Greenhouse Gases Formation and Emission
Antonio C Barbera, University of Catania, Catania, Italy
Jan Vymazal, Czech University of Life Sciences Prague, Praha, Czech Republic
Carmelo Maucieri, University of Padua, Legnaro, Italy
r 2019 Elsevier B.V. All rights reserved.
Glossary
Biogeochemical cycles Natural pathways by which the
chemical elements that are present in the organic matter are
circulated through biological, geological, and chemical
cycles in the biotic and abiotic Earth's compartments.
Global warming A rise in the Earth's standard atmospheric
temperature that causes related changes in climate
and that may result from the greenhouse gases effect.
Introduction
The rising temperature of earth, known as global warming (GW), is mainly the result of the rise of greenhouse gases (GHGs)
concentration in the atmosphere since the beginning of the 20th century, mostly due to anthropogenic activities. Global warming,
as we know, is one of the major threats to the environment because of the resulting climate change. Oxygen (O2) and nitrogen
(N2) are the principal atmosphere gases at concentrations of 21% and 78%, respectively, nevertheless it is thought that they do not
absorb or emit thermal radiation. Water vapor and less abundant gases, such as carbon dioxide (CO2), methane (CH4), and
nitrous oxide (N2O), because of their long atmospheric lives and their relatively high thermal absorption capacities, are generally
known as greenhouse gases. Besides water vapor that is not considered to be a cause of man-made global warming because it does
not persist in the atmosphere for more than few days, the most important greenhouse gas is CO2, followed by CH4 and N2O which
are the other most relevant contributors to GW. Hundred year global warming potential of methane, nitrous oxide and other
GHGs is compared to CO2 in terms of CO2 equivalence (CO2eq). This last is a simple way to normalize all greenhouse gases in a
standard unit based on the radiative forcing of a unit of CO2 over 100 years. For example 1 g of methane has a global warming
potential 28 times higher than 1 g of CO2 and so it is expressed as 28 g of CO2eq. Biological and/or non-biological, as natural and
anthropogenic processes are involved in GHG cycling (most carbon (C) and nitrogen (N) cycles).
Soils can be a source or a sink of CO2, CH4, and N2O depending on the specific conditions. Natural soils are mainly a sink for
natural GHGs and sequester as much C and N they emit, but due to human activities, mainly agriculture, soils can be mainly a
source for GHGs. Infact, intensive agriculture, sustained by mineral fertilizer use, has contributed significantly to the elevation of
atmospheric GHGs, including CO2, CH4, and N2O. Rising GHG emissions usually lead to a decrease in soil C. Currently, soil
organic C is twice that of all standing crop biomass, making it an extremely important player in the C cycle.
Anyhow, agronomic practices have the potential to reduce agricultural GHG emissions. Main management practices that
impact GHG emissions and soil C content include various tillage practices, several N fertilization amounts and typologies
(mineral, manure, or a combination of both), the use of cover crops, aeration, and water levels. Moreover, other agriculture GHGs
sources are intensive livestock such as cattle intensive production systems and rice paddy fields. Furthemore deforestation,
expecially in tropical rainforest, adds more C to the atmosphere than cars and trucks trafic; the reason is that when trees are cut
down or dead, they release their carbon content in the environment. Employing best agricultural management practices (BMPs),
we can promote the sequestration of CO2 plus N2O and the preservation of soil C. Measuring soil C storage and GHG emissions
and using them as metrics to evaluate BMPs are vital in understanding agriculture's role in this topic.
Carbon Greenhouse Gases
Carbon Dioxide (CO2)
CO2 is a colorless gas with a density about 50% higher than that of dry air. In the CO2 molecule (molecular weight of 44 g molÀ1
)
carbon atom is covalently double bonded with two oxygen atoms, and it is naturally present in the earth's atmosphere. Because the
oxygen of CO2 molecule forms intermolecular hydrogen bounding with the hydrogen of water, CO2 is soluble in water, and it
occurs in groundwater, rivers and lakes, ice caps, glaciers, and seawater (CO2 solubility in water is about 1.45 g LÀ1
at 151C) and in
deposits of petroleum and natural gas too. CO2 is odorless at usual concentrations, however its aqueous solution has, at high
concentrations, a sharp and acidic odor and taste.
Among the greenhouse gases, CO2, as presented in the introduction paraghaph is the reference gas with a global warming
potential of 1, lower than all other greenhouse gases. However it contributes 460% to GW due to its huge emission amount. The
concentration of CO2 in the atmosphere has increased from approximately 277 parts per million (ppm) in 1750, at the beginning
Encyclopedia of Ecology, 2nd edition, Volume 2 doi:10.1016/B978-0-12-409548-9.10895-4 329
of the Industrial Era, to 402.8 7 0.1 ppm in 2016. The increase of atmospheric CO2 above the preindustrial levels was primarily
caused by the release from deforestation and other land-use change activities. While emissions from the burning of fossil fuels
started before the Industrial Era, they only became the dominant source of anthropogenic emissions to the atmosphere from
around 1920 and their relative share has continued to increase until present.
Sources of atmospheric CO2 include volcanoes activities, the combustion and decay of organic matter, respiration of aerobic
(oxygen-using) organisms; extraction and burning of fossil fuels, clearing of lands, and production of cement and steel an so on by
human activities.
These sources are at least partially balanced by a set of physical, chemical, or biological processes, called “sinks,” that tend to
remove CO2 from the atmosphere. The main C sink are plants that through the photosynthesis process grab CO2 from
atmosphere.
Plant life is a foundamental natural sink. In the oceans, marine life can absorb dissolved CO2, and some marine organisms
even use CO2 to build skeletons and other structures made of calcium carbonate (CaCO3).
Organic matter respiration, operated by heterotrophic organisms, is the most important path through which the CO2, photosynthetically
fixed as glucose, comes back to the atmosphere:
Glucose þ 6 Oxygen - 6 Carbon dioxide þ 6 Water
6(CH2O) þ 6O2 - 6CO2 þ 6H2O
This organic matter degradation occurs in both soils (primarily in the rooting zone) and oceans (within the surface mixed
layer). Organic matter respiration and CO2 release in the atmosphere are controlled in part by the composition of the organic
matter and the environmental factors (e.g., temperature, soil moisture, and soil porosity, texture/mineralogy).
In total, more than twice as much C is stockpiled in the world's soil than in the vegetation or atmosphere combined.
Considering all the C stored in soil, soil organic carbon (SOC) makes up about 50% of all soil organic matter (SOM).
About a third of the soil organic C occurs in forests, another third is in grasslands and savannas, and the rest is in wetlands,
croplands, and other biomes. SOM is composed by soil microorganisms communities (mainly bacteria and fungi), plant and
animal tissues, fecal material, and products derived from their decomposition. Soil CO2 flux is primarily the result of a
combination of microbial decomposition of SOM and plant root respiration. The main drivers of soil CO2 flux are soil
temperature, soil moisture, and substrate C availability. Temperature affects CO2 flux by speeding up the rate of microbial
decomposition when soils are warm and water is not a limiting factor. Although rising temperatures cause an increase in CO2
flux rate from soils, in some parts of the world there are no clear trends of decreasing soil carbon with increasing mean annual
temperature. This is due, partly, to competing processes within the system, such as SOC rising due to increased primary
productivity that principally occurs through the photosintesis process (better water and nutrient availability), and SOC
decreasing by increased respiration processes. While in the short-term, warming depletes SOC, in the long-term, C losses by
accelerated microbial respiration may be equalized by increases in C inputs to the soil tied to increased net primary production,
as well as any acceleration of soil physico-chemical “stabilization” reactions. Additionally, changes in microbial
community composition or declines in the temperature sensitivity decomposition processes may reduce the response of
microbial respiration to increasing temperature over time (i.e., thermal acclimation cold soil and air temperatures have the
opposite effect on CO2 flux rate, causing it to slow down). Even though slowed, soil microorganisms maintain both catabolic
(CO2 production) and anabolic processes (biomass synthesis) under frozen conditions. Because of this, gaseous exchange
between the atmosphere and soil does not stop even under frozen soil, resulting in the accumulation of CO2 during winter
and its release into the atmosphere during spring thaw events. Another dominant factor controlling the net exchange of GHGs
is soil moisture, which can vary dramatically over time and space. The production and transport of GHGs in soil is strongly
affected by changes in soil moisture through diel 24-h period cycles, wet-up and dry-down events, management practices,
seasonal patterns, and interannual variation in climate. Overall, when water is limiting, plant and microbial availability
increase with soil moisture, thereby increasing soil CO2 flux directly by alleviating plant and microbial desiccation stress and
indirectly by increasing substrate availability (via higher rates of plant growth, photosynthesis, belowground C allocation by
root exudate) and microbial access to substrate for example, increase C diffusion through soil water. Finally, respiration
generally increases with C availability. Plant respiration is largely dependent on C from current photosynthetic activity and,
under non-limiting soil temperatures and moisture availabilities, microbial respiration increases with labile C availability.
Thus, soils with high organic matter inputs and stocks, like those found near the equator, means greater C substrate availability,
which is synonymous with greater flux. Depth and placement of soil carbon is yet another factor to consider when
attempting to make precise conclusions about CO2 flux. For example, in agroecosystems, the bulk of SOM is within the top
10 cm of the soil surface.
Because of this, temporal dynamics of CO2 flux are more intimately related to air temperature than to soil temperature. Also, it
is known that the respiration rates of many soils are strongly linked with the amount of carbon not intimately associated with
minerals. Mineral soil occurs below the litter and organic layer, where soil carbon may be closely associated with mineral particles
—accounting for over 60% of carbon in most forest soils, proposing that the decomposition/respiration rate of mineral soil carbon
is relatively insensitive to temperature. This is because the carbon located here may be protected from microbial mineralization by
stabilization mechanisms, such as occlusion in soil aggregates (physical protection) or interactions with mineral surfaces
(chemical sorption to mineral surfaces).
330 Ecological Processes: Greenhouse Gases Formation and Emission
Methane (CH4)
CH4 is the second most important greenhouse gas in volume after CO2, contributing to global warming potential with 28 times more
infrared radiative heating effect than CO2 on a mole-per-mole basis at a 100 year time horizon. Its atmospheric concentration, due to
human activity, has increased by a factor of 2.5 since preindustrial times, that is, from 722 ppb in 1750 to 1803 ppb in 2011.
Furthermore, after almost one decade of stable CH4 concentrations since the late 1990s, atmospheric measurements have shown a
renewed concentration increase since 2006, probably tied to a rise in natural wetland and fossil fuel emissions.
Generally, CH4 emissions in atmosphere can be due to both human activities or natural processes from biogenic, thermogenic
or pyrogenic sources. Anthropogenic emissions account for 50%–65% of total emissions including coal mining, natural gas use,
agriculture, wastewater and waste treatment.
Considering CH4 budget for the decade of 2000–09 (bottom-up estimates) agriculture and waste sectors (rice, animals and
waste) are the second contributors to biogenic CH4 emissions (with values ranging from 187 to 224 Tg (CH4) yearÀ1
) after natural
wetlands emissions that ranged from 177 to 284 Tg (CH4) yearÀ1
.
Soils play an important role in the CH4 cycle because both methanogenesis (CH4 production) and methanotrophy (CH4
oxidation) take place in them. In view of this soils CH4 fluxes are the net result of the CH4 production by methanogenesis and CH4
oxidation by methanotrophy processes.
Methane production
Methanogenesis is operated by strictly anaerobic bacteria which requires negative oxydo-reduction potentials (Eh o À 200 mV).
Methanogens belong to the domain Archaea which have a limited trophic spectrum comprised of a small number of simple
substrates: H2 þ CO2, acetate, formate, methylated compounds (methanol, methylamines, dimethylsulphur), and primary and
secondary alcohols. This allows to distinguish five trophic groups of methanogens: hydrogenotrophs, formatotrophs, acetotrophs,
methylotrophs, and alcoholotrophs. The two major pathways of CH4 production in most environments where organic matter
decomposition is significant are acetotrophy and CO2 reduction by H2.
The possible pathway for CH4 emission from soil are: (i) diffusion of dissolved CH4 along the concentration gradient, (ii) release of
CH4-containing gas bubbles (ebullition), and iii) transport via the aerenchyma of vascular plants (plant-mediated transport).
The first process, diffusion, takes place because of the formation of a CH4 concentration gradient from deeper soil layers, where the
production of CH4 is large, to the atmosphere, while oxidation of CH4 occurs in upper layers. Diffusion is a slow process compared to
the other two transport mechanisms, that is, ebullition and plant-mediated transport, but it is biogeochemically important because it
extends the contact between CH4 and methanotrophic bacteria in the upper aerobic layer, promoting CH4 oxidation.
The second process, ebullition, takes place when CH4 production is large. Gas bubbles are formed and emigrate to the surface.
As this process is fast, CH4 oxidation is absent or negligible.
The third process, plant mediated transport, takes place through an internal system of continuous air spaces named
aerenchyma, a structure which is developed by vascular plants to adapt to flooded environments. The basic function of this
structure is to transport the O2 necessary for root respiration and cell division in submerged organs, but it is also used for CH4
transportation from the rhizosphere to the atmosphere, bypassing the aerobic, CH4-oxidizing layers. This process involves
two major mechanisms: molecular diffusion and bulk flow. The gradient of CH4 concentration formed inside the aerenchyma
conduits is the driving force for CH4 diffusion from the peat root zone to the aerial parts of the plant. The other
plant-mediated transport mechanism, bulk transportation, involves the migration of CH4 along the plant, also through the
aerenchyma structure, from the leaves to the rhizome and back to the atmosphere through old leaves or horizontal rhizomes
connected to other shoots. The driving force for this process is a pressure gradient generated by differences in temperature or
water vapor pressure between the internal air spaces in plants and the surrounding atmosphere. This is a very efficient and
rapid mechanism of CH4 transportation and, in consequence, it is responsible for most of CH4 emissions (495%) to the
atmosphere from rice paddies.
The factors controlling CH4 production in soil are anaerobic conditions and redox potential, electron acceptors, substrate
availability, temperature, diffusion, water availability and water table, soil pH and salinity, fertilizer and manure additions and
amendments, trace metals, competitive inhibition, vegetation, plant species and cultivars, and elevated CO2 concentrations.
Methane oxidation
The major fraction of CH4 emitted in the atmosphere is submitted to oxidation. Oxidation by OH radicals represent the main sink
of atmospheric CH4 (around 90% of the global CH4 sink) and it takes place mostly in the troposphere according to the reaction:
CH4 þ OH - CH3 þ H2O
The remaining sink is thought to be split roughly equally between the stratosphere (removal by OH and O1[D]) and biological
consumption in near-surface soils. Uptake of methane occurs via oxidation by specialized aerobic bacteria, methanotrophs,
although CH4 oxidation can be also monitored in anaerobic conditions.
The aerobic methane oxidation, operated by methanotrophs, can be summarized as follow:
CH4 þ 2O2 - CO2 þ 2H2O
Ecological Processes: Greenhouse Gases Formation and Emission 331
Aerobic methanotrophic bacteria can be classified in three groups (type I, II, and X) considering morphological features, membrane
structures, guanine and cytosine content, phospholipid fatty acids composition and various other physiological characteristics.
Several mechanisms have been offered to explain anaerobic CH4 oxidation. First, sulfate-driven anaerobic methane oxidation:
CH4 þ SO4
2À
- HSÀ
þ HCO3
–
þ H2O
Other electron acceptors such as oxides of iron, manganese and chrome, could also oxidize methane anaerobically as below reported:
CH4 þ 8Fe(OH)3 þ 16Hþ
- CO2 þ 8Fe2 þ
þ 22H2O
CH4 þ 4MnO2 þ 8Hþ
- CO2 þ 4Mn2 þ
þ 6H2O
CH4 þ 4/3Cr2O7
2À
þ 32/3Hþ
- 8/3Cr3 þ
þ CO2 þ 22/3H2O
Another proposed CH4 oxidation process is the anaerobic oxidation of methane coupled to denitrification with CO2 release as
below summarized:
5CH4 þ 8NO3
À
þ 8Hþ
- 5CO2 þ 4N2 þ 14H2O
3CH4 þ 8NO2
À
þ 8Hþ
- 3CO2 þ 4 N2 þ 10H2O
Biological anaerobic CH4 oxidation is done by a consortium of anaerobic archaea in association with anaerobic bacteria.
Optimal conditions for most of methanotrophs have been found in environments with near neutral pH, temperature in the
mesophilic range (ca. 251C) and low salinity.
Nitrogen Greenhouse Gases
Nitrous Oxide (N2O)
The N2O is one of the most important greenhouse gases with a global warming potential, an index of the total energy added to the
climate system by a component in question relative to that added by CO2, for a time horizon of 100 years of 298 CO2(eq). N2O is
also capable of ozone depletion with negative impact on world life due to the lower capability to shield negative fraction of solar
radiation (UV). The concentration of N2O in the atmosphere increased at a rate of 0.73 7 0.03 ppb yearÀ1
over the last three
decades, mostly caused by nitrification and denitrification reactions of reactive nitrogen in soils and in the ocean.
The major source of N2O in the atmosphere is heterotrophic denitrificatrion, that is, reduction of nitrate to nitrogen gas under
anoxic conditions:
6(CH2O) þ 4NO3
À
) 6CO2 þ 2 N2 þ 6H2O
4(CH2O) þ 4NO3
À
) 4HCO3
À
þ 2N2O þ 2H2O
It is generally agreed that during heterotrophic denitrification, nitrate is reduced to nitrite, which is followed by a stepwise
reduction to nitric oxide, nitrous oxide, and finally nitrogen gas as below schematized:
2NO3
À
) 2NO2
À
) 2NO ) N2O ) N2
This reaction is irreversible and occurs in the presence of available organic substrate mainly under anoxic or anaerobic conditions,
where NO3
À
'nitrogen is used as an electron acceptor in place of oxygen. Nitrate is the first inorganic compound which used by bacteria as
electron acceptor after dissolved oxygen depletion from the water-soil system. Denitrification starts at redox potential values of about
þ 220 mV. Environmental factors known to influence denitrification rates include the absence of O2, redox potential, soil moisture,
temperature, pH value, presence of denitrifiers, soil type, organic matter, nitrate concentration and the presence of overlying water.
Facultative heterotrophic anaerobic bacteria are the main actors of heterotrophic denitrification. Most denitrifying bacteria are
chemoheterotrophs. They obtain energy solely through chemical reactions and use organic compounds both as electron donors
and as a source of cellular carbon. Denytrifying bacteria are aerobes that substitute nitrate for oxygen as the terminal electron
acceptor when there is little or no O2 available. The genera Bacillus, Micrococcus, and Pseudomonas are probably the most
important in soils; Pseudomonas, Aeromonas, and Vibrio in the aquatic environment.
The quantity of N2O evolved during denitrification depends upon the amount of nitrogen denitrified and the ratio of N2 to
N2O produced. The ratio is also affected by aeration, pH, temperature and nitrate to ammonia ratio in the denitrifying system. If
the pH is below 4.5, the denitrification rate is relatively slow and only N2O is produced. At pH 4 5, N2 is the main end product of
denitrification. It has also been shown that certain amount of N2O is formed during nitrification, however, it is difficult to
distinguish between nitrification and denitrification as sources of N2O.
Ozone (O3)
O3 is a short-lived trace gas that either originates in the stratosphere or is produced in situ by precursor gases (monoxide (CO),
CH4, and non-CH4 hydrocarbons in the presence of nitrogen oxides (NOx)) and sunlight. It is a bluish gas with a pungent smell,
332 Ecological Processes: Greenhouse Gases Formation and Emission
dipolar and electrophilic molecule capable of very selective reactions, present in the air at concentrations of 0.01–0.05 ppm. In the
atmosphere the presence of the O3 is crucial for any life because it prevents shortwave solar UV radiation in the UVB
(l 280–315 nm) and UVC (l 200–280 nm) from penetrating the atmosphere and reaching the Earth's surface. On the other hand
O3 exert a greenhouse effect increasing the global warming. As reported by the Intergovernmental Panel on Climate Change in the
fifth assessment report: emissions of carbon monoxide and volatile organic compounds, a not well-defined group of hydrocarbons,
lead to production of ozone on short time scales. By affecting OH and thereby the levels of CH4 they also initiate a
positive long-term ozone effect. The effects via ozone and CH4 cause warming, and the additional effects via interactions with
aerosols and via the O3–CO2 link further increase the warming effect.
See also: Ecological Data Analysis and Modelling: Carbon Biogeochemical Cycle and Consequences of Climate Changes. Ecological
Processes: Decomposition and Mineralization. Global Change Ecology: Microbial Cycles; Emergence of Climate Change Ecology; Global
Negative Emission Land Use Scenarios and Their Ecological Implications. Human Ecology and Sustainability: Carbon Footprint
Further Reading
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Bioprocess Engineering 20 (4), 643–651.
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