Soil respiration refers to the production of

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

when soil organisms respire. This includes respiration of plant roots, the

rhizosphere The rhizosphere is the narrow region of soil or Substrate (biology), substrate that is directly influenced by root secretions and associated soil microorganisms known as the root microbiome. Pore space in soil, Soil pores in the rhizosphere can ...

microbe A microorganism, or microbe, is an organism of microscopic size, which may exist in its single-celled form or as a colony of cells. The possible existence of unseen microbial life was suspected from antiquity, with an early attestation in ...

s and

fauna Fauna (: faunae or faunas) is all of the animal life present in a particular region or time. The corresponding terms for plants and fungi are ''flora'' and '' funga'', respectively. Flora, fauna, funga and other forms of life are collectively ...

. Soil respiration is a key

ecosystem An ecosystem (or ecological system) is a system formed by Organism, organisms in interaction with their Biophysical environment, environment. The Biotic material, biotic and abiotic components are linked together through nutrient cycles and en ...

process that releases

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

from the soil in the form of CO₂. CO₂ is acquired by plants from the atmosphere and converted into organic compounds in the process of

photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...

. 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 A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...

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 A nutrient is a substance used by an organism to survive, grow and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...

content and level of

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

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 affected 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 Present-day climate change includes both global warming—the ongoing increase in Global surface temperature, global average temperature—and its wider effects on Earth's climate system. Climate variability and change, Climate change in ...

is composed of numerous changing factors including rising atmospheric CO₂, increasing temperature and shifting

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...

patterns. All of these factors can affect the rate of global soil respiration. Increased

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

fertilization Fertilisation or fertilization (see American and British English spelling differences#-ise, -ize (-isation, -ization), spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give ...

by humans also has the potential to affect rates over the entire

planet A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...

. 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 The carbon cycle is a part of the biogeochemical cycle where carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of Earth. Other major biogeochemical cycles include the nitrogen cycle and the water cycl ...

as well as other

nutrient cycle A nutrient cycle (or ecological recycling) is the movement and exchange of inorganic and organic matter back into the production of matter. Energy flow is a unidirectional and noncyclic pathway, whereas the movement of mineral nutrients is cyc ...

s. 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

feedback Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause and effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handle ...

with global climate change. Positive feedback is when a change in a system produces response in the same direction of the change. Therefore, soil respiration rates can be affected by climate change and then respond by enhancing climate change.

Sources of carbon dioxide in soil

SRS1000 being used to measure soil respiration in the field

All cellular respiration releases energy, water and CO₂ from organic compounds. Any respiration that occurs below-ground is considered soil respiration. Respiration by plant roots, bacteria,

fungi A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...

and soil animals all release CO₂ in soils, as described below.

Tricarboxylic acid (TCA) cycle

The tricarboxylic acid (TCA) cycle – or

citric acid cycle The citric acid cycle—also known as the Krebs cycle, Szent–Györgyi–Krebs cycle, or TCA cycle (tricarboxylic acid cycle)—is a series of chemical reaction, biochemical reactions that release the energy stored in nutrients through acetyl-Co ...

– is an important step in cellular respiration. In the TCA cycle, a six carbon sugar is

oxidized Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

.Berg J, Tymoczko J, Stryer L. (2002). Biochemistry. WH Freeman and Company. 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

Fermentation Fermentation is a type of anaerobic metabolism which harnesses the redox potential of the reactants to make adenosine triphosphate (ATP) and organic end products. Organic molecules, such as glucose or other sugars, are catabolized and reduce ...

is another process in which cells gain energy from organic compounds. In this

metabolic pathway In biochemistry, a metabolic pathway is a linked series of chemical reactions occurring within a cell (biology), cell. The reactants, products, and Metabolic intermediate, intermediates of an enzymatic reaction are known as metabolites, which are ...

, 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 Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an alcohol, with its formula also written as , or EtOH, where Et is the pseudoelement symbol ...

lactic acid Lactic acid is an organic acid. It has the molecular formula C3H6O3. It is white in the solid state and it is miscible with water. When in the dissolved state, it forms a colorless solution. Production includes both artificial synthesis as wel ...

.Klein D, Prescott L, Harley J. (2005). Microbiology. McGraw-Hill. Due to the lack of oxygen, this pathway is described as

anaerobic respiration Anaerobic respiration is respiration using electron acceptors other than molecular oxygen (O2). Although oxygen is not the final electron acceptor, the process still uses a respiratory electron transport chain. In aerobic organisms undergoing ...

. This is an important source of CO₂ in soil respiration in waterlogged ecosystems where oxygen is scarce, as in

peat bog A bog or bogland is a wetland that accumulates peat as a deposit of dead plant materials often mosses, typically sphagnum moss. It is one of the four main types of wetlands. Other names for bogs include mire, mosses, quagmire, and muske ...

s and

wetland A wetland is a distinct semi-aquatic ecosystem whose groundcovers are flooded or saturated in water, either permanently, for years or decades, or only seasonally. Flooding results in oxygen-poor ( anoxic) processes taking place, especially ...

s. However, most CO₂ released from the soil occurs via respiration and one of the most important aspects of below-ground 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 accounts for approximately half of all soil respiration. However, these values can range from 10 to 90% depending on the dominant plant types in an

and conditions under which the plants are subjected. Thus, the amount of CO₂ produced through root respiration is determined by the root

biomass Biomass is a term used in several contexts: in the context of ecology it means living organisms, and in the context of bioenergy it means matter from recently living (but now dead) organisms. In the latter context, there are variations in how ...

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

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

exudate An exudate is a fluid released by an organism through pores or a wound, a process known as exuding or exudation. ''Exudate'' is derived from ''exude'' 'to ooze' from Latin language, Latin 'to (ooze out) sweat' (' 'out' and ' 'to sweat'). Medi ...

s, into the soil. These exudates include sugars,

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

vitamin Vitamins are Organic compound, organic molecules (or a set of closely related molecules called vitamer, vitamers) that are essential to an organism in small quantities for proper metabolism, metabolic function. Nutrient#Essential nutrients, ...

s, long chain

carbohydrate A carbohydrate () is a biomolecule composed of carbon (C), hydrogen (H), and oxygen (O) atoms. The typical hydrogen-to-oxygen atomic ratio is 2:1, analogous to that of water, and is represented by the empirical formula (where ''m'' and ''n'' ...

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

s 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.Hutsch B, Augustin J, Merbach W. (2002) Plant rhizodeposition – an important source for carbon turnover in soils. Journal of Plant Nutrition and Soil Science. 165, 4, 397–407. These exudates are decomposed primarily by bacteria. These bacteria will respire the carbon compounds through the TCA cycle; however,

fermentation Fermentation is a type of anaerobic metabolism which harnesses the redox potential of the reactants to make adenosine triphosphate (ATP) and organic end products. Organic molecules, such as glucose or other sugars, are catabolized and reduce ...

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.Vance E, Chapin III F. (2001) Substrate limitations to microbial activity in

taiga Taiga or tayga ( ; , ), also known as boreal forest or snow forest, is a biome characterized by coniferous forests consisting mostly of pines, spruces, and larches. The taiga, or boreal forest, is the world's largest land biome. In North A ...

forest floors. Soil Biology and Biochemistry. 33, 2, 173–188. Another important organism in the rhizosphere are root-infecting fungi or

mycorrhizae A mycorrhiza (; , mycorrhiza, or mycorrhizas) is a symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the plant's rhizosphere, the plant root system and its surroundings. Mycorrhizae play ...

. 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.Harrison M. (2005) Peace Talks and Trade Deals. Keys to Long-Term Harmony in Legume-Microbe Symbioses. Plant Physiology. 137, 4, 1205–1210. Fungi, along with bacteria and soil animals, also play a large role in the decomposition of

litter Litter consists of waste products that have been discarded incorrectly, without consent, at an unsuitable location. The waste is objects, often man-made, such as aluminum cans, paper cups, food wrappers, cardboard boxes or plastic bottles, but ...

and

soil organic matter Soil organic matter (SOM) is the organic matter component of soil, consisting of plant and animal detritus at various stages of decomposition, cells and tissues of soil microbes, and substances that soil microbes synthesize. SOM provides numerou ...

Soil animals

Soil animals graze on populations of bacteria and fungi as well as ingest and break up litter to increase soil respiration.

Microfauna Microfauna ( and ) are microscopic animals and organisms that exhibit animal-like qualities and have body sizes that are usually <0.1 mm. Microfauna are represented in the animal kingdom (e.g.

are made up of the smallest soil animals. These include

nematode The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (h ...

s and

mites Mites are small arachnids (eight-legged arthropods) of two large orders, the Acariformes and the Parasitiformes, which were historically grouped together in the subclass Acari. However, most recent genetic analyses do not recover the two as eac ...

. 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 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 , such as

earthworm An earthworm is a soil-dwelling terrestrial invertebrate that belongs to the phylum Annelida. The term is the common name for the largest members of the class (or subclass, depending on the author) Oligochaeta. In classical systems, they we ...

s and

termite Termites are a group of detritivore, detritophagous Eusociality, eusocial cockroaches which consume a variety of Detritus, decaying plant material, generally in the form of wood, Plant litter, leaf litter, and Humus, soil humus. They are dist ...

s. 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.Chapin III F, Matson P, Mooney H. (2002) Principles of terrestrial ecosystem ecology. Springer-Verlag, New York.

Regulation of soil respiration

Regulation of CO₂ production in soil is due to various

abiotic In biology and ecology, abiotic components or abiotic factors are non-living chemical and physical parts of the environment that affect living organisms and the functioning of ecosystems. Abiotic factors and the phenomena associated with them und ...

, or non-living, factors. Temperature, soil moisture and

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 , root respiration begins to shut down completely.Atkin O, Edwards E, Loveys B. (2000) Response of root respiration to changes in temperature and its relevance to global warming. New Phytologist. 147, 141–154. Microorganisms are divided into three temperature groups; cryophiles,

mesophile A mesophile is an organism that grows best in moderate temperature, neither too hot nor too cold, with an optimum growth range from . The optimum growth temperature for these organisms is 37 °C (about 99 °F). The term is mainly applied ...

s and

thermophile A thermophile is a type of extremophile that thrives at relatively high temperatures, between . Many thermophiles are archaea, though some of them are bacteria and fungi. Thermophilic eubacteria are suggested to have been among the earliest bacte ...

s. Cryophiles function optimally at temperatures below , mesophiles function best at temperatures between 20 and and thermophiles function optimally at over . 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.Mikan C, Schimel J, Doyle A. (2002) Temperature controls of microbial respiration in Arctic tundra soils above and below freezing. Soil Biology and Biochemistry. 34, 1785–1795. 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 Soil moisture is the water content of the soil. It can be expressed in terms of volume or weight. Soil moisture measurement can be based on ''in situ'' probes (e.g., capacitance probes, neutron probes) or remote sensing methods. Water that enters ...

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.Xu L, Baldocchi D, Tang J. (2004) How soil moisture, rain pulses, and growth alter the response of ecosystem respiration and temperature. Global Biogeochemical Cycles. 18. Many microorganisms possess strategies for growth and

survival Survival or survivorship, the act of surviving, is the propensity of something to continue existing, particularly when this is done despite conditions that might kill or destroy it. The concept can be applied to humans and other living things ...

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 by restricting access to atmospheric oxygen. With the exception of wetland plants, which have developed specific mechanisms for root aeration, most plants are not adapted to wetland soil environments with low oxygen.Lambers H, Chapin III F, Pons T. (1998) Plant physiological ecology. Springer-Verlag, New York. The respiration dampening effect of elevated soil moisture is amplified when soil respiration also lowers soil

redox Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is t ...

through bioelectrogenesis. Soil-based microbial fuel cells are becoming popular educational tools for science classrooms.

Nitrogen

Nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

directly affects soil respiration in several ways. Nitrogen must be taken in by roots 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 Fick's laws of diffusion describe diffusion and were first posited by Adolf Fick in 1855 on the basis of largely experimental results. They can be used to solve for the diffusion coefficient, . Fick's first law can be used to derive his second ...

. 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 bacterial 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 to assure growth,

reproduction Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. There are two forms of reproduction: Asexual reproduction, asexual and Sexual ...

and long-term survival. Another way nitrogen affects soil respiration is through litter

decomposition Decomposition is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts. The process is a part of the nutrient cycle and is ess ...

. High nitrogen litter is considered high quality and is more readily decomposed by microorganisms than low quality litter. Degradation of

cellulose Cellulose is an organic compound with the chemical formula, formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of glycosidic bond, β(1→4) linked glucose, D-glucose units. Cellulose is an important s ...

, a tough plant structural compound, is also a nitrogen limited process and will increase with the addition of nitrogen to litter.Sinsabaugh R, Carreiro M, Repert D. (2002) Allocation of extracellular enzymatic activity in relation to litter composition, N deposition, and mass loss. Biogeochemistry. 60, 1–24.

Methods of measurement

Different methods exist for the measurement of soil respiration rate and the determination of sources. Methods can be divided into field- and laboratory-based methods. The most common field methods include the use of long-term stand alone soil flux systems for measurement at one location at different times; survey soil respiration systems for measurement of different locations and at different times. The use of

stable isotope ratio The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific element. Hence, the plural form stable isotopes usually refers to isotopes of the same element. The relative abundan ...

s can be used both in laboratory of field measurements. Soil respiration can be measured alone or with added nutrients and (carbon) substrates that supply food sources to the microorganisms. Soil respiration without any additions of nutrients and substrates is called the basal soil respiration (BR). With the addition of nutrients (often nitrogen and phosphorus) and substrates (e.g. sugars), it is called the substrate-induced soil respiration (SIR). In both BR and SIR measurements, the moisture content can be adjusted with water.

Field methods

Long-term stand-alone soil flux systems for measurement at one location over time

These systems measure at one location over long periods of time. Since they only measure at one location, it is common to use multiple stations to reduce measuring error caused by soil variability over small distances. Soil variability may be tested with survey soil respiration instruments. The long-term instruments are designed to expose the measuring site to ambient conditions as much as is possible between measurements.

Types of long-term stand-alone instruments

= Closed, non-steady state systems

= Closed systems take short-term measurements (typically over few minutes only) in a chamber sealed over the soil. The rate of soil CO₂ efflux is calculated on the basis of CO₂ increased inside the chamber. As it is within the nature of closed chambers that CO₂ continues to accumulate, measurement periods are reduced to a minimum to achieve a detectable, linear concentration increase, avoiding an excessive build-up of CO₂ inside the chamber over time. Both individual

assay An assay is an investigative (analytic) procedure in laboratory medicine, mining, pharmacology, environmental biology and molecular biology for qualitatively assessing or quantitatively measuring the presence, amount, or functional activity ...

information and diurnal CO₂ respiration measuring information is accessible. It is also common for such systems to also measure soil temperature, soil moisture and PAR (

photosynthetically active radiation Photosynthetically active radiation (PAR) designates the spectral range (wave band) of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis. This spectral region corresponds more ...

). These variables are normally recorded in the measuring file along with CO₂ values. For determination of soil respiration and the slope of CO₂ increase, researchers have used linear regression analysis, the Pedersen (2001) algorithm, and exponential regression. There are more published references for linear regression analysis; however, the Pedersen algorithm and exponential regression analysis methods also have their following. Some systems offer a choice of mathematical methods.Wayson C, Randolph J, Hanson P, Grimmond P, Schmid H. (2006) Comparison of soil respiration methods in a mid-latitude deciduous forest. Biogeochemistry. 80, 173–189. When using

linear regression In statistics, linear regression is a statistical model, model that estimates the relationship between a Scalar (mathematics), scalar response (dependent variable) and one or more explanatory variables (regressor or independent variable). A mode ...

, multiple data points are graphed and 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

F=bV/A

, 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.Field C, Ball J, Berry J. (1989) Photosynthesis, Principles and field techniques. Plant physiological ecology, field methods and instrumentation. Chapman and Hall, New York. 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.Conen F, and Smith K. (2000) An explanation of linear increases in gas concentration under closed chambers used to measure gas exchange between soil and the atmosphere. European Journal of Soil Science. 51, 111–117.

= Open, steady-state systems

= Open mode systems are designed to find soil flux rates when measuring chamber equilibrium has been reached. Air flows through the chamber before the chamber is closed and sealed. This purges any non-ambient CO₂ levels from the chamber before measurement. After the chamber is closed, fresh air is pumped into the chamber at a controlled and programmable flow rate. This mixes with the CO₂ from the soil, and after a time, equilibrium is reached. The researcher specifies the equilibrium point as the difference in CO₂ measurements between successive readings, in an elapsed time. During the assay, the rate of change slowly reduces until it meets the customer's rate of change criteria, or the maximum selected time for the assay. Soil flux or rate of change is then determined once equilibrium conditions are reached within the chamber. Chamber flow rates and times are programmable, accurately measured, and used in calculations. These systems have vents that are designed to prevent a possible unacceptable buildup of partial CO₂ pressure discussed under closed mode systems. Since the air movement inside the chamber might cause increased chamber pressure, or external winds may produce reduced chamber pressure, a vent is provided that is designed to be as wind proof as possible. Open systems are also not as sensitive to soil structure variation, or to boundary layer resistance issues at the soil surface. Air flow in the chamber at the soil surface is designed to minimize boundary layer resistance phenomena.

= Hybrid Mode Systems

= A hybrid system also exists. It has a vent that is designed to be as wind proof as possible, and prevent possible unacceptable partial CO₂ pressure buildup, but is designed to operate like a closed mode design system in other regards.

Survey soil respiration systems – for testing the variation of CO₂ respiration at different locations and at different times

These are either open or closed mode instruments that are portable or semi-portable. They measure CO₂ soil respiration variability at different locations and at different times. With this type of instrument, soil collars that can be connected to the survey measuring instrument are inserted into the ground and the soil is allowed to stabilize for a period of time. The insertion of the soil collar temporarily disturbs the soil, creating measuring artifacts. For this reason, it is common to have several soil collars inserted at different locations. Soil collars are inserted far enough to limit lateral diffusion of CO₂. After soil stabilization, the researcher then moves from one collar to another according to experimental design to measure soil respiration. Survey soil respiration systems can also be used to determine the number of long-term stand-alone temporal instruments that are required to achieve an acceptable level of error. Different locations may require different numbers of long-term stand-alone units due to greater or lesser soil respiration variability.

Isotope methods

Plants acquire CO₂ and produce organic compounds with the use of one of three photosynthetic pathways. The two most prevalent pathways are the C₃ and C₄ processes. C₃ plants are best adapted to cool and wet conditions while C₄ plants do well in hot and dry ecosystems. Due to the different photosynthetic enzymes between the two pathways, different

carbon isotope Carbon (6C) has 14 known isotopes, from to as well as , of which only and are stable. The longest-lived radioisotope is , with a half-life of years. This is also the only carbon radioisotope found in nature, as trace quantities are formed ...

s are acquired preferentially.

Isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...

s are the same element that differ in the number of neutrons, thereby making one isotope heavier than the other. The two stable carbon isotopes are ¹²C and ¹³C. The C₃ pathway will discriminate against the heavier isotope more than the C₄ pathway. This will make the plant structures produced from C₄ plants more enriched in the heavier isotope and therefore root exudates and litter from these plants will also be more enriched. When the carbon in these structures is respired, the CO₂ will show a similar ratio of the two isotopes. Researchers will grow a C₄ plant on soil that was previously occupied by a C₃ plant or vice versa. By taking soil respiration measurements and analyzing the isotopic ratios of the CO₂ it can be determined whether the soil respiration is mostly old versus recently formed carbon. For example, maize, a C₄ plant, was grown on soil where spring wheat, a C₃ plant, was previously grown. The results showed respiration of C₃ SOM in the first 40 days, with a gradual linear increase in heavy isotope enrichment until day 70. The days after 70 showed a slowing enrichment to a peak at day 100.Rochette P, Flanagan L, Gregorich E. (1999) Separating soil respiration into plant and soil components using analysis of natural abundance of carbon-13. Soil Science Society of America Journal. 63, 1207–1213. By analyzing stable carbon isotope data it is possible to determine the source components of respired SOM that was produced by different photosynthetic pathways.

Substrate-induced respiration in the field using stable isotopes

One problem in the measurement of soil respiration in the field is that respiration of microorganisms can not be distinguished from respiration from plant roots and soil animals. This can be overcome using stable isotope techniques. Cane sugar is a C₄ – sugar which can act as an isotopic tracer. Cane sugar has a slightly higher abundance of ¹³C (δ¹³C ≈ −10‰) than the endogenous (natural) carbon in a C₃ ecosystem (δ¹³C=−25 to −28‰). Cane sugar can be sprayed on the soil in a solution and will infiltrate the upper soil, Only microorganisms will respire the added sugar because roots exclusively respire carbon products that are assimilated by the plant via photosynthesis. By analyses of the δ¹³C of the CO₂ evolving from the soil with or without adding cane sugar, the fraction of C₃ (root and microbial) and C₄ (microbial respiration) can be calculated. Field respiration using stable isotopes can be used as a tool to measure microbial respiration in-situ without disturbing the microbial communities by mixing soil nutrients, oxygen, and soil contaminants that may be present.

Responses to human disturbance

Throughout the past 160 years, humans have changed

land use Land use is an umbrella term to describe what happens on a parcel of land. It concerns the benefits derived from using the land, and also the land management actions that humans carry out there. The following categories are used for land use: fo ...

and industrial practices, which have altered the climate and global

biogeochemical cycle A biogeochemical cycle, or more generally a cycle of matter, is the movement and transformation of chemical elements and compounds between living organisms, the atmosphere, and the Earth's crust. Major biogeochemical cycles include the carbon cyc ...

s. These changes have affected the rate of soil respiration around the planet. In addition, increasingly frequent extreme climatic events such as heat waves (involving high temperature disturbances and associated intense droughts), followed by intense rainfall, impact on microbial communities and soil physico-chemistry and may induce changes in soil respiration.

Elevated carbon dioxide

Since the

Industrial Revolution The Industrial Revolution, sometimes divided into the First Industrial Revolution and Second Industrial Revolution, was a transitional period of the global economy toward more widespread, efficient and stable manufacturing processes, succee ...

, humans have emitted vast amounts of CO₂ into the atmosphere. These emissions have increased greatly over time and have increased global atmospheric CO₂ levels to their highest in over 750,000 years. Soil respiration increases when ecosystems are exposed to elevated levels of CO₂. Numerous free air CO₂ enrichment (FACE) studies have been conducted to test soil respiration under predicted future elevated CO₂ conditions. Recent FACE studies have shown large increases in soil respiration due to increased root biomass and microbial activity.Lipson D, Wilson R, Oechel W. (2005) Effects of Elevated Atmospheric CO₂ on Soil Microbial Biomass, Activity, and Diversity in a Chaparral Ecosystem. Applied and Environmental Microbiology. 71, 12, 8573–8580 Soil respiration has been found to increase up to 40.6% in a

sweetgum ''Liquidambar'', commonly called sweetgum (star gum in the UK), gum, redgum, satin-walnut, styrax or American storax, is the only genus in the flowering plant family Altingiaceae and has 15 species. They were formerly often treated as a part of ...

forest in

Tennessee Tennessee (, ), officially the State of Tennessee, is a landlocked U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders Kentucky to the north, Virginia to the northeast, North Carolina t ...

and poplar forests in

Wisconsin Wisconsin ( ) is a U.S. state, state in the Great Lakes region, Great Lakes region of the Upper Midwest of the United States. It borders Minnesota to the west, Iowa to the southwest, Illinois to the south, Lake Michigan to the east, Michig ...

under elevated CO₂ conditions.King J, Hanson P, Bernhardt E, Deangelis P, Norby R, Pregitzer K. (2004) A multiyear synthesis of soil respiration responses to elevated atmospheric CO₂ from four forest FACE experiments. Global Change Biology. 10, 1027–1042. It is extremely likely that CO₂ levels will exceed those used in these FACE experiments by the middle of this century due to increased human use of

fossil fuel A fossil fuel is a flammable carbon compound- or hydrocarbon-containing material formed naturally in the Earth's crust from the buried remains of prehistoric organisms (animals, plants or microplanktons), a process that occurs within geolog ...

s and land use practices.

Climate warming

Due to the increase in temperature of the soil, CO₂ levels in our atmosphere increase, and as such the mean average temperature of the Earth is rising. This is due to human activities such as forest clearing, soil denuding, and developments that destroy

autotrophic An autotroph is an organism that can convert abiotic sources of energy into energy stored in organic compounds, which can be used by other organisms. Autotrophs produce complex organic compounds (such as carbohydrates, fats, and proteins) us ...

processes. With the loss of photosynthetic plants covering and cooling the surface of the soil, the infrared energy penetrates the soil heating it up and causing a rise in heterotrophic bacteria. Heterotrophs in the soil quickly degrade the organic matter and soil structure crumbles, thus it dissolves into streams and rivers into the sea. Much of the organic matter swept away in floods caused by forest clearing goes into

estuaries An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environm ...

, wetlands and eventually into the open ocean. Increased

turbidity Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of both water clarity and wa ...

of surface waters causes biological oxygen demand and more autotrophic organisms die. Carbon dioxide levels rise with increased respiration of soil bacteria after temperatures rise due to loss of soil cover. As mentioned earlier, temperature greatly affects the rate of soil respiration. This may have the most drastic influence in the

Arctic The Arctic (; . ) is the polar regions of Earth, polar region of Earth that surrounds the North Pole, lying within the Arctic Circle. The Arctic region, from the IERS Reference Meridian travelling east, consists of parts of northern Norway ( ...

. Large stores of carbon are locked in the frozen

permafrost Permafrost () is soil or underwater sediment which continuously remains below for two years or more; the oldest permafrost has been continuously frozen for around 700,000 years. Whilst the shallowest permafrost has a vertical extent of below ...

. With an increase in temperature, this permafrost is melting and aerobic conditions are beginning to prevail, thereby greatly increasing the rate of respiration in that ecosystem. Oechel W, Vourlitis G, Hastings S. (1995) Change in Arctic CO₂ flux over two decades, Effects of climate change at Barrow, Alaska. Ecological Applications. 5, 3, 846–855.

Changes in precipitation

Due to the shifting patterns of temperature and changing oceanic conditions, precipitation patterns are expected to change in location, frequency and intensity. Larger and more frequent storms are expected when oceans can transfer more energy to the forming storm systems. This may have the greatest impact on

xeric Deserts and xeric shrublands are a biome defined by the World Wide Fund for Nature. Deserts and xeric (Ancient Greek 'dry') shrublands form the largest terrestrial biome, covering 19% of Earth's land surface area. Ecoregions in this habita ...

, or arid, ecosystems. It has been shown that soil respiration in arid ecosystems shows dynamic changes within a raining cycle. The rate of respiration in dry soil usually bursts to a very high level after rainfall and then gradually decreases as the soil dries. With an increase in rainfall frequency and intensity over area without previous extensive rainfall, a dramatic increase in soil respiration can be inferred.

Nitrogen fertilization

Since the onset of the

Green Revolution The Green Revolution, or the Third Agricultural Revolution, was a period during which technology transfer initiatives resulted in a significant increase in crop yields. These changes in agriculture initially emerged in Developed country , devel ...

in the middle of the last century, vast amounts of nitrogen fertilizers have been produced and introduced to almost all agricultural systems. This has led to increases in plant available nitrogen in ecosystems around the world due to agricultural runoff and wind-driven

. As discussed earlier, nitrogen can have a significant positive effect on the level and rate of soil respiration. Increases in soil nitrogen have been found to increase plant dark respiration, stimulate specific rates of root respiration and increase total root biomass.Lutze J, Gifford R, Adams H. (2000) Litter quality and decomposition in Danthonia richardsonii swards in response to CO₂ and nitrogen supply over four years of growth. Global Change Biology. 6, 13–24. This is because high nitrogen rates are associated with high plant growth rates. High plant growth rates will lead to the increased respiration and biomass found in the study. With this increase in productivity, an increase in soil activities and therefore respiration can be assured.

Importance

Soil respiration plays a significant role in the global carbon and nutrient cycles as well as being a driver for changes in climate. These roles are important to our understanding of the natural world and human preservation.

Global carbon cycling

Soil respiration plays a critical role in the regulation of

at the ecosystem level and at global scales. Each year approximately 120 petagrams (Pg) of carbon are taken up by land plants and a similar amount is released to the atmosphere through ecosystem respiration. The global soils contain up to 3150 Pg of carbon, of which 450 Pg exist in wetlands and 400 Pg in permanently frozen soils. The soils contain more than four times the carbon as the atmosphere.Sabine C, Hemann M, Artaxo P, Bakker D, Chen C, Field C, Gruber N, Le Quere C, Prinn R, Richey J, Romero-Lankao P, Sathaye J, Valentini R. (2003) Current status and past trends of the carbon cycle. Toward CO₂ Stabilization: Issues, strategies, and consequences. Island Press. Washington DC. Researchers have estimated that soil respiration accounts for 77 Pg of carbon released to the atmosphere each year.Raich J, and Potter C. (1995) Global patterns of carbon dioxide emissions from soils. Global Biogeochemical Cycles. 9, 23–36. This level of release is greater than the carbon release due to

anthropogenic Anthropogenic ("human" + "generating") is an adjective that may refer to: * Anthropogeny, the study of the origins of humanity Anthropogenic may also refer to things that have been generated by humans, as follows: * Human impact on the enviro ...

sources (56 Pg per year) such as

burning. Thus, a small change in soil respiration can seriously alter the balance of atmosphere CO₂ concentration versus

soil carbon Soil carbon is the solid carbon stored in global Soil, soils. This includes both soil organic matter and Inorganic compound, inorganic carbon as carbonate minerals. It is vital to the soil capacity in our ecosystem. Soil carbon is a carbon sink in ...

stores. Much like soil respiration can play a significant role in the global carbon cycle, it can also regulate global

nutrient cycling A nutrient cycle (or ecological recycling) is the movement and exchange of inorganic and organic matter back into the production of matter. Energy flow is a unidirectional and noncyclic pathway, whereas the movement of mineral nutrients is cyc ...

Nutrient cycling

A major component of soil respiration is from the decomposition of litter which releases CO₂ to the environment while simultaneously immobilizing or mineralizing nutrients. During decomposition, nutrients such as nitrogen are immobilized by microbes for their own growth. As these microbes are ingested or die, nitrogen is added to the soil. Nitrogen is also mineralized from the degradation of proteins and

nucleic acid Nucleic acids are large biomolecules that are crucial in all cells and viruses. They are composed of nucleotides, which are the monomer components: a pentose, 5-carbon sugar, a phosphate group and a nitrogenous base. The two main classes of nuclei ...

s in litter. This mineralized nitrogen is also added to the soil. Due to these processes, the rate of nitrogen added to the soil is coupled with rates of microbial respiration. Studies have shown that rates of soil respiration were associated with rates of microbial turnover and nitrogen mineralization. Alterations of the global cycles can further act to change the climate of the planet.

Climate change

As stated earlier, the CO₂ released by soil respiration is a

greenhouse gas Greenhouse gases (GHGs) are the gases in the atmosphere that raise the surface temperature of planets such as the Earth. Unlike other gases, greenhouse gases absorb the radiations that a planet emits, resulting in the greenhouse effect. T ...

that will continue to trap energy and increase the global mean temperature if concentrations continue to rise. As global temperature rises, so will the rate of soil respiration across the globe thereby leading to a higher concentration of CO₂ in the atmosphere, again leading to higher global temperatures. This is an example of a positive

loop. It is estimated that a rise in temperature by 2 °C will lead to an additional release of 10 Pg carbon per year to the atmosphere from soil respiration.Friedlingstein P, Dufresne J, Cox P. (2003) How positive is the feedback between climate change and the global carbon cycle? Tellus. 55B, 692–700. This is a larger amount than current

carbon emissions. There also exists a possibility that this increase in temperature will release carbon stored in permanently frozen soils, which are now melting. Climate models have suggested that this positive feedback between soil respiration and temperature will lead to a decrease in soil stored carbon by the middle of the 21st century.Cox P, Betts R, Jones C, Spall S, Totterdell I. (2000) Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model. Nature. 408, 184–187.

Summary

Soil respiration is a key ecosystem process that releases carbon from the soil in the form of carbon dioxide. Carbon is stored in the soil as organic matter and is respired by plants, bacteria, fungi and animals. When this respiration occurs below ground, it is considered soil respiration. Temperature, soil moisture and nitrogen all regulate the rate of this conversion from carbon in soil organic compounds to CO₂. Many methods are used to measure soil respiration; however, the closed dynamic chamber and use of stable isotope ratios are two of the most prevalent techniques. Humans have altered atmospheric CO₂ levels, precipitation patterns and fertilization rates, all of which have had a significant role on soil respiration rates. The changes in these rates can alter the global carbon and nutrient cycles as well as play a significant role in climate change.

References

* * Su B. (2005
Interactions between ecosystem carbon, nitrogen and water cycles under global change: Results from field and mesocosm experiments.
University of Oklahoma, Norman, OK. * Flanagan L, and Veum A. (1974) Relationships between respiration, weight loss, temperature and moisture in organic residues in tundra. Soil Organisms and decomposition in Tundra. 249–277.

External links

{{soil science topics Soil biology

Sources of carbon dioxide in soil

Tricarboxylic acid (TCA) cycle

Fermentation

Root respiration

Rhizosphere respiration

Soil animals

Regulation of soil respiration

Temperature

Soil moisture

Nitrogen

Methods of measurement

Field methods

Long-term stand-alone soil flux systems for measurement at one location over time

Types of long-term stand-alone instruments

= Closed, non-steady state systems

= Open, steady-state systems

= Hybrid Mode Systems

Survey soil respiration systems – for testing the variation of CO2 respiration at different locations and at different times

Isotope methods

Substrate-induced respiration in the field using stable isotopes

Responses to human disturbance

Elevated carbon dioxide

Climate warming

Changes in precipitation

Nitrogen fertilization

Importance

Global carbon cycling

Nutrient cycling

Climate change

Summary

References

External links

Survey soil respiration systems – for testing the variation of CO₂ respiration at different locations and at different times