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IV. Sulfur Cycling in the Tropics
N. S. PASRICHA AND R. Id. FOX
' a *
Figure 2. Global sulfur cycle (units: lo6tons S/annum) (Robinson and Robbins, 1968).
Another pathway is the atmosphere-plant-soil route. This is called dry
deposition and is important in industrial and residential areas where fossil
fuels are burned. In the tropics, burning of vegetation is relatively more
important. Areas that have a marked wet-dry rainfall pattern giving rise to
savanna-type vegetation that is regularly burned no doubt lose much of the
S that accrues to them in rainfall in this way. Large areas of the tropics are
so affected. Burning is generally done in the dry season. Thus there is little
likelihood that S volatilized by agricultural burning will be redeposited on
land from which it came. A disproportionate quantity will accrue to downwind locations and to nearby areas where soils are moist and vegetation is
green (Fox and Blair, 1986). Burning is likely to be important in the
redistribution of S in tropical Australia. Most native vegetation is subject to
fire (Tothill, 1971), but we were unable to find reports on S losses from
such fires. Losses are likely to be appreciable, if data from burning of
heather (36% loss) are a guide.
Sulfur is one of the main components of atmospheric deposition. Atmospheric deposition impacts S cycling both directly and indirectly. The
inputs of S may be relatively large (60 kg S ha-' yr-l) in some ecosystems
(Johnson, 1984) and very few studies have been done on the effect of these
inputs on S cycling. Increased numbers of S oxidizers have been measured,
PLANT NUTRIENT SULFUR
but effects on S oxidation have been mixed (Wainwright, 1979, 1980). The
sulfur cycle is important in understanding the S budgets of soil. One
estimate of the global S cycle (Robinson and Robbins, 1968) is presented
in Fig. 2. Major components of the S cycle system are gaseous S in the
atmosphere, dissolved S in rainwater, S in surface and groundwater (imgation), inorganic S in soils, organic S in soils and plant residues, and S in
vegetation. In areas of coarse-textured, low-organic-matter soils that lack
significant capacities for surface sorption, the limit of S uptake (S yield) by
crops is approximately equal to dissolved S in precipitation. If incoming S
is in the range of 1 to 4 kg S ha-', sulfur deficiency becomes a major
constraint, even for low levels of production, in areas as diverse as the
United States (Alabama, Nebraska, and Hawaii), Nigeria, Australia, and
New Zealand (Fox and Hue, 1986). Gaseous S may contribute 20% or
more of the S taken up by plants, but this S source is not a significant factor
in most tropical areas.
Because the atmosphere is the major S source for most upland soils, the
S budget can be understood better if it is viewed in the context of the total
environment. It is, therefore, important to know the S compounds in the
atmosphere and their concentration and chemical behavior, the source of S
in the atmosphere, and the quantity of S supplied to soils from the atmosphere.
Atmospheric S is generally present in gaseous form as H,S and SO,, and
as particulate forms as sulfate. Considerations of the global S budget show
that HzS, SO,, and particulate SO, are present as trace constituents. These
are primarily of natural origin except in polluted areas where anthropogenic emission may dominate.
There are few reliable data on atmospheric H,S. Concentrations of this
gas are greatest near natural swamps, anaerobic waters, industrial sources,
volcanoes, geothermal wells, etc. Hydrogen sulfide has a background concentration between 5 and 50 parts in lo', parts of air by volume (Slatt et
al., 1978). It has been proposed that reaction of carbaryl sulfide and CS, is
an important source for both HzS ( McElroy ef af.,1980)and SO, (Logan et
af.,1979). The latter author postulated that the source of atmospheric SOz
formed from CS, and COS can be quite large. A major source of sulfides
from the oceans is dimethyl sulfide produced by phytoplankton. Estimates
of the total quantity of S released to the atmosphere in this way are greater
than estimates of anthropogenic S sources. Although the quantity of this
product is relatively large, the concentration is so low and the half-life is so
N. S. PASRICHA AND R. L. FOX
short that dimethyl sulfide is not an environmental problem. We assume
that sulfide is a major contributor to background S in the tropics, without
which S deficiency would be an even more serious problem than it is
already. Such considerations suggest that S emission control measures such
as are now being instituted, although locally effective, will be of little
significance on a global basis. For SO,, an average concentration in the
troposphere is about I p g m-3 STP. A generalized value for SO, is given as
0.9 ppb, but a value of 0.3 ppb is given for central Brazil. Values are higher
in Panama (Lodge et al., 1973).
Although the United States generates 35% of the world’s electricity and
consumes a corresponding fraction of fossil fuel, its production of SO, may
be less than 35% of the world’s production from energy sources (Kellog et
al., 1972).We will adopt the figure 100 X lo6tons of SO, per year for total
man-made contributions to the atmosphere. This corresponds to I50 X
lo6 tons per year of SO,, into which most of the SO, is converted. This is a
global figure. It is significant that only about 6.5% was produced in the
Southern Hemisphere [Massachusetts Institute of Technology (MIT),
Because seawater contains SO,, SO, concentrations in the air in coastal
areas usually are higher than they are further inland. In polluted surface air
the SO,: SO, ratio is about 10 times greater than in clean air (Junge, 1970).
Thus, SO4 concentration does not vary as much as SO, concentration.
Oxidation products of SO, are further oxidized to SO, by a variety of
processes. The net residence time for SO, in the atmosphere must be
similar to that of SO, because the overall ratio of SO, :SO, concentration
in the atmosphere seems to be close to unity (Georgii, 1970).
Quantitative estimates of global sources of S compounds in the atmosphere are presented in Table 11. The total amount of about 2 X lo8
tons/annum is of the same order as global industrial production or consumption of s. Sulfate aerosols are primarily produced naturally from sea
spray over the ocean. Most of it is deposited over the ocean by rain. The
large figures for H,S production were obtained indirectly by budget considerations and these may not be reliable.
The range of maritime sulfate concentrations, 0.22 - 2.72 p g m-3, determined around Asia by Horvath et al. (1981) are in good agreement with
results obtained over oceans in other parts of the world (Nguyue et al.,
1974a,b). The value of excess SO4 (0.87 pg m-3) is similar to the value of
0.9 pg m-3 reported by Grevenhorst (1978) for the North Atlantic. Although the origin and nature of precursor gas is not well understood, one
possibility is anthropogenic SO,. However, taking into account the residence time of SO, ( 1 -2 days) as well as wind direction observed during
sampling rules SO, out as a major source in favor of submicron sulfate
PLANT NUTRIENT SULFUR
Estimates of Sources of Atmospheric S per Yeara
Land (tons S)
Ocean (tons S)
7 0 X lo6
1 3 X lo6$'
4 4 x 106
3 0 X 106
3 x 106
4 4 x I06
103 X 106
73 x 106
220 x 106
Adapted from Robinson and Robbins (1968).
Total (natural anthropogenic).
particles from a sulfur gas of natural origin, namely, dimethyl sulfide, the
oceanic release of which is estimated at 27 Mt yr-'.
The relationship between soil sulfate and rainfall is complex. Rainfall
amount seems to have influenced soil sulfate in five ways: (1 ) SO, accession, (2) SO, retention, (3) SO, utilization, (4) S immobilization, and (5)
SO, leaching. Sulfate accessions are the combined effects of rainfall quantity and rainfall quality. Muller (1975) analyzed rainwater and lysimeter
leachates for total S for 20 years at the Otara Research Station, Auckland,
New Zealand. Losses of SO, in leachates from fertilized and unfertilized
soils were also determined. An average of 13 kg ha-' S was received
annually in the rainfall, with extremes of 6.6 and 17.6 kg ha-'. The proximity of tidal flats and accessions from aerial top-dressing may have been
responsible for contributions not exceeding 25%.
Sulfur concentration in rainwater in the continental tropics is usually
low. The deposition of S in rainfall was measured at 10 locations in Central
Kenya, monthly for 1 year (1977- 1978) by Bromfield el al. (1980).
Amounts deposited ranged from 1.58 to 3.81 (mean 3.47) kg S ha-'.
Concentrations ranged from 0.10 to 0.17 (mean 0.12) mg S liter-'. Low S
concentration in rainwater and depleted organic matter reserves of soils are
associated with S deficiency in the seasonally dry West African Savanna.
Mean annual S in rainfall for northern Nigeria is about 1.14 kg S ha-'
(Bromfield, 1974), suggesting that yields of cowpea, an important crop in
the seasonally dry savanna, are limited by S deficiency (Fox et al., 1977).
N. S. PASRICHA AND R. L. FOX
Distance (km) x altitude (m)
Figure 3. Influence of distance and elevation from the sea on the concentration of S in
rainwater (Fox el al., 1983).
On the other hand, rainfall, air deposition, and particulate matter contributed approximately 10.7, 1.8, and 3.0 kg S ha-' per year, respectively at
one location in the southern United States (Suarez and Jones, 1982). No
relationship was obtained between applied S and crop response for several
crops. Suarez and Jones (1982) emphasized the need to keep in view
contributions of atmospheric deposited S when making fertilizer recommendations.
Influence of the sea as a S source diminishes in proportion to a product
of distance and elevation from the source (Fox et a/., 1983). Sulfur concentration in rainwater dropped exponentially with increasing distance
from the coast in both New Zealand and Hawaii (Fig. 3). For example, in
Hawaii, rainwater contained 4.5 mg S liter-' 0.5 km from sea, 1.0 mg S
liter-' 3 km inland, and only 0.1 mg S liter-' 24 km from the coast.
Estimated total S inputs from rain were 24, 10, and 1 kg S ha-' at 0.5, 3,
and 24 km distance, respectively (Hue et al., 1990). Although New Zealand is only subtropical in its northern latitudes, data on the S composition
of rainwater should provide a useful example of the importance of the sea
in tropical regions. Maximum distance from the sea in New Zealand was
100 km. At that distance the sea was of little consequence but, even so,
0.2 mg S kg-' in rainwater has some agricultural significance. Fox et af.,
PLANT NUTRIENT SULFUR
1979) demonstrated that 0.2 mg SO,-S liter-’ was sufficient for approximately one-half maximum yield of banana. In the Southern Hemisphere
and in the tropics generally, background atmospheric S is low because
atmospheric S does not move readily across the tropical convergence zone.
Thus, the influence of the oceans can be more readily discerned there than
in the North Temperate Zone, where, because of pollution, the influence of
oceans is relatively less important.
V. EFFECTS OF ACID RAIN
The effects of acid rain (precipitation) are widely debated. Some of the
chemical compounds associated with acid deposition are important nutrients for both plants and animals. Water in equilibrium with CO, in the
atmosphere has a pH of approximately 5.6. As usually defined, “acid rain”
is rain with pH below 5.6 resulting from the solution of other acid-forming
constituents, such as SO,, directly from the atmosphere. This process is
known as “wet deposition.” The term “acid deposition” represents the
total deposition of acid from the atmosphere. With adequate precautions,
the acidity of precipitation can be measured with reasonable ease and
precision, but dry deposition is not so easily determined and little is known
of the magnitude or significance of dry deposition [Council for Agricultural Science and Technology (CAST), 19851.
A. EFFECTON CROPPLANTS
Experiments with simulated acid rain within the observed pH range of
acid precipitation have sometimes decreased crop yields, although Irving
(1983) concluded that the effects appear to be very small, and that when
responses are observed, they may be positive or negative. There is no
convincing evidence that acid precipitation as such is detrimental to crops
in the field. Increased incidence of blossom-end rot of tomatoes has been
associated with volcanic activity in Hawaii, suggesting that acid rain may
have brought on a Ca deficiency.
Faller ( 1 97 l), observed that crop yields increased with increasing concentration of SO, in the atmosphere (Table 111). For tobacco, total dry
weight increased up to 48%. The yield of leaves and stems increased by
SO%, reflecting reduced root yield frequently associated with improved S
status. Additions of SO, increased plant inorganic sulfate. Simulated acidic
rain on radish plants decreased hypocotyl growth but not shoot growth
N. S. PASRICHA AND R. L. FOX
Effect of Atornospheric SO,on Relative Dry Matter Yields of
Sunflower, Cow and Tobacco"
Relative yield, dry weight
(per x mg SO, M-' air)
Adapted from Faller ( 197I); by permission of The Sulphur Institute, Washington, D.C.
1 2 1 5
Internal flux of SO,
(nrnol ern-' hc')
Figure 4. Relationship between the internal deposition of SO, and the inhibition of net
photosynthesis in Viciu fubu (Black and Unsworth, 1979;Reprinted with permission from
Nuiure (London), Macmillan Magazines Limited.)