plant roots, as well as indirectly from the transfer of carbon-enriched compounds from roots to soil microbes. For example, many plants form symbiotic associations between their roots and specialized fungi in the soil known as mycorrhizae; the roots provide the fungi energy in the form of carbon while the fungi provide the plant with often-limiting nutrients such as phosphorus. Decomposition of biomass by soil microbes results in carbon loss as CO2 from the soil due to microbial respiration, while a small proportion of the original carbon is retained in the soil through the formation of humus, a product that often gives carbon-rich soils their characteristic dark color (Fig. 2). These various forms of SOC differ in their recalcitrance, or resistance to decomposition. Humus is highly recalcitrant, and this resistance to decomposition leads to a long residence time in soil. Plant debris is less recalcitrant, resulting in a much shorter residence time in soil. Other ecosystem processes that can lead to carbon loss include soil erosion and leaching of dissolved carbon into groundwater. When carbon inputs and outputs are in balance with one another, there is no net change in SOC levels. When carbon inputs from photosynthesis exceed C losses, SOC levels increase over time.

Figure 2: Carbon balance within the soil (brown box) is controlled by carbon inputs from photosynthesis and carbon losses by respiration. Decomposition of roots and root products by soil fauna and microbes produces humus, a long-lived store of SOC. © 2012 Nature Education All rights reserved.

Photosynthesis, decomposition, and respiration rates are determined partly by climatic factors, most importantly soil temperature and moisture levels. For example, in the cold wet climates of the northern latitudes, rates of photosynthesis exceed decomposition resulting in high levels of SOC (Fig. 3). Arid regions have low levels of SOC mostly due to low primary production, while the tropics often have intermediate SOC levels due to high rates of both primary productivity and decomposition from warm temperatures and abundant rainfall. Temperate ecosystems can have high primary productivity during summer when temperature and moisture levels are highest, with cool temperatures during the rest of the year slowing decomposition rates such that organic matter slowly builds up over time (Fig. 4). While climatic conditions largely generate global patterns of soil carbon, other factors that vary on smaller spatial scales interact with climate to determine SOC levels. For example, soil texture — the relative proportions of sand, silt, and clay particles that make up a particular soil — or the mineralogy of those soil particles can have a significant impact on soil carbon stocks. Additionally, the processes of erosion and deposition act to redistribute soil carbon according to the topography of the landscape, with low-lying areas such as floodplains often having increased SOC relative to upslope positions.

Figure 3: World map showing the quantity of SOC to 1 m depth. © Nature Education Photo courtesy of USDA Natural Resources Conservation Service. All rights reserved.
Figure 4: Dark colored topsoil showing high levels of SOC due to abundant plant roots and their associated soil fauna and microbes in a cultivated soil in central Iowa. © 2012 Nature Education Photo courtesy of Todd Ontl. All rights reserved.

Soil Carbon and the Global Carbon Cycle

The amount of C in soil represents a substantial portion of the carbon found in terrestrial ecosystems of the planet. Total C in terrestrial ecosystems is approximately 3170 gigatons (GT; 1 GT = 1 petagram = 1 billion metric tons). Of this amount, nearly 80% (2500 GT) is found in soil (Lal 2008). Soil carbon can be either organic (1550 GT) or inorganic carbon (950 GT). The latter consists of elemental carbon and carbonate materials such as calcite, dolomite, and gypsum (Lal 2004). The amount of carbon found in living plants and animals is comparatively small relative to that found in soil (560 GT). The soil carbon pool is approximately 3.1 times larger than the atmospheric pool of 800 GT (Oelkers & Cole 2008). Only the ocean has a larger carbon pool, at about 38,400 GT of C, mostly in inorganic forms (Houghton 2007).

Soil Carbon and Climate Change

There is a growing body of evidence supporting the hypothesis that the earth’s climate is rapidly changing in response to continued inputs of CO2 and other greenhouse gases (GHGs) to the atmosphere resulting from human activities (IPCC 2007). While a suite of GHGs exist (e.g., N2O, CH4), CO2 has the largest effect on global climate as a result of enormous increases from the preindustrial era to today. Atmospheric CO2 concentrations have risen from approximately 280 parts per million (ppm) prior to 1850, to 381.2 ppm in 2006 (WMO 2006), with a current annual increase of 0.88 ppm (3.5 GT C/yr) (IPCC 2007). Approximately two-thirds of the total increase in atmospheric CO2 is a result of the burning of fossil fuels, with the remainder coming from SOC loss due to land use change (Lal 2004), such as the clearing of forests and the cultivation of land for food production (Fig. 5).
Figure 5: Deforestation around Rio Branco, Brazil. Light colored areas are where rainforest vegetation has been cleared and burned (see smoke plume) for farming and cattle ranching. © 2012 Nature Education Photo courtesy of NASA Earth Observatory. All rights reserved.

While the carbon released to the atmosphere through deforestation includes carbon emitted from the decomposition of aboveground plant biomass, carbon levels in the soil are also rapidly depleted from the decomposition of SOM. The decomposition of SOM is due to the activity of the microbial decomposer community in the absence of continual rates of carbon input from the growth of forest vegetation, as well as increased soil temperatures that result from warming of the ground once the forest canopy has been removed. Although this soil carbon loss has contributed to increased CO2 levels in the atmosphere, it also is an opportunity to store some of this carbon in soil from reforestation.

Despite the much larger size of the oceanic carbon pool relative to the soil carbon pool, the rate of exchange between the atmosphere and the soil is estimated to be higher than that between the atmosphere and the ocean. Current estimates are that carbon inputs from photosynthesis by terrestrial vegetation fixes more carbon than carbon loss through soil respiration, resulting in a soil storage rate of about 3 GT C/yr. Oceanic carbon flux rates suggest oceans store about 2 GT carbon/yr despite occupying a vastly larger proportion of the earth’s surface. Although there is interest in increasing oceanic carbon storage rates through large-scale nutrient additions, there is skepticism towards this approach due to the unknown consequences on global nutrient cycles and marine ecosystems (Cullen & Boyd 2008). The goal of increased storage of carbon in soil has received much wider acceptance due to a better understanding of the processes involved in SOC storage, more direct control of these processes through human activities, and the other known ecosystem benefits to be obtained by increasing SOC, including benefits to water quality and increased food security.

Soil Carbon Sequestration

Soil carbon sequestration is a process in which CO2 is removed from the atmosphere and stored in the soil carbon pool. This process is primarily mediated by plants through photosynthesis, with carbon stored in the form of SOC. In arid and semi-arid climates, soil carbon sequestration can also occur from the conversion of CO2 from air found in soil into inorganic forms such as secondary carbonates; however, the rate of inorganic carbon formation is comparatively low (Lal 2008).

Since the industrial revolution, the conversion of natural ecosystems to agricultural use has resulted in the depletion of SOC levels, releasing 50 to 100 GT of carbon from soil into the atmosphere (Lal 2009). This is the combined result of reductions in the amount of plant roots and residues returned to the soil, increased decomposition from soil tillage, and increased soil erosion (Lemus & Lal 2005). Depletion of SOC stocks has created a soil carbon deficit that represents an opportunity to store carbon in soil through a variety of land management approaches. However, various factors impact potential soil carbon change in the future, including climatic controls, historic land use patterns, current land management strategies, and topographic heterogeneity.

Continued increases in atmospheric CO2 and global temperatures may have a variety of different consequences for soil carbon inputs via controls on photosynthetic rates and carbon losses through respiration and decomposition. Experimental work has shown that plants growing in elevated CO2 concentrations fix more carbon through photosynthesis, producing greater biomass (Drake et al. 1997). However, carbon loss may also increase due to increased plant respiration from greater root biomass (Hungate et al. 1997), or from accelerated decomposition of SOM through increased microbial activity (Zak et al. 2000). Likewise, increased temperatures may impact the carbon balance by limiting the availability of water, and thus reducing rates of photosynthesis. Alternatively, when water is not limiting, increased temperatures might increase plant productivity, which will also impact the carbon balance (Maracchi et al. 2005). Increased temperatures may also lead to higher rates of SOM decomposition, which may in turn produce more CO2, resulting in positive feedbacks on climate change (Pataki et al. 2003).

At the scale of a watershed or crop field, the carbon sequestration capacity of the soil may be influenced by local controls on ecosystem processes. Processes such as rainfall infiltration, soil erosion and deposition of sediment, and soil temperature can vary on local scales due to landscape heterogeneity — all of which affect carbon input and carbon loss rates (Fig. 6), resulting in differences in SOC contents along topographic gradients (Thompson and Kolka 2005). For example, slope position impacts soil moisture and nutrient levels, with subsequent impacts on the root growth of plants that may have consequences for soil carbon (Ehrenfeld et al. 1992). The combined effects of changes in carbon inputs and losses from land use, land management, and landscape-level effects on carbon input and loss rates result in variation in the carbon sequestration capacity across landscapes.

Figure 6: Landscape heterogeneity due to landscape position along a hill slope and possible effects on biophysical processes that effect carbon inputs and losses. Darker areas on bars indicate higher rates. © 2012 Nature Education Photo courtesy of Todd Ontl. All rights reserved.

Carbon sequestration potential may be determined by an understanding of both the historic SOC stocks under natural vegetation prior to conversion to other uses and the influences of those land uses on carbon loss. Land uses and management that reduce carbon inputs or increase losses compared to natural vegetation result in reductions in SOC over time, creating a soil carbon deficit relative to the levels of carbon that previously existed in the soil. This deficit represents an opportunity to store carbon from conversions in land use and management when those changes result in either increased inputs or decreased losses of carbon. For example, reforestation or grassland restoration on a former crop field can reduce the carbon deficit caused from years of agricultural production and sequester carbon through higher root productivity compared to crops. Likewise, the creation of wetlands and ponds can sequester large amounts of carbon because decomposition is greatly reduced in waterlogged soils from lack of oxygen; this can actually result in carbon gains that exceed the deficits resulting from past land use. Other management practices such as irrigation of pasture or rangelands may also increase carbon levels beyond historic SOC stocks if carbon inputs under new management greatly exceed levels under natural conditions. The effect of land management on SOC levels, especially the impacts of management in agricultural settings, is the subject of much current research (Table 1). These changes in soil carbon, however, typically take many decades to occur, making actual measurements of changes in SOC stocks difficult.

Table 1: Possible management practices for increasing SOC levels through reduced carbon losses and increased carbon inputs in agricultural systems. © 2012 Nature Education All rights reserved.


SOC is a vital component of soil with important effects on the functioning of terrestrial ecosystems. Storage of SOC results from interactions among the dynamic ecological processes of photosynthesis, decomposition, and soil respiration. Human activities over the course of the last 150 years have led to changes in these processes and consequently to the depletion of SOC and the exacerbation of global climate change. But these human activities also now provide an opportunity for sequestering carbon back into soil. Future warming and elevated CO2, patterns of past land use, and land management strategies, along with the physical heterogeneity of landscapes are expected to produce complex patterns of SOC capacity in soil.

The importance of carbon in the soil

The carbon cycle is a fundamental part of life on earth. ‘Soil organic carbon’ (SOC) – the amount of carbon stored in the soil is a component of soil organic matter – plant and animal materials in the soil that are in various stages of decay.

Soil organic carbon is the basis of soil fertility. It releases nutrients for plant growth, promotes the structure, biological and physical health of soil, and is a buffer against harmful substances.

Soil organic carbon is part of the natural carbon cycle, and the world’s soils holds around twice the amount of carbon that is found in the atmosphere and in vegetation. Organic material is manufactured by plants using carbon dioxide from the air and water. Plants (and animals, as part of the food chain), die and return to the soil where they are decomposed and recycled. Minerals are released into the soil and carbon dioxide is released into the atmosphere.

Soil organic carbon accounts for less than 5% on average of the mass of upper soil layers, and diminishes with depth. According to the CSIRO, in rain-forests or good soils, soil organic carbon can be greater than 10%, while in poorer or heavily exploited soils, levels are likely to be less than 1%.1

The amount of soil organic carbon present in soil can vary hugely according to soil and landscape types,an and c climate an change in the same paddock over time depending on climate and farming methods. Temperature, rainfall, land management, soil nutrition and soil type all influence soil organic carbon levels.

In Australia, soil carbon levels have dropped by up to half of pre-agricultural levels in many areas because of activities such as fallowing, cultivation, stubble burning or removal and overgrazing.2

Increasing soil organic carbon has two benefits – as well as helping to mitigate climate change, it improves soil health and fertility. Many management practices that increase soil organic carbon also improve crop and pasture yields.

According to the NSW Department of Primary Industry, although there is a limit on the amount of organic carbon that can be stored in soils, the large losses in the past means that many of Australia’s agricultural soils have the potential for a large increase in soil organic carbon.3

If more carbon is stored in the soil as organic carbon, it will reduce the amount present in the atmosphere, and help reduce global warming.4 The process of storing carbon in soil is called ‘soil carbon sequestration’.

Some of the practices that increase soil organic carbon include conservation farming (reducing or eliminating tillage and retaining stubble from previous crops), improving crop management (e.g. through better rotation), maintaining and improving tree/forestry management, improving grazing management and adding organic materials such as composts and manures. For more details see win-win carbon farming practices.

For further information see NSW Department of Primary Industry’s Increasing Soil Organic Carbon fact sheet and CSIRO’s information about soil organic carbon.

Climate change skeptics have an arsenal of arguments for why humans need not cut their carbon emissions. Some assert rising CO2 levels benefit plants, so global warming is not as bad as scientists proclaim. “A higher concentration of carbon dioxide in our atmosphere would aid photosynthesis, which in turn contributes to increased plant growth,” Rep. Lamar Smith (R–Texas) wrote in an op-ed last year. “This correlates to a greater volume of food production and better quality food.” Scientists and others calling for emission cuts are being hysterical, he contends.

So is it true rising atmospheric CO2 will help plants, including food crops? Scientific American asked several experts to talk about the science behind this question.

There is a kernel of truth in this argument, experts say, based on what scientists call the CO2 fertilization effect. “CO2 is essential for photosynthesis,” says Richard Norby, a corporate research fellow in the Environmental Sciences Division and Climate Change Science Institute of Oak Ridge National Laboratory. “If you isolate a leaf and you increase the level of CO2, photosynthesis will increase. That’s well established.” But Norby notes the results scientists produce in labs are generally not what happens in the vastly more complex world outside; many other factors are involved in plant growth in untended forests, fields and other ecosystems. For example, “nitrogen is often in short enough supply that it’s the primary controller of how much biomass is produced” in an ecosystem, he says. “If nitrogen is limited, the benefit of the CO2 increase is limited…. You can’t just look at CO2, because the overall context really matters.”

Scientists have observed the CO2 fertilization effect in natural ecosystems, including in a series of trials conducted over the past couple decades in outdoor forest plots. In those experiments artificially doubling CO2 from pre-industrial levels increased trees’ productivity by around 23 percent, according to Norby, who was involved in the trials. For one of the experiments, however, that effect significantly diminished over time due to a nitrogen limitation. That suggests “we cannot assume the CO2 fertilization effect will persist indefinitely,” Norby says.

In addition to ignoring the long-term outlook, he says, many skeptics also fail to mention the potentially most harmful outcome of rising atmospheric CO2 on vegetation: climate change itself. Its negative consequences—such as drought and heat stress—would likely overwhelm any direct benefits that rising CO2 might offer plant life. “It’s not appropriate to look at the CO2 fertilization effect in isolation,” he says. “You can have positive and negative things going at once, and it’s the net balance that matters.” So although there is a basic truth to skeptics’ claim, he says, “what’s missing from that argument is that it’s not the whole picture.”

Scientists have also looked specifically at the effects of rising CO2 on agricultural plants and found a fertilization effect. “For a lot of crops, is like having extra material in the atmosphere that they can use to grow,” says Frances Moore, an assistant professor of environmental science and policy at the University of California, Davis. She and other experts note there is an exception for certain types of plants such as corn, which access CO2 for photosynthesis in a unique way. But for most of the other plants humans eat—including wheat, rice and soybeans—“having higher CO2 will help them directly,” Moore says. Doubling CO2 from pre-industrial levels, she adds, does boost the productivity of crops like wheat by some 11.5 percent and of those such as corn by around 8.4 percent.

A lack of nitrogen or other nutrients does not affect agricultural plants as much as wild ones, thanks to fertilizer. Still, research shows plants “get some benefits early on from higher CO2, but that starts to saturate” after the gas reaches a certain level, Moore says—adding, “The more CO2 you have, the less and less benefit you get.” And while rising carbon dioxide might seem like a boon for agriculture, Moore also emphasizes any potential positive effects cannot be considered in isolation, and will likely be outweighed by many drawbacks. “Even with the benefit of CO2 fertilization, when you start getting up to 1 to 2 degrees of warming, you see negative effects,” she says. “There are a lot of different pathways by which temperature can negatively affect crop yield: soil moisture deficit heat directly damaging the plants and interfering with their reproductive process.” On top of all that, Moore points out increased CO2 also benefits weeds that compete with farm plants.

Rising CO2’s effect on crops could also harm human health. “We know unequivocally that when you grow food at elevated CO2 levels in fields, it becomes less nutritious,” notes Samuel Myers, principal research scientist in environmental health at Harvard University. “ lose significant amounts of iron and zinc—and grains lose protein.” Myers and other researchers have found atmospheric CO2 levels predicted for mid-century—around 550 parts per million—could make food crops lose enough of those key nutrients to cause a protein deficiency in an estimated 150 million people and a zinc deficit in an additional 150 million to 200 million. (Both of those figures are in addition to the number of people who already have such a shortfall.) A total of 1.4 billion women of child-bearing age and young children who live in countries with a high prevalence of anemia would lose more than 3.8 percent of their dietary iron at such CO2 levels, according to Meyers.

Researchers do not yet know why higher atmospheric CO2 alters crops’ nutritional content. But, Myers says, “the bottom line is, we know that rising CO2 reduces the concentration of critical nutrients around the world,” adding that these kinds of nutritional deficiencies are already significant public health threats, and will only worsen as CO2 levels go up. “The problem with argument is that it’s as if you can cherry-pick the CO2 fertilization effect from the overall effect of adding carbon dioxide to the atmosphere,” Myers says. But that is not how the world—or its climate—works.

The Carbon Cycle

Carbon (C) is the basis of life on Earth. Scientists consider 99.9% of all organisms on the planet to be carbon based life. Those organisms need carbon to survive. Whether the carbon is in the form of a sugar or carbon dioxide gas, we all need it. Unlike energy, carbon is continuously cycled and reused. The Earth only has a fixed amount of carbon. The carbon cycle is the ultimate form of recycling.

Start With Plants

Plants are a good starting point when looking at the carbon cycle on Earth. Plants have a process called photosynthesis that enables them to take carbon dioxide out of the atmosphere and combine it with water. Using the energy of the Sun, plants make sugars and oxygen molecules. All of the non-photosynthetic creatures on the planet use the oxygen. Every creature on the planet uses the sugars and starches created by plants.

Then Animals Eat The Plants

Animals are the non-photosynthetic creatures of the planet. They are not able to create their own food. Instead, they eat plants or other animals. The sugars and starches they eat are broken down by a process of metabolism. The results are energy for the creature, water, and carbon dioxide molecules. The carbon dioxide then returns to the atmosphere where the plants use it again.

Who Eats The Animals?

There are also decomposers involved in the carbon cycle. They break down organic material such as dead animals, poop, or leaves. Decomposers are able to break down the chemical compounds inside the body. They also release carbon dioxide as well as methane.
Sometimes the decomposers don’t break down organic material. There are great oil fields under the surface that are made of plants that did not decompose millions of years ago. There are also layers of rock made of millions of creatures who had shells. One day this carbon will return to the everyday carbon cycle, but geological processes are much slower than living processes.
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Carbon dioxide’s role and management in the growing environment

It is impossible to overstate the impact and influence of carbon dioxide on all aspects of plant growth. Carbon alone accounts for 45% of the dry weight of any plant. Only oxygen has an equal standing at 45%, with hydrogen at 6% completing the top three elements required for a plant to survive. Carbon is a nutrient, just as surely as nitrogen is a nutrient, but since it is derived from the air, it is often overlooked.

By Geary Coogler, B.S. Horticulture

Carbon comes in a convenient plant-available form, carbon dioxide (CO2), a gas. Plants have become excellent assimilators of carbon, and employ similar – but not identical – systems to ensure they capture enough CO2 to grow and mature. All of this to acquire the simple building blocks that they need to grow and reproduce. Any discussion surrounding carbon dioxide should begin with an understanding of carbohydrates: what they are, and how they are made.

Figure 1: A Tropical mangrove tree is better at storing CO2 than any other type of tree. They store two to four times the carbon that tropical rainforests do. For this reason, cutting down a coastal mangrove forest is much worse for the environment than cutting down any other forest of the same size.

The Product

Carbohydrates are the basic building blocks of all things classified as life. Some life forms produce carbohydrates, some life forms consume carbohydrates. Carbohydrates are a group of organic compounds that contain only carbon, hydrogen and oxygen usually in a 1:2:1 ratio. They include sugars, starches, cellulose, and many others. They are the major energy source for animals and are produced by photosynthesis in plants. Animals obtain their carbon by consuming carbon as carbohydrates and proteins, while plants get their carbon by assimilating a gas, CO2. There are two basic processes that occur to produce basic carbohydrates in a plant: the first is what is known as the light reaction and the second the dark reaction.

Both occur in the chloroplast. The light reaction is, as the name implies, the reaction that occurs in the presence of light photons and converts light energy into chemical energy, ATP (Adenosine-5’-triphosphate) and NADPH (Nicotinamide adenine dinucleotide phosphate). These energy molecules are then used in the dark reaction (more appropriately called the carbon reaction) to produce triose phosphate, the basic carbohydrate. In reality, the dark reaction still requires light to function, not only to produce energy, but for many other light-influenced controls. This dark reaction is known as the Calvin Cycle.

Figure 2: The Calvin Cycle occurs in two stages, the light and dark reactions and makes all life possible, but both require the energy of the sun (light).

In this cycle, a base carbohydrate is used as a seed molecule (known as a substrate molecule) with 5 carbons to capture CO2 with the help of a light-activated enzyme known as Rubisco that also adds water molecules and is ultimately split into 3 carbon molecules that undergo more changes to become a carbohydrate, triose phosphate. The problem is:

  1. 85% of the base carbohydrates produced return to the system; and
  2. Oxygen (O2) is more attractive than CO2 and destroys the seed molecules stopping the process; this is known as photorespiration.

The Process

Air from around the leaves enters the leaf tissue through specialized pores known as stomata. Once inside, the molecules in the air will dissolve into the fluid around the inner plant cells, mesophyll cells, and then on through the cell membranes, across the cytosol, and to the chloroplast in this liquid phase. From the moment the CO2 approaches the stomata, it has to go through several areas of resistance including the boundary at the opening of the stomata, the interior air boundaries including the stomatal cavity, and the intercellular air space. This resistance is caused by the lack of movement in the air, meaning that molecules like CO2 have to diffuse slowly across the opening of their own accord.

Gas molecules will disperse quickly when there are currents of air, but in the calm stillness of a boundary, they have to move under their own vibration or the action of another molecule – a very slow process. Once in a liquid state, the CO2 molecules then have to diffuse across and through the cells until they can be integrated into the Calvin Cycle. The problem remains that there is much more oxygen than carbon dioxide in the air, so the O2 is moving right along with the CO2 but at a higher concentration.

Once they become part of the Calvin Cycle, there will be more O2 available than CO2, unless the concentration is increased or the Calvin Cycle is isolated into areas from which oxygen can be excluded or limited. The process we have described so far is both the basic idea in the conversion of light energy into carbohydrates, and the exact description of one of the three types of metabolism plants employ to do this. This one is known as the C3 cycle (also known as C-3 metabolism); it is the preferred cycle of most dicots but some monocots as well.

The next cycle is the C4 cycle, so named because it captures the CO2 in a 4-carbon sugar in the mesophyll cells, and transports it to the chloroplasts in bundle sheath cells to be processed in the Calvin Cycle, thus limiting the amount of oxygen in the process. C4 plants include all angiosperm plants and most monocots such as grass, maize, and sugar cane. C4 plants do better in hot dry locations because the process that causes the photorespiration mentioned earlier increases with temperature. Both C3 and C4 cycles require light to function. There is one more C4 variant which performs part of the cycle in daylight followed by another part during the night. These are known as CAM (crassulacean acid metabolism) (see figure 3).

Figure 3: Leaf cross section showing boundary layers.

Carbon dioxide needs to be allowed in, to make carbohydrates and to do this the stomata have to open. Once they are open, not only can CO2 enter, but water can escape, and so limiting the opening time of the stomata to the hours of darkness is a clear evolutionary advantage. There is one other important note here, C3 plants work better at higher concentrations of CO2 whereas C4 plants do better at lower concentrations of the gas because they actively pump the gas into the cycle.

The Environment

In plants where the CO2 moves into the Calvin Cycle of its own accord (C3), the concentration of CO2 in the cell will pretty much equal the atmospheric concentration. A growth increase of 30 – 60% has been documented when CO2 is supplied at 600 – 700 ppm in C3 plants. C4 plants, on the other hand, had no such advantage because in these plants the CO2 is actively captured, transported, and concentrated at the same rate, no matter what the atmospheric concentration is. The ideal CO2 level for C4 plants is the naturally occurring ambient level. So some plants do respond to higher levels of CO2, but photosynthesis reaches a maximum rate (saturation) at a given point, and going higher than this is pointless. At really high levels above 3% or 3000 ppm, the rate falls off. The ideal level for C3 plants seems to be 600 – 700 ppm; any more than this will not produce any additional benefit so it will be a waste of money. Keep in mind that this level is what should be available around the stomata.

The boundary resistance we mentioned earlier applies to the area immediately adjacent to the stomata opening. When the air is still, the air under or next to leaves and inside the physical structure of the plant will have lower levels of CO2 and O2 than the air because the plant is removing these elements. This air will also be more humid because the plant is transpiring to cool itself and transport nutrients from the root system. In short there is a microclimate right next to the plant’s leaves.

The lower levels of CO2 will mean that less is available in the cells and photosynthesis will slow down. The higher humidity in the air means that less water can evaporate out of the plant and so there will be less movement of water through the plant. By creating a movement of air, we can make sure that more of the necessary components come close to the stomata, while unnecessary components like water are dispersed.

Putting this into Practice

Clearly, the CO2 cycle in the plant is part of other systems and affected by many factors including temperature, light, and many more. The movement of CO2 is only a small part of what is actually going on. We have sought to simplify a very complex process, but along the way, we have uncovered some very important information.

There are differences in the ways that plants capture carbon dioxide. When CO2 is limited, so is the production of substrates. CO2 is only used during the light cycle, even though some plants can capture it at night to use during the day. Temperature has been shown to be an issue especially at upper ranges. Certain plants (C3) benefit from higher ambient levels of CO2 while for others (C4 and CAM) adding CO2 is a waste of resources. There are many barriers that the CO2 molecule has to cross to become part of the Calvin Cycle. Air movement at all levels is critical to eliminate the micro-climate associated with leaf surfaces.

Figure 4: The CAM plant has an evolutionary advantage of doing part of its cycle at night by storing the precursor Malic acid made during the day, in the vacuole of the cell to be converted at night when the stomata has to open, in order to conserve water loss.

Now, how can we make this information work for us?

Outdoor crops and indoor crops have different capabilities. Outdoors, little can be controlled and crops are basically at the mercy of nature. Indoors, the restrictions of technology and resources apply. The benefits of air movement apply to all plants, so this is the first thing to look at. For indoor plants, fans are essential to move the air inside the growing area, as well as to exchange the old air for CO2 refreshed air. For outdoor plants, increasing the spacing between plants will help control some micro-climate formation, or at least its severity. Enriching the air with CO2 in covered crops (indoor or tented) is an option, but is almost useless for C4 plants.

If a grower has C3 plants and can control the atmosphere, then he or she can add extra CO2. There are two basic ways to apply CO2. The first is to use bottled CO2 gas released through a regulation system and the second is a burner system, whereby gas is burned in the room to produce CO2 as a by-product. Both methods have drawbacks. Bottled systems require many bottles and frequent trips to have them refilled. Gas burners are less expensive and require less effort; however burners can malfunction causing a range of issues including safety.

Temperature affects photosynthesis. In indoor crops, the plants need light to function but lighting increases temperature. Throw a CO2 burner into the mix and overheating can rapidly become an issue. Venting to the outside and drawing in fresh air will take away all the CO2 that has been applied. It is pointless to apply CO2 during the dark when things are cooler because it will not work. So what can be done? Well, it really depends on the money. If the money is there, first arrange for good air movement in the growth zone of the plants in the room, use air conditioning in the room or water chiller units and leave the room closed, switch lights to sealed air-cooled lights, use bottled CO2, or vent when needed and replace the CO2. Any of these measures will work but some are better for hotter environments.

Figure 5: CO2 is a term often used negatively when talking about the excesses of our consumer society. It’s an invisible gas that can cause great harm to the environment. ‘However, in small doses and at the right time, CO2 isn’t that bad.

During periods of high heat stress, the crop can be saved by shading or switching off lights. CO2 use will also fall off so stop supplying it. Watch the humidity because plants can go a long way in cooling themselves when moisture conditions are right for good water movement. However, there needs to be enough water available at all times for this, otherwise leaf wilting and tissue damage can result. Too much water, however, will slow down transpiration enough to reduce the cooling effect. Above all, balance the system by using workable set-points and automated controls when working out the enrichment times and ventilation. Always calibrate the monitoring equipment.

Carbon-skinned balloon

In the end, plants need carbon dioxide. The gas becomes the plant in every way, like a carbon-skinned water balloon. Plants obtain the carbon they need in different ways, or rather the same way with an extra step or two thrown in. Because of the differences, some plants do well with higher concentrations of ambient CO2 while others will be indifferent. Growth can be affected positively by managing the CO2 as a ‘nutrient’. It is possible to benefit from the introduction of additional CO2, but all other factors for growth – such as temperature and humidity – also have to be taken into account.

Basically a plant is a kind of carbonaccumulating machine. As such, carbon dioxide’s role in plant growth cannot be overstated.

Carbon cycle

Carbon is a very important element, as it makes up organic matter, which is a part of all life. Carbon follows a certain route on earth, called the carbon cycle. Through following the carbon cycle we can also study energy flows on earth, because most of the chemical energy needed for life is stored in organic compounds as bonds between carbon atoms and other atoms.
The carbon cycle naturally consists of two parts, the terrestrial and the aquatic carbon cycle. The aquatic carbon cycle is concerned with the movements of carbon through marine ecosystems and the terrestrial carbon cycle is concerned with the movement of carbon through terrestrial ecosystems.
The carbon cycle is based on carbon dioxide (CO2), which can be found in air in the gaseous form, and in water in dissolved form. Terrestrial plants use atmospheric carbon dioxide from the atmosphere, to generate oxygen that sustains animal life. Aquatic plants also generate oxygen, but they use carbon dioxide from water.
The process of oxygen generation is called photosynthesis. During photosynthesis, plants and other producers transfer carbon dioxide and water into complex carbohydrates, such as glucose, under the influence of sunlight. Only plants and some bacteria have the ability to conduct this process, because they possess chlorophyll; a pigment molecule in leaves that they can capture solar energy with.

The overall reaction of photosynthesis is:
carbon dioxide + water + solar energy -> glucose + oxygen
6 CO2 + 6 H2O + solar energy -> C6H12O6 + 6 O2
The oxygen that is produced during photosynthesis will sustain non-producing life forms, such as animals, and most micro organisms. Animals are called consumers, because they use the oxygen that is produced by plants. Carbon dioxide is released back into the atmosphere during respiration of consumers, which breaks down glucose and other complex organic compounds and converts the carbon back to carbon dioxide for reuse by producers.
Carbon that is used by producers, consumers and decomposers cycles fairly rapidly through air, water and biota. But carbon can also be stored as biomass in the roots of trees and other organic matter for many decades. This carbon is released back into the atmosphere by decomposition, as was noted before.
Not all organic matter is immediately decomposed. Under certain conditions dead plant matter accumulates faster than it is decomposed within an ecosystem. The remains are locked away in underground deposits. When layers of sediment compress this matter fossil fuels will be formed, after many centuries. Long-term geological processes may expose the carbon in these fuels to air after a long period of time, but usually the carbon within the fossil fuels is released during humane combustion processes.
The combustion of fossil fuels has supplied us with energy for as long as we can remember. But the human population of the world has been expanding and so has our demand for energy. That is why fossil fuels are burned very extensively. This is not without consequences, because we are burning fossil fuels much faster than they develop. Because of our actions fossil fuels have become non-renewable recourses.
Although the combustion of fossil fuels mainly adds carbon dioxide to air, some of it is also released during natural processes, such as volcanic eruptions.
In the aquatic ecosystem carbon dioxide can be stored in rocks and sediments. It will take a long time before this carbon dioxide will be released, through weathering of rocks or geologic processes that bring sediment to the surface of water.
Carbon dioxide that is stored in water will be present as either carbonate or bicarbonate ions. These ions are an important part of natural buffers that prevent the water from becoming too acidic or too basic. When the sun warms up the water carbonate and bicarbonate ions will be returned to the atmosphere as carbon dioxide.
Schematic representations of the aquatic and terrestrial part of the carbon cycle are shown here:

1) The aquatic carbon cycle

2) The terrestrial carbon cycle

Carbon dioxide (greenhouse gas)

As many people know carbon dioxide is a greenhouse gas, which basically means that too much carbon dioxide in air causes the earth to warm up.
Humans emit great amounts of carbon dioxide during combustion processes and because of this, the greenhouse effect consisted. The greenhouse effect means that the climate is affected by the concentrations of greenhouse gasses on earth.
In the past few decades a warmer climate has developed, because of the large amounts of carbon dioxide and other greenhouse gases that we emit. This warmer climate can cause problems, such as the melting of large ice formations at the Arctic’s.

For more information on CO2, move to the carbon dioxide page

For more information on carbon, move to the periodic chart

Back to main page of matter cycles

To the matter cycles pollution page

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