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III. Environmental Factors Affecting the Evolution of the Gaseous Hydrocarbons in Soil
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
GASEOUS HYDROCARBONS IN SOIL
of a wet soil. Another influence is caused by the fact that the heat capacity of a
dry soil (0.2 cal/g "C) is much lower than that of water (1 cal/g "C). Moist
soils are therefore slow in warming up because the heat can easily dissipate by
water evaporation. It should therefore be clear that an increase in the
moisture content of a soil will result in a lowering of the soil temperature.
Consequently, this will cause changes in the rate and even type of metabolism
and distribution of the soil microorganisms, having a direct effect on the
amount of gases produced.
Yamane and Sat0 (1961) stated that the CH, production from muck and
alluvial paddy waterlogged soils attained a maximum at 40°C. Above this
temperature, its formation decreased and stopped at 60°C. Moreover, below
20°C its formation was very small. The same authors (1967) showed that the
amount of CH, from a flooded soil treated with different organic substances
increased by increasing the soil temperature above 20°C. Johnston et al.
(1974) showed that from core samples taken from Esthwaite water in the
English Lake District, the production of CH, at 20°C was approximately 40
times greater than at 7.5"C. Vogel et aZ. (1982) found that the production of
hydrocarbon gases out of a San Francisco Bay sediment was important at
27"C, was inhibited at 4"C, and was completely prevented at -10°C. In a
field experiment, Smith and Dowdell (1974) found in two different clayey
soils that the concentration of ethylene rose logarithmically with the soil
temperature during the spring. A temperature increase from 4 to 11°C
resulted in a 20-fold increase in the ethylene concentration. In laboratory
experiments carried out at temperatures between 5 and 3 5 T , Smith and
Restall (1971) found an important production of C2H4, especially at the
beginning of the incubation and mostly pronounced at higher temperatures.
Sutherland and Cook (1980) found that 50°C and apparently even 70°C
temperatures were as favorable for C2H4 production as 30°C.
Formation of C2H, in soil at 50°C and above could be due either to
decomposition of organic matter (Rovira and Vendrell, 1972) or to growth of
thermophilic microorganisms. These authors also showed that sterilization of
the soil by using 6oCo gamma irradiation gave a higher production of
ethylene than when autoclaved at 120°C for 20 m or even kept unsterile. It is
possible that the high radiation dose required for sterilization created many
free radicals in the soil organic matter, which could cause a continuing
synthesis of ethylene (Glegg and Kertesz, 1957; Allen and Scharz, 1958;
Marples and Glew, 1958; Bridges and Horne, 1958). Lindberg et al. (1979)
studied the effect of temperature at 160% H,O on ethylene formation in
root-free humus from a Scots pine forest. They showed that no C2H, was
formed at and below 10°C. The incubations above 24°C had a strong
stimulating effect on C2H, formation. Moreover, the formation rate at 37°C
was about 8 times that at 24°C.
0. VAN CLEEMPUT AND A. S. EL-SEBAAY
It is generally agreed that the majority of the methane formers are only
efficient in a very narrow range of pH, around neutrality. Pohland (1969)
stated that the methane biosynthesis within an anaerobic ecosystem results
from the conversion of complex organic substances through a series of
biologically mediated oxidation-reduction reactions and is pH dependent.
According to Jenkins (1963), the optimum pH range from nine different
species of methanogens is 6.4 to 7.8. Skinner (1968), on the other hand, found
a more tolerant range for CH, production, namely, from 5.5 to 9.0. However,
according to Cappenberg (1979, methanobacteria have an optimum pH of
7.1. In a suspension of a Crowley soil, Jakobsen et al. (1981) found that the
optimum pH for CH, formation was 6.7.
Goodlass and Smith (1978) found that the relationship between C,H,
production and soil pH was not significant in freshwater and grassland soils.
With dried arable soils, it was highly significant over the pH range from 5.0 to
7.3. It was concluded that increasing the organic matter of soils increases the
production of CH,, C2H,, C2H4, and C3Hs, especially when the pH is
neutral or higher (El-Sebaay, 198 1). During sewage sludge digestion, Kotze et
al. (1969) reported that the optimal pH range for CH, production was
6.4-7.2. McCarty (1963) stated that CH, production was possible in acid
bogs, but he gave no range of tolerable pH.
Takai et al. (1956) demonstrated that the redox potential of a soil must be
low, generally less than +200mV, in order to have CH, production.
According to Yamane and Sato (1964), the evolution of CH, from flooded
paddy soils does not commence until the Eh, falls below -200 mV. Also,
Ponnamperuma et al. (1966) showed that methane production occurs in the
Eh range of - 250 to - 300 mV. Moreover, Cappenberg (1974) found that, at
3 to 6 cm depth in the mud of Lake Vechten (the Netherlands), the maximum
methane concentration occurred at redox potential values of -250 to
- 300 mV. In a suspension of a Crowley soil, a small amount of methane was
formed at - 120 mV (Jakobsen et al., 1981).
Almost no research has been carried out on the relation between the redox
potential and the production of ethane, propane, or ethylene from soils. The
relation between ethylene and oxygen is reported by many authors. This
discussion follows in Section III,F.
GASEOUS HYDROCARBONS IN SOIL
E. ADDITIONOF CHEMICALS
Generally, there are at least four mechanisms by which a compound added
to the soil might affect the gaseous hydrocarbon production: (1) by interfering with the development of anaerobic microsites (Smith and Cook, 1974);
(2) by preventing production of a substrate necessary for gas production
(Lynch and Harper, 1974); (3) by directly inhibiting gas-producing microorganisms; or (4) by stimulating gas-producing microorganisms.
Because nitrate can act as a terminal electron acceptor in the absence of
oxygen for anaerobic respiration and because it poises the redox potential of
soils at values preventing the activity of strict anaerobes (Laskowski and
Moraghan, 1967; Bell, 1969; Ponnamperuma, 1972), addition of NO; to
waterlogged soils might suppress the production of gaseous hydrocarbons.
Takai et al. (1956) and Yamane (1957) demonstrated that small amounts of
native NO; disappeared from certain incubated waterlogged soils before the
production of CH, was observed. The effect of nitrate appeared to be
twofold: first, it delayed methane formation until the reduction of nitrate was
complete and the redox potential was low enough for further anaerobic
reactions to proceed (MacGregor and Keeney, 1973); second, it exerted a
toxic effect on methane formation (Cappenberg, 1975; Balderston and Payne,
1976; Winfrey and Zeikus, 1979). When air dried or pre-incubated before the
treatment, addition of KNO, (500 ppm N) to a waterlogged Tetonka soil
(North Dakota) was found to reduce the rate of CH4 accumulation (Laskowski and Moraghan, 1967). Balderston and Payne (1976) found that 1 mM
nitrate was necessary to inhibit methane production in marsh soils, whereas
Winfrey and Zeikus (1979) found that a concentration of nitrate as low as
10 ,ug/liter already inhibited methane completely in freshwater sediments.
Jakobsen et al. (1981) showed that in a Crowley soil treated with 400 pg
NO;-N and 400 pg SO:--S/g of soil, the suppression of methane production
was complete, as compared to a depression of 50% upon adding only 400 pg
SO;--S/g of soil. Bollag and Czlonkowski (1973) showed that in a soil kept
anaerobic, nitrate had a stronger inhibitory effect on CH, formation than
nitrate followed by nitric oxide and nitrous oxide.
It is shown by Smith and Restall (1971) that a very high concentration of
nitrate (2000 pg/g soil) greatly reduced the rate of C2H4evolution but did not
suppress the C,H, production completely, even at the start of the anaerobic
incubation. Smith and Cook (1974) found that the ethylene production was
highest when the soils were leached free of nitrate. When such soils were reamended with nitrate at a level of 20-200ppm N, the C2H4 production
started only when all nitrate had disappeared. Goodlass and Smith (1978)
found no significant relationship between the amount of native NO;-N in
0. VAN CLEEMPUT AND A. S. EL-SEBAAY
fresh soils and the quantities of C2H4evolved. For arable air-dried soils, these
authors found a significant decrease of C2H, formation with an increasing
NO;-N concentration. Moreover, addition of nitrate to these soils up to
lOOpg/g soil showed no influence. However, when higher amounts of
NO;-N (500 and 2000 pg NO;-N/g soil) were added, the C2H4 production
was reduced after 10 days of incubation by 69 and 85% (respectively). In a
series of 24 soils, Van Cleemput et al. (1981) showed that addition of 100 pprn
NO;-N retarded the production of methane, ethane, propane, and ethylene.
One of the final products of nitrate reduction in soil is N 2 0 . This
compound can also affect the production of gaseous hydrocarbons. However,
this has not been investigated intensively. Laskowski and Moraghan (1967)
showed that addition of 500 ppm N20-N to Tetonka loam soil (North
Dakota) clearly suppressed CH, production. Moreover, they found that
when NO;-N or N 2 0 - N was added in the same quantity, the inhibitory
effect of NO;-N was higher than that of N 2 0 . Van Cleemput et al. (1983b)
also found that addition of N 2 0 (7.5% in air space above the soil suspension)
to waterlogged soils, in a closed system, retarded the production of methane,
ethane, propane, and ethylene. In addition to that, the effect of NO;-N and
N,O added together was not fully cumulative. In a field experiment carried
out at the Khok Samrong Rice Experimental Station in Thailand, Araragi
and Tangchan (1979) showed that addition of ammonium phosphate (32 kg
N and 40 kg P,O,/ha) inhibited the formation of CH,, while addition of
phosphorus alone (40 kg P,O,/ha) increased its production as compared to
the control. Addition of ammonium sulfate (50, 100, and 500 ppm) to a soil
kept anaerobic did not cause a clear influence on CH, production (Bollag
and Czlonkowski, 1973). Thermodynamically, ferric iron should be reduced
before methane is formed. Smith (1976b) found that addition of Fe2+to a soil
markedly stimulated ethylene production. Further investigations on this
matter are still required.
Addition of sulfate to a soil also influences the production of CH, either by
its effect on raising the redox potential (MacGregor and Keeney, 1973) or by
the toxic effect of its reduction product (Cappenberg, 1975; Balderston and
Payne, 1976; Jakobsen et al. 1981). Again, according to thermodynamic
principles, sulfate should be reduced before methane is formed (Stumm, 1967;
Claypool and Kaplan, 1974). This indeed has been observed in rice fields
(Takai and Kamura, 1966). But in water sediments as well as in pure cultures,
it has been found that sulfate reducers can compete with CH, producers for
substrates (Winfrey and Zeikus, 1977; Bryant et al., 1977; Abram and
On the other hand, the presence of methane has been observed in
conjunction with sulfide (Martens and Berner, 1974, 1977; Cappenberg,
1975; Barnes and Goldberg, 1976; Reeburgh, 1976; Oremland and Taylor,
GASEOUS HYDROCARBONS IN SOIL
1978; Kosiur and Warford, 1979; Warford et al., 1979). Several explanations
have been put forward. First, methane is formed in anaerobic microsites, after
which it diffuses into sulfate reduction zones (Martens and Berner, 1974,
1977). Second, a syntrophic relationship exists between sulfate reducers and
methane producers. The sulfate reducers yield acetate and hydrogen to the
methanogens (Cappenberg, 1975; Bryant et al., 1977). These conditions,
predominant in environments of low sulfate concentration (Bryant et al.,
1977), may be extended to those of high concentration, where anaerobes
other than sulfate reducers may interact synergistically with the methane
producers (Oremland and Taylor, 1978). Third, sulfide and methane production are in fact not mutually exclusive. And finally, sulfate reducers are able to
oxidize anaerobically in a very low rate some of the formed CH, in the sulfate
reduction zones of water sediments (Barnes and Goldberg, 1976; Reeburgh,
1976; Kosiur and Warford, 1979; Warford et al., 1979). Almost no research
has been done on the influence of sulfate on the production of ethylene,
ethane, or propane in soils.
In bottom deposits of Lake Vechten (the Netherlands), Cappenberg (1974)
found that addition of fluoroacetate reduced the production of CH, by about
75%. In a laboratory experiment, Bollag and Czlonkowski (1973) found no
influence of hydroxylamine on the production of methane from soils kept
anaerobic. Sutherland and Cook (1980) showed that addition of chemicals
such as novobiocin, chloroamphenicol, sodium azide, and sodium cynamide
inhibited the production of C2H4 in a silt loam soil from Pullman (United
States). Addition of chlorogenic acid, ethionine, and ethylenediaminetetraacetic acid (EDTA) stimulated its production. Addition of methionine or
cycloheximide showed no influence. In contrast, Lynch (1983) found no
influence of novobiocin on the production of C,H, in a soil taken from the
same area (Pullman) but with a lower organic matter content. In pure
culture, Considine and Patching (1975) found that addition of phenolic acids
as a carbon source stimulated C2H4 production. Yamane and Sat0 (1963)
found that the decomposition of sodium salts of organic acids, including
acetate, propionate, butyrate, and lactate, after being added to a flooded soil,
increased the CH, production. However, addition of sodium formate showed
no influence. In a waterlogged soil, addition of glucose (in low concentration), peptone or cellulose (Bell, 1969), starch, and alfalfa (Bollag and
Czlonkowski, 1973) clearly increased the amount of CH, produced. Addition
of glucose at relatively high concentration (10 mg/g soil), however, inhibited
CH, production. This is in contrast to the effects of starch and alfalfa when
added in comparable concentration (Bollag and Czlonkowski, 1973).
Plants can be affected by C,H, when significantly produced in coniferous
forest soils (Lindberg et al., 1979; Nohrstedt, 1983); in soils containing fresh
quackgrass, dry quackgrass, oat straw, alfalfa hay, smooth brome-grass
0. VAN CLEEMPUT AND A. S. EL-SEBAAY
leaves and quackgrass leaves (Harvey and Lindscott, 1978); in litter of Pinus
radzata (Lill and McWha, 1976); and in soils containing native high organic
matter contents (Smith and Cook, 1974). Production of CH,, C,H,, C,H,,
and C2H4 from wheat straw, added to pure sand and kept under waterlogged
conditions, was much higher than from humic acid, even when the latter was
added in an amount 10 times higher than the amount of wheat straw (ElSebaay, 1981). Vogel et al. (1982) found that some bactericides inhibited
methane formation but not ethene formation. Their results depended on the
concentration of the sediment slurry.
F. GASES:H,, O,, CO,
Gases such as H,, CO,, and 0, in the atmosphere of flooded soils can
affect the appearance of gaseous hydrocarbons. Yamane and Sato (1964)
found that when an easily decomposable organic material such as glucose is
added to soils, H,, is evolved immediately, causing a delay in the appearance
of CH, since CH, formation occurs after reabsorption of H, into the soil.
However, H, can interact with CO,, producing CH, as shown before. Since
it is well known that CH, formation is a strictly anaerobic phenomenon,
exposure of CH,-producing microorganisms to even a small amount of
oxygen will lead to rapid death (Games and Hayes, 1976). In laboratory
experiments, Cornforth (1975) showed that C,H, was highly resistant to
environments containing elevated amounts of CO,.
The oxygen-ethylene relationship or the so-called 02-C2H4 cycle in soil
which has been proposed by Smith and Cook (1974) is not yet clear. They
proposed that this cycle has far-reaching implications for the biological
activity of the soil. According to their hypothesis, anaerobic microsites are
continually formed in soil as a result of aerobic microorganisms utilizing the
available oxygen. Anaerobes would develop in these microsites and produce
ethylene, which would diffuse through the soil and inactivate the aerobes.
This, in turn, would lead to a drop in oxygen consumption, so that oxygen
would diffuse back into the microsites and prevent anaerobic ethylene
However, there are several experimental observations which are in conflict
with the concept of the cycle. First, soils rich in organic matter produce the
highest amount of ethylene, but even this amount is not sufficient to inhibit
fungi development (Smith, 1976b). How often would sufficiently high levels of
C2H4 be reached? Furthermore, Smith (1978) has shown that ethylene, even
at concentrations far higher than those commonly observed in the field, has
an extremely small or even no effect on aerobic respiratory activity, regardless of the organic status of the soil. Second, Smith and Cook (1974) and
Smith (1976a) completely ignored the existence of ethylene-oxidizing bac-
HYDROCARBONS IN SOIL
teria, whose activities would prevent the accumulation of ethylene in anaerobic habitats. Despite the fact that such bacteria have only recently been
isolated (De Bont, 1976; Heyer, 1976), their activities in soil have been well
documented (Abeles and Heggestad, 1973; Cornforth, 1975). Third, most
microorganisms have better ethylene production in aerobic environments
(Lynch and Harper, 1974; Primrose, 1976). From this discussion it should be
clear that the available evidence does not support the concept of the
C,H,-02 cycle as proposed by Smith and Cook (1974).
G . ADSORPTION
Adsorption phenomena on some soil constituents can also play a role in
the evolution of gaseous hydrocarbons. Vanloocke et al. (1975) and Rohn
(1977) showed that soils can retain hydrocarbons from gases and liquids
passing through them. The retention appears to increase with the molecular
weight of the hydrocarbon (Bohn, 1972). Also, for dry subsoils serving as the
stationary phase in gas-solid chromatography, the retention of gaseous
hydrocarbons increased with molecular weight and unsaturation (Bohn et al.
1980). In an acidic soil of Portsmouth (North Caroline), Witt and Weber
(1975) found that gaseous hydrocarbons can be absorbed on soil constituents
in the following quantities: CH,, 19 nmole/g; CzH6, 13 nmole/g; C3He,
13 nmole/g; and C,H,, 63 nmole/g. They also found that the adsorption of
ethylene was 10 pmole/g on pure montmorillonite, 1.8 pmole/g on pure
kaolinite, and 2.2 pmole/g on organic matter (muck, 94% 0.m.).
Growing plants on the soil may also affect the evolution of gaseous
hydrocarbons. Yamane and Sato (1963) found that flooded soils planted to
rice frequently evolve less methane than the corresponding uncropped sites.
Bare cultivated soils produced less ethylene than undisturbed or cropped
soils, regardless of whether it was measured in situ or in laboratory experiments with dried and sieved soils (Smith and Cook, 1974).
Moisture content and temperature of the soil change continuously
throughout the year. Consequently, their influence on the evolution of
gaseous hydrocarbons will vary importantly. Additional parameters having a
varying influence are the stage of development of plants and the nutrient
0. VAN CLEEMPUT A N D A. S. EL-SEBAAY
status of the soil. These factors are the reasons for a considerable variation in
the relative and absolute amounts of hydrocarbons between different sampling points in the same soil and between different soils.
IV. SAMPLING A N D ANALYSIS OF THE GASEOUS
HY D R 0CAR B 0 NS
To sample the soil atmosphere, several types of devices have been
described (Dowdell et al., 1972; Belford, 1979; Blackwell, 1983; Van Cleemput and Baert, 1983; McGarity and Rajaratnam, 1973; Burford and Stefanson, 1973; Buyanovsky and Wagner, 1983). They all consist of a diffusion
equilibrium reservoir connected to a sampling point at the soil surface via a
low-dead-space tube. Useful information on continuous-flow gas chambers is
given by McGarity and Rajaratnam (1973). Several types of reservoirs can be
used, but it was found by Taylor and Abrahams (1953) that a small ratio of
volume of reservoir to contact area with the soil is the most desirable to
obtain a representative soil atmosphere sample. Also, the withdrawal of gas
volumes should not be considerably in excess of the reservoir volume, in
order to avoid gas flow from the surface soil toward the sampling point. It is
also important that the reservoirs have a pore size permitting gas and water
to enter or to leave but excluding soil particles. Care should be taken of the
type of material used because some synthetic materials either absorb or
release hydrocarbons (Smith and Restall, 1971). The optimum volume of
discarded gas for flushing can be verified by sequential withdrawing of small
samples until a constant concentration is recorded.
Dissolved gases can be determined by equilibrating a water sample of
known volume with a known volume of inert or noninterfering gas in a sealed
bottle and sampling the gas phase with a hypodermic syringe. The initial
content is then calculated from the distribution coefficient of the particular
gas and the volumes of gas and water (Smith and Restall, 1971).
Analysis of C, -C, hydrocarbons can be done according to the technique
described by Smith and Restall (1971). It consists of isothermic (1 10°C)
separation on a 1.5 x 6 mm alumina column, using nitrogen as carrier gas.
The alumina must be partly deactivated by treatment with sodium iodide.
The detection occurs by flame ionization and the limit is about 0.01 ppm in
the gas phase.
Some Physical and Chemical Properties of the Gaseous Hydrocarbons
Melting point ("C)b
Boiling point ("C)b
Solubility in water (m1/100 ml)*
Gas phase dipole moment (D)"
Ionization potential (eV)"
Thermal conductivity [cal/(sec)(cm')("C/cm) x
Heat of combustion (kg cal/g mol at 1 atm and 25°C)"
Limits of inflamation in air (%V/V) at 1 atm and room temp."
Thermodynamic constants at 298.15 Ka
AH; (kcal/g mole)
AF; (kcal/g mole)
log,, K , (kcal/g mole)
s" (cal/K g mole)
From Weast (1979).
*From Hodgman et al. (1962).