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II. Formation, Transformation, and Importance of Gaseous Hydrocarbons
GASEOUS HYDROCARBONS IN SOIL
(fatty acids, amino acids, sugars, etc.)
p-Oxidation of fatty acids
FIG. 1. Generalized degradation scheme of organic materials in an anaerobic environment.
(From Games and Hayes, 1976.)
stances such as glucose, cellulose, hemicellulose, proteins, organic acids,
alcohols, etc. The different steps of this process and the corresponding
materials are schematically shown in Fig. 1.
Not only is CH, produced by simple organic materials and plants in
anaerobic environments, but it can also be generated under feedlots, where
decomposition of the animal manure provides microbial nutrients and results
in 0, depletion (Elliott and McCalla, 1972). Mancinelli et al. (1981) showed a
statistically significant correlation between the methane content and the
number of methanol-oxidizing bacteria in soil.
The biosynthesis of CH, is limited to a rather specialized physiological
group of bacteria living in waterlogged soils, marshes, swamps, manure piles,
the intestines of higher animals, and marine and freshwater sediments. These
microorganisms, responsible for the conversion of organic substances into
CH,, are collectively termed “methane-producing bacteria” (Doelle, 1969).
All of these bacteria are strictly anaerobic and will not proliferate in the
presence of 0,.
Alexander (1977) reported that a variety of reactions can lead to the
production of CH, in 0,-depleted environments. These processes have been
0. VAN CLEEMPUT AND A. S. EL-SEBAAY
identified in mixtures of populations as well as in pure cultures. The following
reactions are typical:
CH4 + 2 H z 0
CH, + 3 C 0 2 + 2 H , 0
CH, + C O ,
It is noted that the major products of the decomposition of simple
molecules are CO, and CH,. The ratio of these two gases varies with the
substrate in question. A common method of CH, formation is the reduction
of CO, : the simple organic molecule is fermented and utilized as a n electron
(or hydrogen) acceptor. In the case of acetic acid, this occurs as follows:
CH,COOH + 2 H 2 0
2 C 0 , + 8(H)
CH4 + 2 H 2 0
A generalized equation for the CO, reduction process can be formulated as
follows: 4H,R + CO, -+ 4R + CH, + 2H2. The CH, arises by reduction of
CO, at the expense of the hydrogen donor, H,R, which is thereby converted
to the product R. Should the CO, supplied be radioactive, the CH4 recovered
would contain the isotope. On the other hand, should the carbon of the
organic molecule be tagged, then the CH, would not bear the isotopic label.
The substrate represented as H,R is frequently but not necessarily organic.
As early as 1920, Harrison [mentioned by Alexander (1977)l stated that
the H, production in paddy fields is small because it reacts with CO, to form
A species of Methanosarcina can decompose methanol and acetate, whereby CH, arises from the methyl group of the alcohol or the acetate. This has
been confirmed by the use of 14C:
This pathway of acetate fermentation is essentially a simple decarboxylation
(Stadtman and Barker, 1951).
Methane is biologically unique among the gaseous hydrocarbons in two
ways. First, it is the only product formed in large quantities through
microbial action in anaerobic environments. Second, it is metabolized by
microorganisms that are often inactive to larger hydrocarbon molecules.
Oxidation of CH, seems to be the most important transformation. As
reported by Alexander (1977), methane oxidizers also occur in well-drained
GASEOUS HYDROCARBONS IN SOIL
soils, particularly in surroundings containing both CH, and CO,. Paddy
soils are examples of an environment having both oxidized and reduced
zones. They frequently contain a surface film that brings about the oxidation
of CH,. The responsible organisms live on the anaerobically mineralized
CH, released from the lower regions of the soil profile. They seem to be more
abundant there than near the surface. Methane oxidation, however, has been
studied more intensively in lake water, anoxic sediments, sewage sludge, and
pure cultures than in soil (Whittenbury et al., 1970; Ferenci et al., 1975;
Wilkinson, 1975).Davis and Yarborough (1966) showed that methane can be
oxidized at a very low rate by the sulfate-reducing bacteria. Zehnder and
Brock (1980) found eight different strains of methanogenic bacteria able to
oxidize methane anoxically in sediments and sewage sludge. Zn situ rates of
methane oxidation have been measured in marine sediments by Reeburgh
(1980). He injected I4CH4 into sediment cores and showed that the formed
CO, accounts for a substantial amount of the carbon necessary for sulfate
reduction in the sulfate-containing strata of a sediment. A similar approach
to the study of the rate of methane oxidation in in-shore marine sediments (at
Jutland, Denmark) was used by Iversen and Blackburn (1981). They found
that methane was produced, in addition to being oxidizzd (mostly anoxically), in a 0-18-cm sediment stratum. Wilkinson (1975) stated that the
oxidation of CH, starts with the hydroxylation of the substrate. Also,
methane itself can be hydroxylated to form methanol (Pate1 et al., 1982).
Beside the anaerobic oxidation, methane can also be oxidized aerobically.
In this case, 0, is consumed and CO, is produced. For each mole of CH,
that disappears, 2 mole of 0, are theoretically required: CH,
2 0 , -+
CO, + 2H,O. Since CH, is also a carbonaceous nutrient for organisms,
some of the gas will be incorporated into the cell substrate. Because of this
carbon assimilation, the experimental values of the above-mentioned reaction do not necessarily agree with the theoretical equation. Studies on the
oxidation of this one-carbon compound resulted in the establishment of the
following oxidation pathway:
The production of methane in soils is considered an indicator for the
existence of anaerobic conditions and hence an indicator for the biological
and chemical processes occurring under these conditions. As previously
mentioned, methanogenic bacteria as well as sulfate-reducing bacteria can
utilize methane as a carbon source (Reeburgh, 1980). The presence of CH, in
the root zone can have a significant negative effect on the growth of certain
0. VAN CLEEMPUT A N D A. S. EL-SEBAAY
tree roots (Gilman et al., 1982). However, information on this matter is not
As an air pollutant, a total amount of probably more than 10" kg of this
gas is discharged each year from paddy fields and swamps (Ehhalt, 1973).
Because its production in soils can be eliminated by autoclaving, ethylene
is considered to be of microbiological origin (Smith and Restall, 1971; Smith
and Cook, 1974). As early as 1954, Davis and Squires showed that ethylene
can be produced by Penicillium digitatum through fermentation of organic
materials such as glucose, ethanol, sodium acetate, sodium butyrate, paper
tissues, and sewage sludge. The maximum production was obtained by using
glucose and ethanol. Also, Considine and Patching (1975) showed that in
soils containing high levels of degradable phenolic polymers, phenolic acidutilizing organisms may contribute to the total production of ethylene.
Considine et al. (1977) found that ethylene can be produced out of certain
media containing different carbon sources such as glucose, methionine, yeast
extract, malic acid, glutamic acid, vanillic acid, ferulic acid, acetate, citrate,
and succinate when two Penicillium species were used.
FIG.2. Proposed scheme for the biosynthesis of ethylene. (From Primrose, 1977.)
GASEOUS HYDROCARBONS IN SOIL
The formation of C2H4 is also common in plant tissues. It functions as an
important plant-growth regulator (Abeles, 1973). Plants, fungi, and bacteria
active in this respect generally require methionine as a precursor to form
C,H, (Abeles, 1972; Lynch, 1972; Primrose, 1976), but ethanol (Abeles, 1972)
or phenolic acids (Considine and Patching, 1975) can also be used by some
fungal species. A proposed scheme for the biosynthesis of ethylene out of Lmethionine, in the presence and absence of light, made by Primrose (1977) is
shown in Fig. 2. It seems, however, that the capacity to synthesize ethylene is
not limited to a certain type of microorganism, because Ilag and Curtis (1968)
found that 25% of 228 fungi species tested were able to produce ethylene in
soils. The following species were identified : Agaricus, Aspergillus, Cephalosporium, Chaetomium, Fusarium, Mucor, Penicillium, and Scopulariopsis
(Davis and Squires, 1954; Ilag and Curtis, 1968; Lynch, 1972, 1974, 1975;
Smith, 1973; Dasilva et al., 1974; Considine et al., 1977; and many others).
Besides these fungi, the yeasts Candida and Trichosporon, the spore-forming
bacteria Pseudomonas (Smith and Cook, 1974; Smith, 1976b), and the
actinomycetes also have the ability, in oitro at least, to produce ethylene.
However, the agents responsible for the biosynthesis of this hydrocarbon in
nature are not fully known.
The amount of ethylene found in the soil is the result of its production and
removal processes. Up until now, very little attention has been given to the
removal processes of C,H, out of the soil atmosphere.
Ethylene in soil may undergo important transformations, which are mainly
enzymatic. Products such as ethane, ethanol, and ethylene oxide can be
formed (De Bont, 1976; De Bont and Albers, 1976). Moreover, ethylene oxide
can be produced from ethylene during oxidation of 1,3-butadiene (Watkinson, 1973) or oxidation of l-alkene (Abbott and Hou, 1973),as well as during
the oxidation of a number of aromatic compounds (Gibson, 1971). It is
pointed out by De Bont (1976) that the oxidation of CzH4 when present at
concentrations of 50ppm or less can be carried out by different bacteria
strains. However, long periods of time (a week or more) were required in
order to allow for growth of the microorganisms responsible for this process.
It was found that the soluble methane monooxygenase from Methylobacterium sp. can catalyze the ethylene epoxidation (Pate1 et al., 1982; Colby et al.,
1977). Lukins (1962) showed that Mycobacterium smegmatis is able to
oxidize ethylene as a nongrowth substrate.
In addition to the biological transformations, C2H4 can also be adsorbed
on soil organic matter and on soil minerals (Witt and Weber, 1975; Bohn et
al., 1980). More information on this point will be given in Section II1,G.
0. VAN CLEEMPUT AND A. S. EL-SEBAAY
Furthermore, C,H, can be removed by soil. Experiments with sterilized soils
by Smith et al. (1973) and Abeles et al. (1971) showed that soil microorganisms are responsible for this uptake. Smith et al. (1973) stated that the rates of
ethylene removal by their soils ranged from 0.14 to 0.97 nmol/g day compared with the 13-14 nmole/g day observed by Abeles et al. (1971). Many
factors influence the rate of this removal. The most important ones are
temperature, soil type (De Bont, 1976; Abeles and Heggestad, 1973), and any
experimental pretreatment (Cornforth, 1975).
During the last 20 years, soil scientists have discovered that soils, particularly those under anaerobic conditions, can release ethylene in quantities
sufficient to cause crop damage (Osborne, 1968; Pratt and Goeschl, 1969;
Smith and Russell, 1969; Smith and Restall, 1971; Rovira and Vendrell,
Some of the physiological effects on plants are breaking of dormancy,
regulation of swelling and elongation, hypertrophy, promotion of adventitious roots, modification of root growth, promotion of root hairs, epinastry,
hook closure, inhibition of leaf expansion, control of flower induction,
exudation, ripening, senescence, and abscission. Besides that, ethylene in soil
causes fungistasis and affects the growth of bacteria, actinomycetes, and some
nematodes (Smith, 1973). Consequently, it has an important influence on
many soil biological processes (Smith and Cook, 1974).
With regard to air pollution, ethylene is one of the major hydrocarbon
pollutants of the atmosphere, and millions of metric tons are discharged to
the air, chiefly from the combustion of gasoline of automobiles. Despite the
vast quantities generated, ethylene concentrations in nonurban areas in the
United States do not increase and remain below 5 ppb due to either oxidation
by ozone or reaction with an oxide of nitrogen in light (Leighton, 1961).
c . ETHANE
Next to methane and ethylene, other volatile hydrocarbons can also be
produced in the soil. Ethane and propane have been detected in the
atmosphere of soils maintained under anaerobiosis (Smith and Restall, 1971;
Van Cleemput et al., 1981, 1983a,b; El-Sebaay, 1981). Davis and Squires
(1954) showed that by fermentation through Penicillium digitatum of organic
materials such as glucose, ethanol, sodium putyrate, sodium acetate, and
paper, ethane can be produced; propane was produced only out of glucose.
GASEOUS HYDROCARBONS IN SOIL
Similar results were obtained by De Boks et al. (1982) using a strain of
Zymonomas mobifis. El-Sebaay (1981) found that both gases were produced
when humic acid or wheat straw was added to pure sand under waterlogged
conditions. Moreover, with wheat straw, the production was much higher
than with humic acid. According to Oremland (1981), small quantities of
ethane can be formed by certain methanogenic bacteria.
Information on transformations of ethane and propane in soil are rather
scarce. As early as 1962, Lukins showed that Mycobacterium smegmatis was
able to oxidize ethane and propane as a nongrowth substrate. Later, Johnson
and Frederick (1971) found that many microorganisms were able to do the
same. Many of the bacteria isolated from soils contaminated by oil are also
able to utilize both gases (Kvasnikov et al., 1971).
As forlthe case of ethylene, ethane and propane undergo enzymatic attack
by soil microorganisms. Pate1 et a/. (1982) showed that hydroxylation of
ethane and propane to form, respectively, ethanol and 1-propano1 can occur
through the soluble methane monooxygenase of Methylobacterium sp. Besides the microbiological transformations, ethane and propane can also be
adsorbed on soil organic matter as well as on (and in) soil minerals (Abeles et
al., 1971; Witt and Weber, 1975; Bohn et al., 1980).
Concerning the potential of ethane and propane to pollute the atmosphere,
out of laboratory experiments with 24 different soils, Van Cleemput et al.
(1981) found that a mean total amount of about 200 g/ha can be produced.
Ferman (1981) found in North Carolina that the amount of these gases in the
ambient atmosphere above rural areas was less than 3 ppb.
In general, ethane and propane have no known importance in the
soil-plant system. They are produced in smaller quantities than methane and
ethylene (Van Cleemput et a/., 1981).
1 1 1 . ENVIRONMENTAL FACTORS AFFECTING THE
EVOLUTION OF GASEOUS HYDROCARBONS I N SOIL
Because the gaseous hydrocarbons in soil are produced by biological
processes, mostly occurring under anaerobic conditions, any factor affecting
the physical, chemical, and biological characteristics of the soil will have, in
general, an influence on the production and evolution of these gases.
0. VAN CLEEMPUT A N D A. S. EL-SEBAAY
Due to the fact that in soil ethylene is about the only gaseous hydrocarbon
having a significant effect on plant growth, most of the soil literature is
focused on this gas. Because of its importance as an indicator for microbial
degradation of organic matter in soils under anaerobic conditions, a lot of
attention has also been given to methane. The important factors having an
influence on the concentration of these and some other gaseous hydrocarbons in the soil are discussed below.
Moisture is required for the activity of microorganisms, and it is likely that
within certain limits the activity increases with moisture content. Baker and
Cook (1974) stated that the activity of soil bacteria is limited at a water
potential of - 15 bar. However, the prime factor by which the moisture
content of soils affects the microorganisms, and hence the produced gases, is
probably its influence on the state of aeration of the environment (Griffin,
1963). Since a gas, i.e., oxygen, moves 10,000 times faster through a gas than
through a liquid phase, the capacity of a soil to exchange gases with the
atmosphere will decrease as the soil becomes waterlogged.
Skinner (1968) reported that by waterlogging soils the CH, production
was increased. In a sandy loam there is a clear relationship between the high
moisture content and both the production of ethylene and the depression of
oxygen (Smith and Dowdell, 1974). The same results were obtained by Smith
and Restall (1971). They stated that the capacity of the soil to produce
ethylene was lower when stored aerobically at field capacity rather than
under waterlogged conditions. Moreover, the ethylene production is favored
by a low oxygen tension, which is mainly achieved by a high moisture
content, making gas exchange with atmospheric air more difficult. Fully
reduced conditions are not necessary. Thus, the ethylene production reaches
a maximum before the main buildup of methane. The evolution of methane is
known to occur in strictly anaerobic, reduced environments. Smith (1976b)
found that C,H, was produced in soil at - 5 bar and wetter, but only slightly
or not at all at - 15 bar or drier. Recently, El-Sebaay (1981) showed that the
production of methane, ethane, ethylene, and propane from soils was higher
under waterlogged conditions than at field capacity in closed systems at 25°C.
Soil temperature is known to be an important factor in controlling the
activity of soil microorganisms. This is to a certain extent related to the soil
moisture content because the heat conductivity of a dry soil is lower than that