Final Report
THE CAPACITY OF THE SOIL AS A NATURAL
SINK FOR CARBON MONOXIDE
Prepared for:
COORDINATING RESEARCH COUNCIL
NEW YORK, NEW YORK
CONTRACT CAPA-4-68 (1-71)
and
THE ENVIRONMENTAL PROTECTION AGENCY
DURHAM, NORTH CAROLINA
CONTRACT 68-02-0307
STANFORD RESEARCH INSTITUTE
Menlo Park, California 94025 • U.S.A.
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STANFORD RESEARCH INSTITUTE
Menlo Park, California 94025 • U.S.A.
Final Report
December 1972
THE CAPACITY OF THE SOIL AS A NATURAL
SINK FOR CARBON MONOXIDE
By: R. B. INGERSOLL
Prepared for:
COORDINATING RESEARCH COUNCIL
NEW YORK, NEW YORK
CONTRACT CAPA-4-68 (1-71)
and
THE ENVIRONMENTAL PROTECTION AGENCY
DURHAM, NORTH CAROLINA
CONTRACT 68-02-0307
SRI Project LSU-1380
Approved by:
W. A. SKINNER, Executive Director
Life Sciences Division
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CONTENTS
INTRODUCTION ,
SUMMARY AND CONCLUSIONS
BACKGROUND
METHODS AND MATERIALS 8
Field Studies 8
Test Sites 8
Test System 13
Field Gas Analyzing System 14
Test Procedure 14
Laboratory Studies ..... 15
Test System 15
Test Soils and Procedures 15
Water Control 17
RESULTS 18
Field Test Sites 18
Roadside Study 30
DISCUSSION 35
REFERENCES 37
ii
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ILLUSTRATIONS
1 Field Test Sites
2 Effect of Temperature on the CO Uptake Rate of
Potting Soil 23
3 Influence of Agricultural Chemicals on CO Uptake
by Soil 29
4 Effect of Continuous CO Exposure on the CO Uptake
Rate of Soils 32
TABLES
1 Vegetation Characteristics of Test Sites 10
2 CO Uptake Rates of Soil at Selected Sites Throughout
North America 19
3 CO Uptake Rates of Vegetative Areas Corrected for
Temperature Variation 21
4 Effects of Temperature on Uptake of Carbon Monoxide by
Southern North American Soils under Laboratory Condi-
tions When Exposed to Atmospheres of About 100 ppm CO ... 25
5 Effect of Soil Moisture on the Uptake of Carbon Monoxide
by Potting Soil Exposed to 100 ppm CO at 25°C 26
6 Comparison of the CO Uptake Capacity of Soils Under
Cultivation and Soils under Natural Vegetation 27
7 Effect of Type and Quantity of Organic Matter in a
Soil on Its CO Uptake Capacity 28
8 Soil Activity at Sites Alongside a 20-Mile Stretch of
the Bayshore Freeway Preselected on the Basis of
Representative Vegetation or Ground Cover 31
9 Potential CO Uptake Rates of the Soils of the
Conterminous United States 33
10 Potential CO Uptake Rates of the Soils of the World .... 34
iii
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INTRODUCTION
For many years, carbon monoxide (CO) has been considered to be an
important atmospheric pollutant. Recent estimates indicate that over
400 million metric tons of CO are produced annually due to man's activi-
ties alone (6). At this rate of production, the ambient concentration
of CO could be expected to double every two to three years; however,
ambient levels have apparently not changed appreciably in the last
decade (6). The fate of CO liberated into the atmosphere is poorly
defined. A number of possible mechanisms for its removal have been
postulated with little confirming evidence. Due to the potential health
hazard of increased levels of CO and ever-increasing emissions, the
search for natural sinks for CO has become a field of increasing in-
terest during the last few years.
A series of research contracts was awarded to Stanford Research
Institute by the Coordinating Research Council and the National Air
Pollution Control Association (later transferred to the Environmental
Protection Agency) to provide insight into the fate of atmospheric CO.
The objective was to investigate the biosphere as a possible sink for
atmosphere CO. Research conducted under the initial contract by
Mrs. Elaine Levy (10) showed that nonsterile soil depleted CO from test
atmospheres, whereas steam-sterilized soil did not; this finding sug-
gested a role for soil or soil microorganisms as a sink for CO. During
the second year of the study, laboratory experiments conducted by
Drs. R. E. Inman and R. B. Ingersoll (5) showed that, potentially,
soil had a capacity to serve as a sink for all the CO produced globally
and that this activity was due to soil microorganisms—in particular,
a number of soil fungi. Estimates of the soil sink potential obtained
from that study were based on a limited number of laboratory observa-
tions and, since soils were shown to vary widely in their activity,
the estimates could be significantly different from the actual poten-
tial of soils in their natural state.
The research described in this report, conducted under a third
contract, was designed as an extension of the first two studies. The
objectives were to:
(1) Determine the potential CO uptake of soils under
natural conditions in the major ecological habitats
of North America.
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(2) Determine what influence environmental variables
exert on the potential CO uptake rates of soils.
(3) Estimate the potential of soils of North America to
serve as a sink for atmospheric CO.
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SUMMARY AND CONCLUSIONS
Studies were conducted to determine the potential magnitude of the
soils of North America to serve as a sink for atmospheric carbon mon-
oxide. In a series of field studies, soils were exposed in situ to ex-
perimental atmospheres containing CO. The influence of environmental
factors was studied in the laboratory. The results are summarized below:
(1) The uptake of CO by soils in situ is highly variable,
with rates ranging from 7.5 to 109.0 rag CO/hr/m .
Generally, the tropical soils were the most active
and the desert soils were the least active in CO
uptake. The forest soils were generally more active
than the grassland soils.
(2) The rate of CO uptake by soils was greatest at a con-
centration of 100 ppm and decreased as the concentra-
tion decreased. This effect was most pronounced for
the soils with the slowest rates of CO uptake.
(3) The rate of CO uptake by soils under cultivation was
significantly less than that of the same soil under
natural vegetation. Laboratory studies indicate that
this difference was not due to agricultural chemicals,
which have little influence, but was probably due to
a lack of organic matter in the cultivated soils,
which limited growth of the microflora. The laboratory
studies showed that the type as well as the amount of
organic matter in a soil significantly influences the
CO uptake by that soil.
(4) The uptake rate of soils adjacent to a major highway
was greater for soils with a dense surface cover of
vegetation than for bare soils. Laboratory studies
indicated that soils constantly exposed to high
levels of CO have greater CO uptake rates, which
could explain what appeared to be higher rates for
soils near freeways. The ground cover phenomenon
was probably due to increased organic matter in the
soil under vegetation.
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(5) Soils removed from test sites for testing in the
laboratory had much reduced CO uptake rates from
those found in the field and did not retain their
ranking in relative activity. This was most prob-
ably due to the long confinement of the soil in
sealed containers during transport and storage.
(6) The uptake capacity of the soils tested in the field
was adjusted for temperature variation, using labora-
tory data, and vegetative regions were assigned the
average value of the tests in that region. The CO
uptake potential of the soils of the conterminous
United States and the world was estimated to be
505 million and 14.3 billion tons/year, respectively.
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BACKGROUND
Carbon monoxide is the most widespread and common air pollutant
emitted by man's activities. Although ambient concentrations of CO
seldom appear to exceed 1 part per million (ppm), especially in remote
regions, more CO is produced annually by man than all the other man-
made air pollutants combined. Ambient levels of CO vary considerably,
probably due to the uneven distribution of man over the surface of the
earth and the action of natural sinks. Swinnterton et al. (19) recorded
concentrations over the Atlantic Ocean between Chesapeake Bay and Puerto
Rico that ranged between 0.075 and 0.44 ppm, with concentrations generally
decreasing with increased distance from urban areas. Robinson and Robbins
(15) found values of 0.04 to 0.2 ppm over different regions of the Pacific
Ocean. Higher concentrations occurred over the northern than over the
southern hemisphere, probably due to a larger number of air pollution
sources in this hemisphere. Seiler and Junge (17)t in a study of the
global tropospheric distribution of CO, found the average for the north-
ern hemisphere to be 0.1-0.15 ppm. Robbins et al. (14) measured 0.3-
0.9 ppm of CO at ground level in Greenland, 0.8 ppm at one spot on the
north coast of California, 0.03-0.3 ppm at Crater Lake, Oregon, and
concentrations up to 0.8 ppm at Patrick Point, California. They con-
cluded that the average CO concentration in the northern hemisphere is
approximately 0.05 ppm.
Both man-made and natural sources contribute to the ambient CO
levels. However, in urban areas the levels of CO are always higher
than ambient levels in remote areas and the CO concentration is ob-
served to rise and fall with the intensity of motor vehicle traffic.
During a recent study in Los Angeles, the CO level at one station
along the Harbor Freeway measured 3 ppm at 4:00 a.m. when traffic in-
tensity was lowest and 15 ppm at 8:30 during the morning rush hour.
Similar patterns have been observed in other large metropolitan areas.
During prolonged periods of air stagnation, the CO levels in Los Angeles
have exceeded 30 ppm for an 8-hour period. In London, CO concentrations
at street level on a calm day have reached 360 ppm.
Jaffe (6) has estimated that the world-wide production of CO by
man exceeds 400 million metric tons each year. According to Jaffe, over
70% of the CO liberated by man in the United States is due to the burning
of gasoline by motor vehicles.
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There are also numerous natural sources of CO. The extent to which
these may contribute to the total ambient concentration is not yet under-
stood. Recently, it was postulated that the oxidation of atmospheric
methane produces 10 times as much CO as do man's activities and that this
oxidation is accompanied by atmospheric reaction of CO with hydroxyl
radicals, resulting in a large natural sink for CO (13). This large
natural turnover of CO due to atmospheric reactions would explain the
calculated short lifetime of CO in the atmosphere (23,24). However, some
researchers in this field (9) feel that the concentrations of the initial
reactants are too low to account for these reactions; in the atmosphere,
this theorized CO turnover cycle cannot be substanti-ated. Swinnerton
et al (20) have observed that sea water is supersaturated with CO and that
the net gas transport was from the sea to the air. Seller and Junge (17)
found CO concentrations on surface waters to be 10-40 times higher than
expected. The ocean has been estimated to be a source of CO varying in
magnitude from 5 to 100% of the man-made source (1,20).
The catabolism of haem-like compounds is known to result in CO pro-
duction within living or autolyzing systems (2). Wilks (25) has shown
that macerated tissues of several green plants (most notably, alfalfa)
evolve CO. Recently, workers at Argonne Laboratories have suggested,
on the basis of isotopic studies, that the decay of chlorophyll may be
a source of CO at least one-third as large as the man-made sources (3,12,
18).
The residence time of CO in the atmosphere has been variously esti-
mated. The most recent calculations range from 0.1 to 0.3 years (4,11,
23). This relatively short residence time is circumstantial evidence
for the existence of sizable sinks for CO. Jaffe (6) has suggested
several possible sinks, including various elements of the biosphere
and atmospheric reactions. Seiler and Junge (17), noting a rapid de-
crease of CO in the tropopause, considered the stratosphere as a major
sink for CO due to CO oxidation there by OH, H202, and H02 radicals.
Evidence also exists that the atmosphere is a natural source of CO as
well as a sink. Swinnerton et al. (21) have found that raindrops may
show up to 200-fold supersaturation with CO in respect to attendant
atmosphere, and they postulate that the source of CO in this instance
is in rain-forming clouds. They suggest, as a plausible CO-formation
mechanism, the photo-oxidation of dissolved organic matter in the rain
water or the dissociation of COs by electric discharge in storm clouds.
The involvement of the biosphere in the turnover of CO has been
postulated for a number of years. In 1926, Wehmer (22) showed that the
microbial activity in the soil was capable of reducing the CO content
of illuminating gas. However, at that time CO was not known to be a
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component of the atmosphere, so no attempt was made to analyze lower
concentrations of CO. Several anaerobic methane bacteria are known
to oxidize CO to CO2 in the absence of H2 or to reduce CO directly to
methane in the presence of H2 (8). In the 1930s, Jones and Scott (7)
reported that certain bacteria present in sealed coal mines were cap-
able of removing CO from the mine atmosphere.
In 1969 (5) we postulated that, based on our laboratory experiments,
soil is potentially a large sink for atmospheric CO. This study indi-
cated that, potentially, soil could serve as a sink for 6.5 times as
much CO as is produced annually by man. Seiler and Junge (17) had made
similar observations for garden soil, but gave no estimate of the rate
of CO uptake.
Although the literature indicates that CO is involved in various
biological mechanisms that could influence its concentration and resi-
dence time in the atmosphere, more precise information was needed before
the role of the biosphere as a sink for CO could be properly assessed.
This report presents data relating to the action of soil in situ in re-
moving CO from experimental atmospheres.
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METHODS AND MATERIALS
Studies were conducted in the field over soils in situ at selected
test sites, and in the laboratory over amended natural soils or artificial
soil mixtures. Field studies comprised an extensive series of tests at
selected sites over the North American continent and a separate study of
various roadside situations in the lower San Francisco Bay area. Labora-
tory studies involved the determination of the effects of various soil
amendments and treatments on the rate of CO uptake by experimental soils.
Field Studies
Test Sites
The continental field test sites were selected throughout North
America to represent as many of the major ecological biotypes as could
be tested within the time and funds available. Each site was selected
on the basis of its location within a region of a particular type of
vegetation. Maps contained in "Potential Natural Vegetation of the
Conterminous United States," "Forest Regions of Canada," "Readers Digest
Great World Atlas," and "The Odyssey World Atlas"*.were used to define
the major vegetation regions and select the test sites. Vegetation class
or type was used as the major guideline for test site selection because
it is a better overall indicator of different ecological situations than
any other single environmental parameter, including soil type. The con-
tinental field test sites selected are shown on the map in Figure 1 and
are characterized in Table 1.
*
Potential Natural Vegetation of the Conterminous United States.
A. W. Ktichler. Special Publication #36, American Geographical Society,
Washington, D.C., 1964.
Forest Regions of Canada. Canadian Department of Fisheries and Forestry,
1970.
Readers Digest Great Itorld Atlas, The Readers Digest Association,
Pleasantville, New York, 1963.
The Odyssey World Atlas. Odyssey Books, New York, 1966.
8
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s where soils under cultivation wer* tesied as wll as soiis under natural vegetation
FIGURE 1 FIELD TEST SITES
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Table 1
VEGETATION CHARACTERISTICS OF TEST SITES
Vegetation Zone
Montane Forest
General Characteristics
Steppe
Temperate
Grassland
Coastal Forest
Most extensive and important of western forest
climaxes; characterized by dominance of ever-
greens Pinus ponderosa, Pseudosuga mucronata and
Abies concolor, along with other species of Pinus.
Dry dense to medium dense grassland, with gen-
erally few woody plants; dominated by Bromus
spp., Avena spp., Fescue ssp,, Agropyron spp.,
and Stipa spp.
Grouping of tall, mid and short grass prairies
with generally mixed populations of grasses,
dominated by species in the genera Stipa,
Agropyron, Andropogon, Bromus, Bouteloua, and
Bulbilis
"Westcoastal forest stretching from N. Calif.
to British Columbia, with dominance, generally,
of douglas fir (Pseudosuga menziesii) and
redwoods.
Site
Number
E-2
E-17
A-5
A-5B
A-6
E-22
A-l
A-IB
A-2
A-2B
A-16
A-16B
A-17
A-14
E-20
M-16
M-17
E-3
E-4
E-15
E-21
A-3
A-4
A-18
A-18B
Dominant Species
Pinus edulis, Juniperus monosperma
P. ponderosa, Pinus contorta
P. ponderosa, Pseudosuga taxlfolia
P. ponderosa, Pseudosuga taxifolia
P. contorta
P. contorta
Bromus spp.
Avena fatua. Fescue spp.
Stipa cernua
Stipa pulchra
Fescue spp., Agropyron spp.
Agropyron spp.
Ag. spicatum, B\ idahoensis
Bulbilis dactyipides
Bouteloua gracilis
Bouteloua eriopoda
Bouteloua eriopoda
Bromus spp.
Bromus rubens, Poa pratensis
Bo. breviseta, An. hallii
Opuntia spp., Lupines
P. menziesii
Tsuga plicata, P. menziesii
Tsuga heterophylla, P_. menziesii
Tsuga heterophylla, P. menziesii
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Table 1 (Continued)
Vegetation Zone
Southern Flood
Plain Forest
Appalachian
Forest
Southern Mixed
Forest
General Characteristics
Desert
Tropical Rain
Forest
Tropical
Deciduous
Forest
Forests on the flood plain of southeastern U.S.,
with dominance of tupelo (Nyssa sylvatica), oaks
(Quercus spp.), and bald cypress (Taxodium
distichum)
Forests covering Appalachian Mountains region;
dominated by white oak (Quercus alba) and northern
oak (Quercus rubra)
Tall mixed deciduous and evergreen broadleaf and
needle leaf in southeastern U.S.; dominated by
oaks (Quercus spp.), pines (Pinus spp.), magnolia
(Magnolia grandifolia), beech (Fagus spp.), and
gums (Liquidamber spp.)
Major region in southwestern U.S. and northern
Mexico; dominated by black brush (Coleogyne
Mixed species of a very large number of
families.
Mixed forest of scrub and deciduous trees;
dominated by Acacia spp., Albizzia spp., and
many trees in the Leguminosae family
Site
Number
E-13
Dominant Species
E-10
E-5
E-18
M-l
M-2
ramosissima), greasewood (Sarcobatus vermiculatus), M-3
cactus (Opuntia spp.), creosote bush (Larrea
divaricatta), and mesquite (Prosopis juliflora)
M-4
E-l
M-9
M-9B
M-13
M-7
M-10
M-ll
M-15
T. distichum
Q. alba, Fagus grandifolia
Betula lenta
Q. alba, Q. rubra
Garya cordiformis
Quercus alba
Quercus laurifolia
Pinus taeda
_L. divaricata, Opuntia spp.
L. divaricata, Yucca baccata
Bouteloua spp.
Acacia spp.
Artemisia tridentata
Unidentified
Unidentified
Unidentified
Mimosa, palms, ferns, legumes
Leguminous trees, vines, shrubs
Leguminous trees, no grasses
Opuntia spp., Albizzia spp.
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Table 1 (Concluded)
Vegetation Zone
Broadleaf and
Mixed Forest
General Characteristics
Boreal Forest
Tropical
Grassland
Tundra
Large deciduous forest spreading over much of
northeastern U.S.; dominated by associations of
maple (Acer) - Beech (Fagus), oak (Quercus)-
chestnut (Castanea) and oak (Quercus) - hickory
(Carya), with occasional stand of pine (Pinus)
Broad band of forest lying just south of the
tundra; dominated by aspens (Populus), birches
(Betula), and pines (Pinus)
Grasslands ranging from those with no woody
species to those with scattered shrubs (Acacia)
and legumes dominated by grasses (Graminae) and
sedges (Cyperaceae)
Artie tundra ground cover on areas of extreme
winters; dominated by sedges, grasses, forbs,
lichens and mosses
Site
Number
E-8
E-12
E-5
E-7
E-10
E-ll
E-19
A- 9
A-1O
A-ll
A-12
A-12B
A-13
M-14
M-15B
A-7
A-7B
A- 8
Dominant Species
Acer spp,, Quercus spp.
Quercus spp., Carya spp., Pinus fp
Quercus spp., Carya
Carya ovata, Quercus spp.
Quercus spp.
Quercus spp., Carya spp., Pinus ssp
Quercus spp., Carya spp., Fraxinus
spp.
Populus tremuloides
Populus tremuloides
Populus t remu1oides
Pinus banksiana
Pinus banksiana
Pinus banksiana
Numerous unidentified grasses and
small mints
Numerous unidentified grasses,
legumes, wild citrus
Numerous unidentified grasses and
small sedges
Curex - Cladonia spp.
Curex - Cladonia spp.
Curex - Cladonia spp.
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Other factors that determined the exact location of each site in
a region were the availability of accurate weather information for the
site and the accessibility of the site. Test sites were selected where
major roads approached the weather station sites found in the Klimadia-
gramm Weltatlas.* Also, wherever possible, the sites were chosen in
areas where a comparison of a given soil under natural vegetation could
be made with the same soil under cultivated or agronomic conditions.
In addition to the continental field test sites, 12 roadside sites
were selected along the Bayshore Freeway (Highway 101) in California
between Menlo Park and San Jose. The sites were selected along both
sides of the freeway in areas where different types of vegetation were
planted as ground cover, or in fields adjacent to the freeway. These
roadside test sites are described in Table 8.
Test System
A bottomless, gas-tight field atmospheric chamber (FAC) positioned
over the soil to be tested in situ was used to contain the test atmo-
sphere. The FAC was collapsible, consisting of a rigid base and top,
with flexible sides for easy storage and transport. The base was con-
structed of four aluminum plates, each 10" X 36" X 5/16". The base of
the FAC was forced or dug into the soil to a depth of 4-6 inches to
provide a good seal with the soil surface. The collapsible sides at-
tached to the base were constructed of fiberglass cloth backed with
neoprene rubber on the inside, making it gas-tight, and aluminized on
the outside to reflect light and help prevent heat build-up inside the
chamber. The top was constructed of 3/4" plywood, with a thin sheet
of aluminum on the outer surface; the inner surface was painted with
white enamel. A squirrel-cage fan was attached to the top inside the
chamber to provide circulation of the enclosed atmosphere. Support
rods were used to hold the chamber top up when the chamber was expanded
for testing. The volume of the chamber was about 0.9 m when in test-
ing position. Ports in the base provided access for probes to monitor
temperature during testing and for the inlet and outlet lines from the
gas analyzer.
*
Klimadiagramm Weltatlas, Walter and Lieth, Gustav Fisher Verlag,
Jena, 1967.
13
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Field Gas Analyzing System
The concentration of CO in the FAC was monitored nondestructively
and continuously by a Beckman Infrared Gas Analyzer Model 315B and a
recorder. Chamber gas was pumped out through tubing and passed through
a column of drierite, then pumped through the analyzer and returned to
the FAC. The flow rate was maintained at 2 liter/minute during testing.
The IR gas analyzer was calibrated using ambient air as zero and 100 ppm
CO as the up-scale calibration gas before each test. The analyzer was
located in a mobile laboratory unit provided by modifying a Winnebago
22-ft "motor inn".
Test Procedure
The continental field sites were reached by means of the mobile
field laboratory. Test equipment included drying and weighing instru-
ments for soil moisture determinations, a pH meter, and the field IR-
test chamber system for monitoring CO uptake by soils in situ. The
vehicle was also equipped with living accommodations for the research
personnel. This mobile laboratory provided the capacity to reach and
test CO uptake by soils in areas, some of which were rather remote,
throughout the continent.
The actual test sites were selected by visual inspection of the
region preselected on the map. Sites were selected to be representa-
tive of the vegetation type characteristics of the region. Natural
vegetation (e.g., small shrubs, grasses, and herbs) was left intact
on the test site as the FAC was positioned over the soil to be tested.
To begin a test, the concentration of CO in the FAC was brought to ~95 ppm
by injecting pure CO into the chamber. The level of CO in the chamber
was continuously monitored during a two- to four-hour test period. Re-
sults were expressed as the average milligrams of CO removed from the
test atmosphere by the test soil per hour per square meter of soil sur-
face. A total of 59 continental sites were tested. During the test
period, the moisture content of the soil and the soil pH were determined.
Soil moisture was determined by weighing duplicate 10-g samples before
and after oven-drying them at 250°F for two hours and cooling them in
a desiccator. The percent weight loss was calculated to be the percent
moisture. The pH was determined on a soil slurry of 10 g of soil and
20 ml of water.
The same test procedure was used to determine the uptake potential
of soils at the 12 selected roadside sites in the lower bay region south
of Menlo Park, California.
14
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Laboratory Studies
Test System
Experimental soils were contained in 250- or 1000-ml filter flasks
or in plastic atmospheric chambers (PACs) of 11 liters capacity which
were described in previous reports. The side arms of the filter flasks
and PACs were used to introduce test atmospheres and to remove atmospheric
samples for analysis. Temperatures were controlled by placing the con-
tainers in an environmental growth chamber or incubator. Analysis of
atmospheric samples removed periodically from atmospheres over the test
soils was conducted via gas chromatography, using a Varian Aerograph
unit and a flame ionization detector. The procedure involved the cataly-
tic reduction of CO to methane, as described in previous reports to this
client.
To begin a test, the container with soil was flushed with the test
atmosphere containing the desired concentration of CO. After a period
suitable to achieve constant CO concentration within the container,
flushing was terminated, the container was sealed, and the test period
was begun. Samples of the test atmosphere (usually 1 ml) were removed
at "zero time" and at various intervals following the beginning of the
test and were analyzed to monitor CO concentrations. A Hamilton, gas-
tight hypodermic syringe was used to remove samples for analysis.
Test Soils and Procedures
A potting soil mixture consisting of loam, sand, leaf mold, peat
moss and steer manure (25:25:16.6:16.6:16.6) was used as the base test
soil in all experiments except those involving organic matter variables.
Also, soil samples taken at various continental test sites were used in
the laboratory to study effects of temperature variables on CO uptake
rates.
Effects of soil moisture on CO uptake activity were studied using
50 g of the base soil mixture contained in 250-ml filter flasks. Start-
ing with air-dried soil, sufficient water was added to the respective
samples in duplicate to establish a series of soil moisture levels.
Soils so amended were incubated at 30°C for 24 hours prior to testing
at 30°C.
15
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Studies of temperature effects on CO uptake rates were conducted
on soil samples taken at field test sites visited during the southern
portion (Mexico and southwestern United States) of the field testing
phase of the research. These samples were collected at the time field
tests were conducted at the respective field sites, and were placed in
sealed one-gallon tins for storage and transport to the laboratory.
Hence, microbial balances and soil conditions existing at the time
these soil samples were tested in the laboratory could not be considered
as characteristic of conditions prevailing in the field at the time the
samples were taken. A soil sample volume of 50 ml contained in a 250-ml
filter flask was incubated at the test temperature for 24 hours prior
to testing. Test temperatures were 5°, 10°, 15°, 20°, 25°, 30°, 35°,
40° and 45°C, maintained by a series of incubators and water baths. The
control consisted of the base potting soil. Following incubation, the
test soils were exposed to a test atmosphere containing an initial con-
centration of 100 ppm CO, and uptake rates at the test temperatures were
monitored during test periods ranging from 2 to 4 hours. The length of
the test period used for a given temperature treatment depended largely
on the rapidity with which the soil depleted atmospheric CO at that
temperature—the slower the rate of depletion, the longer the test
period.
Effects of amending the base soil with selected agricultural chemi-
cals on CO uptake rates were studied in 250-ml filter flasks. One-
kilogram sample of potting soil were treated with 100 ml of one of the
following:
Triox - (1.86%) 2-methoxy-4,6-bis(isopropyl amino s-triazine)
(<1%) parachlorophenol and other chlorinated
phenols (11 ml/100 ml)
Benlate - (50%) benomyl(methyl l-(butylcarbamoyl)-2-
benzimidazole carbamate (0.2 g/100 ml)
Isotox - (5%) gamma benzene hexachloride
(10%) £,o-dimethyl dithiophosphate of diethyl
mercaptosucclnate
(5%) dichloro diphenyl trichloroethane
(3%) 2,4,5-tetrachlorodiphenyl sulphone
(20%) aromatic petroleum solvent (0.4 ml/100 ml)
Diazinon - (25%) 0,0-diethyl-0-(2-isopropyl-4-methyl-6-pyrimidinyl
(57%) aromatic petroleum solvent (0.26 ml/100 ml)
Fungicide - (25%) pentachloronitrobenzene
(25%) N-trichloromethylthio-4-cyclohexene-l,2-
dicarboximide
(10%) zinc ethylene bisdithiocarbamate (0.37 g/ml)
16
-------
The soil samples were thoroughly mixed and allowed to equilibrate
and dry for 24 hours. They were again thoroughly mixed, and 50-g
aliquots were taken from each and placed in 250-ml flasks, with four
replicates per treatment. The soils were held at 30°C and tested for
their uptake rate after 2, 6, 7, 8, 10, 14, and 22 days of incubation.
Organic matter amendments of a base soil consisting of loam and
sand (50:50) were studied for possible effects on CO uptake. A 5-kg
sample of base soil was amended (8% and 25% by weight) with commercial
preparations of peat moss, steer manure, and leaf mold and maintained
in PACs at 10% soil moisture level during the course of the tests.
Uptake activity by the various soils was monitored over a period of
9 weeks at 25°C. During the intervals between tests, the soils were
maintained at 25°C in an incubator, with the PAC lids placed askew over
the soils to provide for aeration. Tests were conducted by sealing the
lids in place, flushing the PACs for 5 minutes with the test atmosphere
containing initially 100 ppm CO, then sealing the PACs and monitoring
CO concentrations over a 2- to 4-hour test period.
The effects of constant exposure to soil to various levels of CO
on the rate of CO uptake by soil was studied using 175-g samples of the
base soil in 1000-ml filter flasks. Duplicate flasks were continually
flushed (100 ml/min) with a test atmosphere containing 0, 5, 28, or
100 ppm CO. The gas stream was bubbled through a water column prior
to entry into the flasks to prevent excessive drying of the soil during
the flushing process. Soils were maintained at their original moisture
content (ca. 10%) by daily weighings, and were held at 25°C throughout
the test period, which lasted for 45 days. Uptake capabilities of the
test soils exposed to different CO levels were determined every 2 to 3
days via the usual procedure as described above, and changes in uptake
capabilities were plotted over time.
17
-------
RESULTS
Field Test Sites
The CO uptake rates of the soil at selected sites throughout North
America are presented in Table 2. The sites are listed according to their
map coordinates, and designated by numbers preceded by M for sites in the
U.S. Southwest and Mexico, A for those in the U.S. Pacific Northwest
and Canada, and E for those in the remainder of the United States. As
can be readily seen, the CO uptake rates measured in the field showed a
great deal of variation, ranging from 7.5 to 109 mg/hr/m2. Although
soils with low pH and moderate moisture content tended to be more active
in CO uptake, there were a great many exceptions. Table 3 shows the up-
take rates when corrected for the changes in temperature throughout the
year. These corrections were made by comparing the field rate to the
laboratory rate shown in Figure 2. The following equation was used to
make the corrections.
. JJL
rft 12
where C is the corrected value, R is the observed field value, rmt is
the rate of uptake on Figure 2 for the average yearly temperature for
the test site, rft is the rate of uptake on Figure 2 of the temperature
during the actual field test, and m is the number of months during the
year when the temperature at the test site averages above 0°C
This correction is an approximation based on fluctuations of the
test temperature above and below the average temperature for the site,
since each site could be visited only once and the temperature during
testing was limited to whatever it happened to be at the time of testing.
Although the surface temperature of the soil in many areas warms to above
freezing during the day in months in which the average monthly temperature
is below freezing and therefore some CO uptake may occur, there are also
nights in months in which the average temperature is above freezing but
the soil temperature may be 0°C or below. These two would tend to balance
each other out. As can be seen from the values in Tables 2 and 3, the
soils tested at the higher latitudes were most influenced by this correc-
tion, since the testing in these areas was all done in the summer months
18
-------
Table 2
CO UPTAKE RATES OF SOIL AT SELECTED SITES THROUGHOUT NORTH AMERICA
Site
No.
Ml
M2
M3
M4
M5
M7
M8
M9A
M9B
M10
Mil
M13
M14
M15
M15B
M16
M17
El
E2
E3
E4
E5
E7
E8
E10
Ell
E12
E13
E14
E15
E17
E18
E19
E20
E21
E22
A-l
A -2
A-2B
A-3
Location
Gila Bend, Ariz.
El Paso, Tex.
Chihuahua, Chi.
Durango, Dur.
Etla, Oax.
Santo Domingo, Oax.
Palengue, Chi.
Palengue, Chi.
Palengue, Chi.
Champoton , Camp .
Merida, Yuc.
Mlnatitlan, V.C.
Poza Rica, V.C.
Tampico, V.C.
Tampico, V.C.
Monterrey, N.L.
Fort Stockton, Tex.
Winnemucca, Nev.
Monticello, Utah
Douglas, Wyo.
Sabetha , Kans .
Columbia, Mo.
Athens, Ohio
Bingharaton, N.Y.
Harrisburg, Pa.
Charlottesville, Va.
Columbus, Ga.
Shreveport, La.
Wichita Falls, Tex.
Albuquerque, N.M.
Donner Pass, Calif.
Mt. Olive, Miss.
Stonelick, Pa.
Prairie View, Kans.
Egbert, Wyo.
Yellowstone Pk. , Wyo.
Stockton, Calif.
Red Bluff, Calif.
Red Bluff, Calif.
Grants Pass, Ore.
Vegetation Type
Desert scrub
Desert scrub
Desert scrub
Desert scrub
Montane forest
Tropical deciduous forest
Equatorial grassland
Tropical rain forest
Tropical rain forest
Tropical deciduous forest
Tropical deciduous forest
Tropical rain forest
Equatorial grassland
Tropical deciduous forest
Equatorial grassland
Equatorial grassland
Grassland
Sagebrush
Montane forest
Mixed prairie
Tall grass prairie
Oak-hickory forest
Mixed deciduous forest
Deciduous forest
Appalachian oak
Mixed deciduous forest
Deciduous forest
Flood plain forest
Grassland
Grassland
Montane forest
Southern mixed deciduous
forest
Oak-hickory forest
Bluestem prairie
Mixed prairie
Montane forest
California steppe
California steppe
California steppe
Coastal forest
Field CO Uptake
(mg/hr/mE )
8.7
9.6
24.6
9.2
24.1
74.6
28.8
14.0
28.3
35.1
43.1
58.0
13.5
83.5
109.0
21.1
14.6
7.5
15.6
43.2
30.4
38.2
47.8
43.6
19.8
20.6
33.3
36.9
41.3
15.4
17.6
15.2
29.1
24.3
45.4
30.5
16.8
10.5
13.0
30.2
Soil
pH
8.3
8.1
7.1
6.4
5.8
6.8
5.6
7.7
7.8
7.7
7.7
6.8
7.6
7.9
7.9
—
—
7.4
7.7
7.6
7.6
7.4
4.9
6.3
4.8
3.9
4.9
7.7
7.8
8.0
5.2
6.2
5.4
7.9
6.7
5.8
7.6
5.5
7.2
6.4
Soil
Moisture
(%)
8.4%
6.2
3.3
2.0
11.4
16.4
3.9
54.9
24.5
14.6
23.3
30.0
32.3
20.8
20.2
17.1
4.0
1.6
2.0
7.1
11.0
24.5
20.0
33.6
30.7
45.8
10.0
30.5
16.8
0.2
33.2
24.0
23.5
4.6
2.5
16.5
2.5
0.1
1.9
6.0
Air Tem-
perature
(°F)
75
64
57
75
71
77
85
79
78
91
83
79
81
78
80
66
70
92
79
88
85
83
78
88
71
81
79
71
91
84
63
79
68
78
88
55
88
94
84
68
19
-------
Table 2 (Concluded)
Site
No.
A-4
A-4B
A -5
A-5B
A-6
A-7
A-7B
A -8
A-9
A-10
A-ll
A-12
A-12B
A-13
A-14
A-15
A-16
A-16B
A-17
A-18
Location
Bellingham, Wash.
Bellingham, Wash.
Prince George, B.C.
Prince George, B.C.
Watsons Lake, Yuk.
Haines Junction, Yuk.
Haines Junction, Yuk.
Haines Junction, Yuk.
Grouard, Alb.
McMurray, Alb.
St. Walburg, Sask.
LaRonge, Sask.
LaRonge, Sask.
Hudson Bay, Man.
Moosejaw, Sask.
Fort Benton, Mont.
Spokane, Wash.
Spokane, Wash.
Burns, Ore.
Prince Rupert, B.C.
Vegetation Type
Coastal forest
Coastal forest
Montane forest
Montane forest
Montane forest
Tundra
Tundra
Tundra
Boreal forest
Boreal forest
Boreal forest
Boreal forest
Boreal forest
Boreal forest
Grassland
Mixed prairie
Steppe
Steppe
Steppe
Coastal forest
Field CO Uptake
(mg/hr/m3 )
51.8
25.8
38.4
26.0
28.0
29.7
22.0
20.3
36.6
44.6
32.0
19.6
20.6
17.3
39.2
35.5
21.3
34.2
12.8
24.0
Soil
pH
5.7
5.2
6.4
6.0
5.2
5.6
5.8
5.9
5.8
6.3
6.4
4.9
4.8
5.9
8.1
7.5
6.6
7.3
7.2
5.2
Soil
Moisture
(%)
16.9%
15.5
14.2
30.0
59.0
27.0
2.4
41.0
17.8
34.0
3.6
3.8
4.2
11.7
22.0
3.4
7.1
2.0
25.7
Air Tem-
perature
(°F)
71
74
72
67
59
63
43
56
77
72
69
76
78
72
85
96
89
99
89
62
20
-------
Table 3
CO UPTAKE RATES OF VEGETATIVE AREAS CORRECTED FOR TEMPERATURE VARIATION
Corrected Rate Average Rate
Vegetation Type Site # (mg CO/hr/m2) (ing CO/hr/m2)
Montane forest
Steppe
Temperate grassland
Coastal forest
E-2
E-17
A-5
A-5B
A-6
E-22
A-l
A- IB
A-2
A-2B
A16
A-16B
A-17
A-14
E-20
M-16
M-17
E-3
E-4
E-14
E-15
D-21
A-3
A-4
A-18
A-18B
5-67
10.76
2.63
4.15
7.70
15.55
4.03
5-29
3.98
3.0?
3-37
3.18
0.77
2.71
2.24
8.77
3-75
3.93
3.60
11.86
3.10
1.96
14.05
8.09
9-39
3.81
3.38
3.81
8.84
Southern flood plain E-13 26.25 26.25
Appalachian forest E-10 17.48 13.82
E-5 10.16
Southern mixed forest E-l8 3.81 3-8l
Desert M-l 1.89 5-45
M-2 6.58
M-3 11.36
M-4 6.86
E-l 0.56
21
-------
Table 3 (Concluded)
Corrected Rate Average Rate
Vegetation Type Site # (mg CO/hr/m3) (mg C0/hr/ms)
Tundra A-7 7.04 5.42
A-7B 4.83
A-8 4.4l
Tropical rain forest M~9 17.64 35-49
M-9B 35.65
M-13 53-17
Tropical deciduous forest M-7 64.81 48.72
M-10 33.43
M-ll 40.19
M-15 56.44
Southern conifer forest E-13 26.25 22.28
E-18 8.31
Broadleaf and mixed forest E-8 26.16 13-87
E-12 10.07
E-5 10.16
E-7 21.33
E-io 17.48
E-ll 2.59
E-19 9.25
Boreal forest A-9 4.17 5-64
A-10 6.78
A-n 6.86
A-12 5.36
A-12B 5.63
A-13 3.71
Tropical grassland M-8 23.28 39-07
M-14 78.41
M-15B 15.52
22
-------
120
100
x
_ 80
"E
o
o
a, 60
LU
< 40
CL
8
20
0 5 10 15 20 25 30 35 40 45 50
TEMPERATURE — °C
FIGURE 2 EFFECT OF TEMPERATURE ON THE CO UPTAKE RATE OF POTTING SOIL
23
-------
when the temperature was well above the average and uptake rates were
probably near maximum.
The corrections for temperature were based on the laboratory studies
on potting soil, diagrammed in Figure 2. Originally it was hoped that
these corrections could be based on the laboratory studies of the soil
samples collected in the field at each test site. The results of the
first study of this kind are shown in Table 4. There was no correlation
between the values found in the field and those obtained in the labora-
tory. Rates of CO uptake by soils in the laboratory were much lower than
the rates observed in the field, and soils that were most active in the
field were not always the most active in the laboratory. This discrepancy
was probably due to changes in the soil microflora during the trip back
to the laboratory, during which time the soil samples were stored in the
sealed cans. Because of this lack of correlation, it was decided to use
the data from the study on potting soil for all the corrections.
No correction was made for soil moisture since the laboratory study
on the influence of soil moisture on the CO uptake rate (Table 5) shows
that the rate is only affected at very high and low moisture contents
and the regions of extreme moisture (e.g., the tropical rain forest,
>50%) and low moisture (deserts, <5%) generally always have more or less
the same moisture content.
Wherever possible, field test sites were chosen at locations where
two tests could be made for comparison—one on the soil under natural
vegetation and the other on the same soil under cultivation. The results
of this testing program are shown in Table 6. In all cases, the culti-
vated soils were significantly less active than the corresponding soils
under natural vegetation. The average uptake rates for the natural and
cultivated soils were 11.7 and 2.6, respectively.
It was suspected that the lack of organic matter in the agricultural
soil might be the reason for the lower activity of that soil. Hence, a
study on the influence of organic matter in soil was conducted. The re-
sults of this study (Table 7) show that both the amount and type of or-
ganic matter in a soil influence the CO uptake rate of a soil. Soils
containing leaf mold or steer manure had higher CO uptake rates than
soils containing peat moss. These rates are low compared to rates found
in some of the active natural soils.
It was also suspected that the treatment of agricultural fields with
chemicals such as herbicides and fungicides might be the reason for the
lower CO uptake. Therefore, a laboratory study (Figure 3) was done on
that parameter. All the compounds were applied in their recommended
24
-------
Table 4
EFFECTS OF TEMPERATURE (°C) ON UPTAKE OF CARBON MONOXIDE (mg/hr/m2)
BY SOUTHERN NORTH AMERICAN SOILS UNDER LABORATORY CONDITIONS
WHEN EXPOSED TO ATMOSPHERES OF ABOUT 100 ppm CO
CO
en
Location
10
15
20
25
30
35
40
45
Gila Bend
El Paso
Chihuahua
Durango
Oaxaca (Etla)
Santa Domingo
Palenque (Tepa )
Palenque
Palenque
Champoton
Merida
Minatitlan
Posa Rica
Tampico
Tampico
Monterrey
Ft. Stockton
Ft. Stockton
0.27
1.24
0.59
0.46
1.53
1.37
0.72
0.70
2.52
0.76
1.69
2.22
2.59
0.60
-0.33
0.89
0.34
0.45
0.0
1.60
0.25
0.0
1.30
1.84
1.07
1.49
3.98
1.93
2.17
2.90
3.18
3.56
0.92
1.33
0.28
1.18
-0.58
2.36
0.19
0.42
2.31
3.96
0.21
3.91
6.78
2.54
5.04
4.00
5.42
2.93
0.07
2.76
1.24
1.32
0.11
3.91
-0.16
0.08
5.44
2.48
1.37
2.27
9.22
3.71
6.11
7.18
10.49
5.49
0.44
2.26
0.94
2.27
1.10
2.86
0.65
-0.38
3.99
6.29
2.24
3.71
9.13
2.79
7.17
6.73
7.79
4.17
1.16
5.71
1.99
3.10
0.25
5.52
0.04
0.16
7.37
7.79
4.29
4.39
15.36
5.49
14.93
11.37
9.02
6.36
1.13
7.99
1.60
2.97
-0.80
8.57
-1.72
0.0
6.52
8.44
4.28
6.75
11.54
5.54
10.45
11.77
11.25
8.63
3.48
7.38
0.75
3.94
0.10
4.48
0.35
-1.15
6.17
8.75
3.94
2.60
12.67
5.44
10.37
9.46
9.33
8.42
2.25
6.34
1.75
2.78
1.22
3.03
0.97
-0.03
6.26
6.15
1.77
3.47
8.49
4.49
6.21
11.01
6.73
6.77
2.13
4.39
1.33
4.65
-------
Table 5
EFFECT OF SOIL MOISTURE ON THE UPTAKE OF CARBON MONOXIDE
BY POTTING SOIL EXPOSED TO 100 ppm CO AT 25°C
Soil
Moisture
(%)
5
10
16
19
22
28
CO Uptake
(mg/hr/m2)*
6.44
7.01
7.08
7.00
7.04
6.64
*
Average of 3 replicates.
26
-------
Table 6
COMPARISON OF THE CO UPTAKE CAPACITY OF SOILS
UNDER CULTIVATION AND SOILS UNDER NATURAL VEGETATION
Test Site
Grouard, Alb.
Athens, Ohio
Ft. McMurray, Alb.
Chenango, N.Y.
Wichita Falls, Tex,
Regina, Sask.
Van Meter, Mo.
Shrevesport, La.
Sabetha, Kans.
Grants Pass, Ore.
Cincinnati, Ohio
Prairie View, Kans.
Soils Under
Natural Vegetation
Vegetation Type Rate"
Boreal forest 4.2
Mixed deciduous 21.3
forest
Boreal forest 6.8
Deciduous forest 26.2
Grassland 11.9
Grassland 2,7
Oak-hickory forest 10.2
Flood plain forest 26.3
Grassland 3.6
Coastal forest 14.1
Oak-hickory forest 21.3
Bluestem prairie 2.3
Soils Under
Cultivation
Type Rate*
Plowed field
Plowed field
Plowed field
Plowed field
Wheat stubble
Plowed field
Newly planted
milo field
Plowed field
Milo stubble
Plowed field
Fallow field
Plowed field
1.1
3.4
2.4
1.8
7.5
1.6
2.9
4.3
2.9
2.2
3.4
1.2
mgCO/hr/m2, corrected for annual average temperature at test sites.
27
-------
Table 7
EFFECT OF TYPE AND QUANTITY OF ORGANIC MATTER
IN A SOIL ON ITS CO UPTAKE CAPACITY
Average CO Uptake
Organic
Treatment
Leaf mold
Peat Moss
Steer Manure
Organic
(%)
0
8
25
0
8
25
0
8
25
(mg/hr/m2) After:
2 Weeks
9.03
13 . 30
27.94
10.74
10.17
13.65
8.03
13.51
21.09
4 Weeks
7.47
19.13
30.22
9.46
15.86
12.23
8.53
16.57
21.47
9 Weeks
14.50
23.53
28.01
20.97
20.90
15.71
20.69
24.74
25.03
28
-------
70
60
50
| 40
I
E 30
Q.
a
£ 20
t
o 10
o
-10
-20
Fungicide
I
10 15
TIME — days
20
25
FIGURE 3 INFLUENCE OF AGRICULTURAL CHEMICALS ON CO UPTAKE BY SOIL
29
-------
dosages. Only Triox, a vegetation-killer containing pentachlorophenol
and prometone, was effective in reducing the soil CO uptake capacity,
and after a period of three weeks it appeared that even the soil treated
with Triox was resuming its capacity to take up CO.
Roadside Study
Since soils alongside roads and, in particular, major highways are
constantly exposed to high levels of CO, a study on the rates of CO up-
take by these soils was conducted. Table 8 depicts the results of this
study. In general, soils that were covered by a ground cover (e.g., ice
plant, ivy, or grass) had higher CO uptake rates. All the soils had gen-
erally higher rates than,soils would be expected to have at that latitude.
To test the hypothesis that the constant exposure of these soils to CO
was responsible for the high rates, a laboratory experiment was performed
in which soils were constantly exposed to various levels of CO and their
CO uptake capacity was monitored for several weeks. As shown in Figure 4,
soils exposed to 100 and 28 ppm CO developed higher rates of CO uptake
than did soils exposed to 5 ppm or to ambient air (control). The rate
for the soil exposed to 5 ppm was only slightly higher than that for the
control soil.
Tables 9 and 10 summarize the CO uptake potential of the soils of
the conterminous United States and the world. The values of CO uptake
for the various vegetative regions were averaged and corrected for tem-
perature variations, then multiplied by the area of that region. The
respective totals for the U.S. and the world potential uptake rates are
505 million and 14.3 billion tons annually. An inspection of these two
tables indicates that, potentially, the forest regions of the United
States could serve as major sinks due to the large proportion of the area
they cover and, on a global basis, the tropical regions are potentially
the largest sinks due to high year-round uptake rates.
30
-------
Table 8
SOIL ACTIVITY AT SITES ALONGSIDE A 20-MILE STRETCH OF THE
BAYSHORE FREEWAY (US 101) PRESELECTED ON THE BASIS
OF REPRESENTATIVE VEGETATION OR GROUND COVER
Test Site
CO Uptake
Vegetation (mg/hr/m3)
Avg.
Temp.
Soil
Moisture
Soil pH
Embarcadero Road
interchange
San Antonio Road
interchange
Mathilda Avenue
interchange
W. Bayshore Road*
+
Frontage Road"1"
±
Dirt road
San Tomas Express-
way interchange
Trimble Road
interchange
U.S. 85
interchange
Mathilda Avenue
off-ramp
W. Bayshore Road*
W. Bayshore Road*
Iceplant
Iceplant
Ivy
Ivy
Grass
Barley
Grass
Grass
Bare
Bare
Plowed
Plowed
64.5
47.9
52.5
43.0
63.4
32.7
36.9
17.8
22.6
16.2
23.1
15.0
65
85
66
60
69
81
80
76
95
82
77
71
7.6
7.8
8.2
8.3
7.8
7.6
7.4
8.0
7.7
7.7
7.0
8.7
19.6
5.7
3.9
19.2
10.0
2.5
5.1
7.3
6.1
3.7
4.2
3.5
Soil temperature inside test chamber.
Percent moisture of surface soil.
Site off freeway right-of-way, but within 100 ft of freeway.
31
-------
to
to
10 15 20 25 30
TIME OF CONTINUOUS EXPOSURE TO CO — days
35
40
FIGURE 4 EFFECT OF CONTINUOUS CO EXPOSURE ON THE CO UPTAKE RATE OF SOILS
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Table 9
POTENTIAL CO UPTAKE RATES OF THE SOILS
OF THE CONTERMINOUS UNITED STATES
Soil-Vegetation Type
Cropland
Pasture
Coastal forest
Deciduous forest
Montane forest
Southern mixed forest
Appalachian forest
Southern flood plain
forest
Sagebrush steppe
Sagebrush
Desert scrub
Paved roads
Covered area
Lakes, rivers, etc.
Total
Area
(mi2)
468,000a
a
616,674
b
329,390
b
339,485
b
87,150
b
48,644
b
265,519
b
58,782
b
264,867
b
145,047
b
220,723
c
28 , 100
d
26,500
d
78,267
CO Uptake1
(tons/yr/mi3)
86
179
200
254
242
86
313
595
76
52
0
0
0
.0
.3
.7
.8
.0
.5
.7
.0
.7
.2
Total CO Uptake
(tons X
40
110
83
86
21
4
84
34
20
19
0
0
0
106/yr)
.25
.57
.93
.50
.10
.21
.17
.95
.32
.09
2,977,128
505.12
Areas based on figures found in: (a) Statistical Abstract of the United
States, 1971, 92nd Ed., U.S. Department of Commerce, Washington, D.C.;
(b) Map of U.S. Forests, U.S. Geological Survey; (c) 1969 Highway Sta-
tistics, U.S. Department of Transportation, Washington, D.C.; (d) 1971
World Almanac, Newspaper Enterprise Assoc., Inc., New York, N.Y.
Corrected for annual temperature variations.
33
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Table 10
POTENTIAL CO UPTAKE RATES OF THE SOILS OF THE WORLD
Soil-Vegetation Type
Area Average CO Uptake"1" Total CO Uptake
(10s mi2)* (tons/yr/mi2) X 10 (tons X 10s/yr)
Agricultural
Pasture
Tropical grassland
Temperate grassland
Steppe
Montane forest
Taiga forest
Mixed and broad leaf forest
Southern pine forest
Tropical deciduous forest
Tropical rain forest
Tundra
Desert
Covered by ice, water,
roads, structures, etc.
Total
4.60
1.84
3.45
3.10
3.45
4.66
4.71
1.32
0.29
1.78
6.55
1.15
10.86
9.20
56.96
86.0
179.3
886.9
86.5
76.7
175.7
127.5
254.8
505.8
1,105.9
805.6
123.0
52.2
0
395.4
329.7
3,062.5
268.4
264.5
817.8
600.9
410.0
145.2
1,969.6
5,277.5
141.3
567.0
0
14,250.0
Areas based on figures derived by integrating vegetation areas found in map
types of natural vegetation in Readers Digest World Atlas (pp. 146 and 147)
and the 1971 World Almanac.
t
Corrected for annual temperature variations.
34
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DISCUSSION
The potential rates of CO uptake by the soils of the United States
and the world are estimated in this report to be 505 million and 14.3 bil-
lion tons per year, respectively. These rates are based on studies con-
ducted at a uniform exposure of 100 ppm CO and are thus an estimate of
the potential of the soil if exposed to this high level of CO. Recent
laboratory studies by Seller (16) indicate that the rate of CO uptake by
soils exposed to an ambient level of CO (0.2 to 1.0 ppm) is one-tenth of
the values obtained in this study. The actual CO uptake rates for the
soils of the United States and the world are thus probably somewhere
around 50 and 1.4 billion tons of CO annually—still a considerable frac-
tion of the estimated annual 407 million tons of CO produced by man glob-
ally.
The higher CO uptake rates for the soils in tropical regions of
Mexico are quite probably due to the higher microbial populations in
these soils due to favorable climate and high amounts of organic matter
in these soils. The rates of CO uptake in the tropical rain forests were
somewhat lower than expected, probably due to the high moisture content
of this soil, which may have limited diffusion of the gas. Also, the
rates in the arctic tundra were somewhat higher than expected, indicating
that the microbial populations in these soils increase during the summer
months when this soil warms.
The uptake of CO by agricultural soils was, in all cases, much lower
than the uptake rate for the same soil under natural vegetation. This
was most probably due to a lack of organic matter in the surface layer
of soils that are cultivated.
A study on the influence of agricultural chemicals, another sus-
pected cause of the reduced activity in cultivated soils, indicates that
these chemicals have only a minor effect, if any at all.
The study alongside the freeway in California indicates that the
uptake rate is highly dependent on the ground cover over it. This is
quite probably due to an increase in organic matter and friability of
soils with luxuriant ground cover, thus leading to a more active micro-
floral population in the soil. Prolonged gassing studies conducted in
the laboratory indicate that soils that are constantly exposed to CO con-
centrations above 5 ppm have relatively high CO uptake capacities. Soils
35
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alongside the freeway are constantly exposed to higher than ambient levels
of CO, and this may explain why they appear to be slightly more active in
CO uptake, on the average, than other soils tested. Since soils along-
side freeways are exposed to CO at higher levels on a more or less con-
tinuous basis, the rate of uptake of CO by these soils is increased due
to the higher starting levels of CO and also probably due to an induced
higher uptake rate for the soil.
An interesting aspect of this study is the evolution of CO by soils
treated with Triox during the first two weeks after treatment. This is
consistent with findings in previous studies that soils sterilized by
autoclaving evolved CO. Studies by Seller and Junge have indicated that
at very low levels of atmospheric CO (<0.2 ppm), an equilibrium is es-
tablished over a soil where CO uptake equals CO evolution by the soil.
Thus, while soils on a net basis are a large sink for CO, they also quite
probably are a natural source for CO as well. Information on the char-
acter of this CO equilibrium would help to answer questions that arise
when isotope determinations are used to calculate the sources of CO in
the atmosphere and the residence time of CO in the atmosphere.
It also appears from the roadside studies that the capacity of the
soil to remove CO from the atmosphere is inducible, since soils adjacent
to a major highway appeared to have higher than normal CO uptake rates
and laboratory studies showed soils exposed .continuously to 100 ppm CO
developed a CO uptake rate almost 10 times that of soils exposed to CO-
free air. This indicates that soils in areas of high ambient CO such as
the Los Angeles Basin may be contributing more as a CO sink than other soils.
It is clear from this study that the CO picture in respect to soils
is a highly variable and complex one and that future studies on the CO
equilibrium and CO uptake at ambient levels of CO should be conducted .
36
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