Soil respiration
Encyclopedia
Soil respiration refers to the production of carbon dioxide
when soil organisms respire. This includes respiration of plant roots, the rhizosphere
, microbes and fauna
.
Soil respiration is a key ecosystem process that releases carbon from the soil in the form of CO2. CO2 is acquired from the atmosphere and converted into organic compounds in the process of photosynthesis
. Plants use these organic compounds to build structural components or respire them to release energy. When plant respiration occurs below-ground in the roots, it adds to soil respiration. Over time, plant structural components are consumed by heterotroph
s. This heterotrophic consumption releases CO2 and when this CO2 is released by below-ground organisms, it is considered soil respiration.
The amount of soil respiration that occurs in an ecosystem is controlled by several factors. The temperature, moisture, nutrient content and level of oxygen in the soil can produce extremely disparate rates of respiration. These rates of respiration can be measured in a variety of methods. Other methods can be used to separate the source components, in this case the type of photosynthetic pathway (C3/C4), of the respired plant structures.
Soil respiration rates can be largely effected by human activity. This is because humans have the ability to and have been changing the various controlling factors of soil respiration for numerous years. Global climate change is composed of numerous changing factors including rising atmospheric CO2, increasing temperature and shifting precipitation patterns. All of these factors can effect the rate of global soil respiration. Increased nitrogen fertilization by humans also has the potential to effect rates over the entire Earth.
Soil respiration and its rate across ecosystems is extremely important to understand. This is because soil respiration plays a large role in global carbon cycling as well as other nutrient cycles. The respiration of plant structures releases not only CO2 but also other nutrients in those structures, such as nitrogen. Soil respiration is also associated with positive feedbacks with global climate change. Positive feedbacks are when a change in a system produces response in the same direction of the change. Therefore, soil respiration rates can be effected by climate change and then respond by enhancing climate change.
– is an important step in cellular respiration. In the TCA cycle, a six carbon sugar will be oxidized. This oxidation produces the CO2 and H2O from the sugar. Plants, fungi, animals and bacteria all use this cycle to convert organic compounds to energy. This is how the majority of soil respiration occurs at its most basic level. Since the process relies on oxygen to occur, this is referred to as aerobic respiration.
is another process in which cells gain energy from organic compounds. In this metabolic pathway, energy is derived from the carbon compound without the use of oxygen. The products of this reaction are carbon dioxide and usually either ethyl alcohol or lactic acid. Due to the lack of oxygen, this pathway is described as anaerobic respiration. This is an important source of CO2 in soil respiration in water logged ecosystems where oxygen is scarce, as in peat bogs and wetlands. However, most CO2 released from the soil occurs via respiration and one of the most important aspects of belowground respiration occurs in the plant roots.
is a zone immediately next to the root surface with its neighboring soil. In this zone there is a close interaction between the plant and microorganisms. Roots continuously release substances, or exudates, into the soil. These exudates include sugars, amino acids, vitamins, long chain carbohydrates, enzymes and lysates which are released when roots cells break. The amount of carbon lost as exudates varies considerably between plant species. It has been demonstrated that up to 20% of carbon acquired by photosynthesis is released into the soil as root exudates. These exudates are decomposed primarily by bacteria. These bacteria will respire the carbon compounds through the TCA cycle, however fermentation is also present. This is due to the lack of oxygen due to greater oxygen consumption by the root as compared to the bulk soil, soil at a greater distance from the root. Another important organism in the rhizosphere are root-infecting fungi or mycorrhizae. These fungi increase the surface area of the plant root and allow the root to encounter and acquire a greater amount of soil nutrients necessary for plant growth. In return for this benefit, the plant will transfer sugars to the fungi. The fungi will respire these sugars for energy thereby increasing soil respiration. Fungi, along with bacteria and soil animals, also play a large role in the decomposition of litter and soil organic matter.
Another way nitrogen affects soil respiration is through litter decomposition. High nitrogen litter is considered high quality and is more readily decomposed by microorganisms than low quality litter. Degradation of cellulose, a tough plant structural compound, is also a nitrogen limited process and will increase with the addition of nitrogen to litter.
When these multiple data points are graphed, the points can be fitted with a linear regression equation, which will provide a slope. This slope can provide the rate of soil respiration with the equation
, where F is the rate of soil respiration, b is the slope, V is the volume of the chamber and A is the surface area of the soil covered by the chamber. It is important that the measurement is not allowed to run over a longer period of time as the increase in CO2 concentration in the chamber will also increase the concentration of CO2 in the porous top layer of the soil profile. This increase in concentration will cause an underestimation of soil respiration rate due to the additional CO2 being stored within the soil.
Instruments that use this closed dynamic chamber mode of operation are the most common method researchers use to measure soil respiration today.
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
when soil organisms respire. This includes respiration of plant roots, the rhizosphere
Rhizosphere
The rhizosphere is the narrow region of soil that is directly influenced by root secretions and associated soil microorganisms. Soil which is not part of the rhizosphere is known as bulk soil. The rhizosphere contains many bacteria that feed on sloughed-off plant cells, termed rhizodeposition, and...
, microbes and fauna
Fauna
Fauna or faunæ is all of the animal life of any particular region or time. The corresponding term for plants is flora.Zoologists and paleontologists use fauna to refer to a typical collection of animals found in a specific time or place, e.g. the "Sonoran Desert fauna" or the "Burgess shale fauna"...
.
Soil respiration is a key ecosystem process that releases carbon from the soil in the form of CO2. CO2 is acquired from the atmosphere and converted into organic compounds in the process of photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
. Plants use these organic compounds to build structural components or respire them to release energy. When plant respiration occurs below-ground in the roots, it adds to soil respiration. Over time, plant structural components are consumed by heterotroph
Heterotroph
A heterotroph is an organism that cannot fix carbon and uses organic carbon for growth. This contrasts with autotrophs, such as plants and algae, which can use energy from sunlight or inorganic compounds to produce organic compounds such as carbohydrates, fats, and proteins from inorganic carbon...
s. This heterotrophic consumption releases CO2 and when this CO2 is released by below-ground organisms, it is considered soil respiration.
The amount of soil respiration that occurs in an ecosystem is controlled by several factors. The temperature, moisture, nutrient content and level of oxygen in the soil can produce extremely disparate rates of respiration. These rates of respiration can be measured in a variety of methods. Other methods can be used to separate the source components, in this case the type of photosynthetic pathway (C3/C4), of the respired plant structures.
Soil respiration rates can be largely effected by human activity. This is because humans have the ability to and have been changing the various controlling factors of soil respiration for numerous years. Global climate change is composed of numerous changing factors including rising atmospheric CO2, increasing temperature and shifting precipitation patterns. All of these factors can effect the rate of global soil respiration. Increased nitrogen fertilization by humans also has the potential to effect rates over the entire Earth.
Soil respiration and its rate across ecosystems is extremely important to understand. This is because soil respiration plays a large role in global carbon cycling as well as other nutrient cycles. The respiration of plant structures releases not only CO2 but also other nutrients in those structures, such as nitrogen. Soil respiration is also associated with positive feedbacks with global climate change. Positive feedbacks are when a change in a system produces response in the same direction of the change. Therefore, soil respiration rates can be effected by climate change and then respond by enhancing climate change.
Sources of carbon dioxide in soil
All cellular respiration releases releases energy, water and CO2 from organic carbon compounds. Any respiration that occurs below-ground is considered soil respiration. Respiration by plant roots, bacteria, fungi and soil animals are all sources of 2 to 20 mm (0.078740157480315 to 0.78740157480315 in) in soil.Tricarboxylic acid (TCA) cycle
The tricarboxylic acid (TCA) cycle – or citric acid cycleCitric acid cycle
The citric acid cycle — also known as the tricarboxylic acid cycle , the Krebs cycle, or the Szent-Györgyi-Krebs cycle — is a series of chemical reactions which is used by all aerobic living organisms to generate energy through the oxidization of acetate derived from carbohydrates, fats and...
– is an important step in cellular respiration. In the TCA cycle, a six carbon sugar will be oxidized. This oxidation produces the CO2 and H2O from the sugar. Plants, fungi, animals and bacteria all use this cycle to convert organic compounds to energy. This is how the majority of soil respiration occurs at its most basic level. Since the process relies on oxygen to occur, this is referred to as aerobic respiration.
Fermentation
FermentationFermentation (biochemistry)
Fermentation is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. In contrast, respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen,...
is another process in which cells gain energy from organic compounds. In this metabolic pathway, energy is derived from the carbon compound without the use of oxygen. The products of this reaction are carbon dioxide and usually either ethyl alcohol or lactic acid. Due to the lack of oxygen, this pathway is described as anaerobic respiration. This is an important source of CO2 in soil respiration in water logged ecosystems where oxygen is scarce, as in peat bogs and wetlands. However, most CO2 released from the soil occurs via respiration and one of the most important aspects of belowground respiration occurs in the plant roots.
Root respiration
Plants respire some of the carbon compounds which were generated by photosynthesis. When this respiration occurs in roots, it adds to soil respiration. Root respiration usually accounts for approximately half of all soil respiration. However these values can range from 10–90% depending on the dominate plant types in an ecosystem and conditions under which the plants are subjected. Thus the amount of CO2 produced through root respiration is determined by the root biomass and specific root respiration rates. Directly next to the root is the area known as the rhizosphere, which also plays an important role in soil respiration.Rhizosphere respiration
The rhizosphereRhizosphere
The rhizosphere is the narrow region of soil that is directly influenced by root secretions and associated soil microorganisms. Soil which is not part of the rhizosphere is known as bulk soil. The rhizosphere contains many bacteria that feed on sloughed-off plant cells, termed rhizodeposition, and...
is a zone immediately next to the root surface with its neighboring soil. In this zone there is a close interaction between the plant and microorganisms. Roots continuously release substances, or exudates, into the soil. These exudates include sugars, amino acids, vitamins, long chain carbohydrates, enzymes and lysates which are released when roots cells break. The amount of carbon lost as exudates varies considerably between plant species. It has been demonstrated that up to 20% of carbon acquired by photosynthesis is released into the soil as root exudates. These exudates are decomposed primarily by bacteria. These bacteria will respire the carbon compounds through the TCA cycle, however fermentation is also present. This is due to the lack of oxygen due to greater oxygen consumption by the root as compared to the bulk soil, soil at a greater distance from the root. Another important organism in the rhizosphere are root-infecting fungi or mycorrhizae. These fungi increase the surface area of the plant root and allow the root to encounter and acquire a greater amount of soil nutrients necessary for plant growth. In return for this benefit, the plant will transfer sugars to the fungi. The fungi will respire these sugars for energy thereby increasing soil respiration. Fungi, along with bacteria and soil animals, also play a large role in the decomposition of litter and soil organic matter.
Soil animals
Soil animals graze on populations of bacteria and fungi as well as ingest and break up litter to increase soil respiration. Microfauna are made up of the smallest soil animals. These include nematodes and mites. This group specializes on soil bacteria and fungi. By ingesting these organisms, carbon that was initially in plant organic compounds and was incorporated into bacterial and fungal structures will now be respired by the soil animal. Mesofauna are soil animals from 0.1 to 2 mm (0.00393700787401575 to 0.078740157480315 in) in length and will ingest soil litter. The fecal material will hold a greater amount of moisture and have a greater surface area. This will allow for new attack by microorganisms and a greater amount of soil respiration. Macrofauna are organisms from 2 to 20 mm (0.078740157480315 to 0.78740157480315 in), such as earthworms and termites. Most macrofauna fragment litter, thereby exposing a greater amount of area to microbial attack. Other macrofauna burrow or ingest litter, reducing soil bulk density, breaking up soil aggregates and increasing soil aeration and the infiltration of water.Regulation of soil respiration
Regulation of CO2 production in soil is due to various abiotic, or non-living, factors. Temperature, soil moisture and nitrogen all contribute to the rate of respiration in soil.Temperature
Temperature affects almost all aspects of respiration processes. Temperature will increase respiration exponentially to a maximum, at which point respiration will decrease to zero when enzymatic activity is interrupted. Root respiration increases exponentially with temperature in its low range when the respiration rate is limited mostly by the TCA cycle. At higher temperatures the transport of sugars and the products of metabolism become the limiting factor. At temperatures over 35 degrees Celsius, root respiration begins to shut down completely. Microorganisms are divided into three temperature groups; cryophiles, mesophiles and thermophiles. Cryophiles function optimally at temperatures below 20 degrees Celsius, mesophiles function best at temperatures between 20 and 40 degrees Celsius and thermophiles function optimally at over 40 degrees Celsius. In natural soils many different cohorts, or groups of microorganisms exist. These cohorts will all function best at different conditions so respiration may occur over a very broad range. Temperature increases lead to greater rates of soil respiration until high values retard microbial function, this is the same pattern that is seen with soil moisture levels.Soil moisture
Soil moisture is another important factor influencing soil respiration. Soil respiration is low in dry conditions and increases to a maximum at intermediate moisture levels until it begins to decrease when moisture content excludes oxygen. This allows anaerobic conditions to prevail and depress aerobic microbial activity. Studies have shown that soil moisture only limits respiration at the lowest and highest conditions with a large plateau existing at intermediate soil moisture levels for most ecosystems. Many microorganisms possess strategies for growth and survival under low soil moisture conditions. Under high soil moisture conditions, many bacteria take in too much water causing their cell membrane to lyse, or break. This can decrease the rate of soil respiration temporarily, but the lysis of bacteria causes for a spike in resources for many other bacteria. This rapid increase in available labile substrates causes short term enhanced soil respiration. Root respiration will increase with increasing soil moisture, especially in dry ecosystems, however individual species root respiration response to soil moisture will vary widely from species to species depending on life history traits. Upper levels of soil moisture will depress root respiration with the exception of wetland plants, which have developed specific mechanisms for root aeration. Soil moisture levels regulate root respiration, which is essential for nitrogen uptake.Nitrogen
Nitrogen directly affects soil respiration in several ways. Nitrogen must be taken in by roots in order to promote plant growth and life. Most available nitrogen is in the form of NO3−, which costs 0.4 units of CO2 to enter the root because energy must be used to move it up a concentration gradient. Once inside the root the NO3− must be reduced to NH3. This step requires more energy, which equals 2 units of CO2 per molecule reduced. In plants with mycorrhizal symbionts, which fix atmospheric nitrogen, the energetic cost to the plant to acquire one molecule of NH3 from atmospheric N2 is 2.36 CO2. It is essential that plants uptake nitrogen from the soil or rely on symbionts to fix it from the atmosphere in order to assure growth, reproduction and long term survival.Another way nitrogen affects soil respiration is through litter decomposition. High nitrogen litter is considered high quality and is more readily decomposed by microorganisms than low quality litter. Degradation of cellulose, a tough plant structural compound, is also a nitrogen limited process and will increase with the addition of nitrogen to litter.
Methods of measurement
Many different methods exist for the measurement of soil respiration rate and the determination of sources. The closed dynamic chamber method and the use of stable isotope ratios represent two of the most widely used techniques.Closed dynamic chamber method
The closed dynamic chamber method involves placing a closed chamber over the soil surface. Tubes running from the top of the chamber will pass the air in the chamber through an infrared gas analyzer (IRGA), which continuously measures CO2 concentration. The air will then be pumped back into the chamber. An initial CO2 measurement is taken and subsequent measurements are taken at regular intervals over the next few minutes until a final CO2 concentration is recorded at a predetermined end time.When these multiple data points are graphed, the points can be fitted with a linear regression equation, which will provide a slope. This slope can provide the rate of soil respiration with the equation
, where F is the rate of soil respiration, b is the slope, V is the volume of the chamber and A is the surface area of the soil covered by the chamber. It is important that the measurement is not allowed to run over a longer period of time as the increase in CO2 concentration in the chamber will also increase the concentration of CO2 in the porous top layer of the soil profile. This increase in concentration will cause an underestimation of soil respiration rate due to the additional CO2 being stored within the soil.Instruments that use this closed dynamic chamber mode of operation are the most common method researchers use to measure soil respiration today.