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II. Formation, Transformation, and Importance of Gaseous Hydrocarbons

II. Formation, Transformation, and Importance of Gaseous Hydrocarbons

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GASEOUS HYDROCARBONS IN SOIL



161



Monomeric forms

(fatty acids, amino acids, sugars, etc.)

<



Decarboxylation

Deamination

Sugar metabolism

p-Oxidation of fatty acids



Anaerobic degradation



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



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0. VAN CLEEMPUT AND A. S. EL-SEBAAY



identified in mixtures of populations as well as in pure cultures. The following

reactions are typical:



-



CO,+4H,

4HCOOH

CH,COOH

2CH,CH,OH



CH4 + 2 H z 0

CH, + 3 C 0 2 + 2 H , 0

CH, + C O ,



+ CO,



3CH,



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

8(H) CO,



+



Net: CH,COOH



-



2 C 0 , + 8(H)

CH4 + 2 H 2 0

CH,



+CO,



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

CH,.

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:

'4CH3COOH



l4CH,



+ CO,



This pathway of acetate fermentation is essentially a simple decarboxylation

(Stadtman and Barker, 1951).

2. Transformation



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



163



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:



+



CH,



-



CH,OH



---+



HCHO



-



HCOOH



---+



CO,



3. Importance

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



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0. VAN CLEEMPUT A N D A. S. EL-SEBAAY



tree roots (Gilman et al., 1982). However, information on this matter is not

clear.

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).

B. ETHYLENE

1. Formation

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.

L-Methionine



c



L-Methionine

2-Keto-4-methylthiobutyric

acid



1

'

1



Peroxidase



Flavin



2 -Keto-4-methylthiobutyric

acid



Light



Ethylene



Ethylene



FIG.2. Proposed scheme for the biosynthesis of ethylene. (From Primrose, 1977.)



GASEOUS HYDROCARBONS IN SOIL



165



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.

2. Transformation

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.



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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).

3. Importance



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,

1972).

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

AND PROPANE

1. Formation



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



167



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.

2. Transformation



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).

3. Importance



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.



168



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.



A. MOISTURE

CONTENT

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.

B. TEMPERATURE

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



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