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Possibility for carbon sequestration in tropical and subtropical soils

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Soil organic matter is a key component of all terrestrial ecosystems, and any variation in its composition and abundance has important effects on many of the processes that occur within the system. The role of soil organic matter in soil nutrient cycling and soil gaseous emissions is discussed in the context of agricultural sustainability and global environmental change. Recent data on organic carbon and nitrogen reserves in the soils of the world are presented, with special reference to the subtropical and tropical regions. Possibilities for long-lasting, enhanced sequestration of carbon in the soil through management of the land and water resources are reviewed. Finally, the need is stressed for an up-to-date database on soil resources and for a global monitoring system in order to permit the study of changes in soil organic matter quantity and quality over time, as determined by changes in land-use and climate.
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Global Change Biology (1997) 3, 161–173
Minireview
Possibilities for carbon sequestration in tropical and
subtropical soils
N.H. BATJES and W.G. SOMBROEK
International Soil Reference and Information Centre (ISRIC), P.O. Box 353, 6700 AJ Wageningen, The Netherlands
Abstract
Soil organic matter is a key component of all terrestrial ecosystems, and any variation
in its composition and abundance has important effects on many of the processes that
occur within the system. The role of soil organic matter in soil nutrient cycling and soil
gaseous emissions is discussed in the context of agricultural sustainability and global
environmental change. Recent data on organic carbon and nitrogen reserves in the soils
of the world are presented, with special reference to the subtropical and tropical regions.
Possibilities for long-lasting, enhanced sequestration of carbon in the soil through
management of the land and water resources are reviewed. Finally, the need is stressed
for an up-to-date database on soil resources and for a global monitoring system in order
to permit the study of changes in soil organic matter quantity and quality over time, as
determined by changes in land-use and climate.
Keywords: carbon sequestration, global change, land management, soils, subtropics, tropics
Received 31 May 1996; revision accepted 29 July 1996
Introduction
Increased concentrations of radiatively-active trace gases to maintain and improve the organic carbon content and
nutrient supply in the soil, and to avoid land degradation.in the atmosphere are now recognized to modify global
climate, affecting terrestrial ecosystems both functionally In the 1980s, the average global releases of carbon to
the atmosphere were 1.1 Pg C y
–1
from land-use changes,and structurally. The importance of the soil as a sink and
source of atmospheric-C and the need for additional 5.5 Pg C y
–1
from fossil fuel combustion, with the
sequestration of carbon in terrestrial agro-ecosystems atmosphere gaining 3.2 Pg C y
–1
and the oceans absorbing
through sound management is a major point of discus- 2.0 Pg C y
–1
(Schimel 1995), reflecting the importance of
sions among scientists and policy makers. Alexandratos land-use change/conversion in the global carbon budget.
(1988) projected that from the mid-eighties to the year About 80% of the radiative forcing created by agriculture
2000 agricultural production in the developing countries arises in tropical agro-ecosystems, with CO
2
being an
will increase by about 60%, of which total 63% will important component (Duxbury 1995). Current budgets
come from increased yield, 15% from increased cropping for human-induced perturbation of CO
2
point to a small
intensity and 22% from a net extension of arable land. inferred terrestrial uptake of about 1.4 Pg C y
–1
, part of
Much of the predicted increase in harvested area will which is due to the ‘CO
2
-fertilization effect’ (1.0 Pg C
result from expansion into tropical rainforests and sub- y
–1
), forest regrowth in Northern Hemisphere (0.5 Pg C
tropical rangelands, much of which are inherently mar- y
–1
), and increased N deposition (0.6 Gt C y
–1
) (Schimel
ginal for crop production (FAO 1993). Tropical agriculture 1995). Bazzaz & Fajer (1992) described the physiological
is faced with the challenge of feeding 70% of the world’s aspects of the ‘CO
2
fertilization effect’.
population (Lal & Sanchez 1992). As the reliance on Land-use and management options that optimize sus-
marginal lands for food production increases, there will tainable production and enhanced sequestration of carbon
be an increasing need for developing sustainable systems in the soil are discussed in this paper, with special
reference to the tropics and subtropics. First, the functions,
properties and fractions of soil organic matter (SOM) are
Correspondence: Niels H. Batjes, fax 131/317-471700, e-mail
batjes@isric.nl
reviewed in relation to land-use and predicted climate
© 1997 Blackwell Science Ltd.
161
162 N.H. BATJES & W.G. SOMBROEK
changes. Secondly, estimates of contemporary soil carbon flora, through its effect on substrate quality and quantity,
and the nature of the fauna that causes litter decomposi-and nitrogen pools are discussed. It is then argued that
functional fractionation schemes of organic matter are tion (Eijsackers & Zehnder 1990). The chemical composi-
tion, amount and spatial distribution of fresh organicneeded in order better to understand and model changes
in the pool size and dynamics of SOM. This type of data residues haveavaryingeffect on thecontent and composi-
tion of organic matter in the soil. In some studies thereshould be collected for the main eco-regions of the world
within the framework of a global terrestrial monitoring was no correlation between the vegetational background
and the chemical structure of SOM (Oades et al. 1988;system. Linkage of such a monitoring system to spatial
data sets on the environmental factors and driving socio- Krosshavn et al. 1990), while in others the chemical
composition of plant debris was found to influence theeconomic variables would provide the necessary concep-
tual framework for studying changes in SOM pools in a rate of decomposition of fresh organic residue and of
humified materials in soil (Cuevas & Medina 1988;‘pressure-state-impact-response’ framework.
Hopkins et al. 1988; Chone
´et al. 1991). Horizontal patterns
of SOM distribution are related to differences in initial
Soil organic matter
soil organic matter content and to differences between
successional tree species, especially in the amounts of
Physical and chemical properties
woody litter produced (Pastor & Post 1986). There is a
great variation in the amount and vertical distributionSoil organic matter (SOM) is defined as the sum of all
organic substances in the soil. It consists of ‘a mixture of of SOM in tropical and subtropical soils (Sombroek
et al. 1993).plant and animal residues at various stages of decomposi-
tion, of substances synthesized microbiologically and/or
chemically from the breakdown products, and of the
Labile and stable fractions
bodies of live and microorganisms and small animals and
their decomposing products’ (Schnitzer 1991). Organic SOM in the topsoil will contribute most actively to
gaseous exchanges with the atmosphere and in nutrientconstituents of tropical soils do not possess any special
qualities that differentiate them from those of temperate cycling in the soil-water-plant system. The breakdown of
microbial biomass, which accounts for about 2–5% ofregions, nor do soils of the tropics necessarily contain
lower contents of SOM than thoseof the temperate region total C, is a rate-determining factor for these processes.
The mean turnover time for microbial biomass-C is 0.13–(Lal & Sanchez 1992).
The favourable effects of SOM on the physical, chemical 0.24 y in soils of the humid tropics and 1.4–2.5 y in
temperate European soils (Houot et al. 1991), reflectingand thermal properties of the soil and on biological
activity, and thus in sustaining soil productivity, are well the important role of temperature in controlling soil
carbon dynamics. Growing plant roots, a source of readilyknown. SOM has a stabilizing effect on soil structure,
improves the moisture retention and release character- decomposable photosynthates for microbial biomass,
often have a priming-effect on the breakdown of organicistics of soil, and protects the soil against erosion. Decom-
posing organic matter releases nutrients, such as N, P, S residues added to soil (Lynch 1990). The introduction of
graminaceous vegetation in succession to forest mayand K, essential for plant and microbial growth. Soil
organic matter is further an important determinant of stimulate microorganisms during the initial years, as a
result of which they will also decompose residues andthe cation exchange capacity, particularly in coarse tex-
tured soils and in ‘low activity clay’ soils (including even humified material from the original forest (Chone
´
et al. 1991; de Moraes et al. 1996).Ferralsols and some Acrisols and Luvisols, as prevalent
in the tropics). SOM further plays an important role in Globally,the mean residence time of soil organicmatter
is about 22 years (Post et al. 1992). The turnover time ofbinding metal-ions and in retention of non-ionic organic
compounds and pesticides. SOM increases with depth in the soil, ranging from
several years for litter to 15–40 y in the upper 10 cm and
over 100 y below a depth of about 25 cm (Harrison et al.
Main controls of SOM formation
1990; Lobo et al. 1990). Mean residence times for organic
carbon of 2000–5000 y have been reported for soilsMajorenvironmental controls of organic matter behaviour
in soil are moisture status, soil temperature, oxygen rich in allophane (Wada & Aomine 1975), reflecting the
importance of mineralogy and active aluminum and ironsupply (drainage), soil acidity, soil nutrient supply, clay
content and mineralogy. All other conditions being equal, hydrous oxides in ‘preserving’ organic materials in the
soil. Similarly, Ferralsols derived from basic or ultrabasicorganic matter accumulates most at low temperatures in
acid parent materials and in anaerobic conditions. parent materials have consistently higher organic matter
contents than those derived from acid materials. Numer-Biotic controls of SOM formation include the type of
© 1997 Blackwell Science Ltd., Global Change Biology,3, 161–173
C SEQUESTRATION IN TROPICAL AND SUBTROPICAL SOILS 163
Table 1. World soil carbon and nitrogen pools (in Pg of C and
ous studies have shown that the fractionation of soil
N, respectively)
separates gives significantly different C values for the
sand, silt and clay fractions, with the fine-textured frac-
Depth range (cm)
tions containing the oldest and highest amounts of carbon.
Carbon dating based on naturally occurring
14
C and
Region 0–30 0–100 0–200
use of the
13
C signal supplied by photosynthetic discrim-
Tropical regions
ination by C
3
and C
4
plants can be effectively used for
Soil carbon
determination of root versus soil respiration, plant residue
Organic-C 201–213 384–403 616–640
decomposition rates, the source of mineralized CO
2
, and
Carbonate-C 72–79 203–218 –
the turnover rate of different SOM fractions (Volkoff &
Total 273–292 587–621 –
Cerri 1987; Martin et al. 1990; Paul et al. 1995).
Soil nitrogen 20–22 42–44
SOM is often divided into various pools, ranging from
Other regions
biologically active to inert SOM, for modelling purposes
Soil carbon
(see Powlson et al. 1996). Adequate methods to experi-
Organic-C 483–511 1078–1145 1760–1816
Carbonate-C 150–166 492–530 –
mentally establish the partitioning of SOM over the
Total 633–677 1570–1675 –
different pools conceptualized in many of these modelling
Soil nitrogen 43–45 91–96
studies, however, are still lacking (Verberne et al. 1990;
World
Cheng & Molina 1995).
Soil carbon
Models for predicting the effects of different rotations,
Organic-C 684–724 1462–1548 2376–2456
tillage methods, and timing of organic residue addition
Carbonate-C 222–245 695–748 –
on nitrogen release, as reviewed in Powlson et al. (1996),
Total 906–969 2157–2296 –
now need to be tested for tropical and subtropical systems,
Soil nitrogen 63–67 133–140
incorporating the effects of annuals and trees where
* The tropics have been defined as the region bounded by
appropriate (Greenland et al. 1994).
latitude 23.5° N and 23.5° S. The first estimate is for soils is
‘with’ and the second ‘without’ a correction for stones. Source:
Batjes (1996).
Estimates of contemporary soil C and N reserves
findings, but greater depths should be used wherever
Organic carbon
appropriate.
Figure 2 shows data on organic carbon mass to a depthGlobal pool size. From the introductory review it follows
that soil databases which only hold data on total organic of 1 m by FAO-Unesco (1974) soil unit obtained using
the WISE database (Batjes 1996). Global averages rangecarbon content and limited information on land-use
(history) can provide only limited information on the from 3.0 kg m
–2
for the Yermosols of arid regions to
25.4 kg m
–2
for Andosols formed on volcanic ash, anddynamics of carbon in different agro-ecosystems upon
land-use or climate-induced changes. Nonetheless, they 77.6 kg m
–2
for Histosols in which the high organic
carbon content is the determining classification criterion.remain critical in estimating the size of the global soil C
and N pools. Estimates by soil unit and depth range still vary markedly
between different studies depending largely on the num-Four recent estimates of soil organic carbon mass in
the upper 1 m include 1220 Pg C (Sombroek et al. 1993), ber and location of profiles considered (see Sombroek
et al. 1993; Eswaran et al. 1995; Batjes 1996).1431 Pg C (Houghton 1995), 1555 Pg C (Eswaran et al.
1995), and 1462–1548 Pg C of which about 26% is stored Table 1 confirms the importance of considering the
organic carbon stored below 1 m depth in global carbonin soils of the Tropics (Batjes 1996; see Table 1). On
average, the soil contains about three times more organic budgets, especially for Histosols and deeply weathered
tropical soils. Generally, it has been assumed that thiscarbon than the vegetation (µ610 Pg C) and about twice
as much carbon than is present in the atmosphere (µ750 ‘deeper’ carbon is old and refractory so that its mass was
considered largely irrelevant in gaseous exchanges withPg C). Thus the possibility of enhanced carbon storage
in standing vegetation and in the soil is of great interest. the atmosphere. Radio-isotopic work by Davidson et al.
(1993), however, has shown that more than half of theFigure 1 shows there is little scientific justification to
consider only the first 1 m of the profile in calculations carbon released after conversion from forest to pasture
is from depths greater than 1 m, pointing at the need forof the soil carbon pool; many soils are deeper and contain
considerable amounts of organic and inorganic carbon in further research on the dynamics of this ‘deeper’ carbon.
Similarly, Nepstad et al. (1991) observed that roots andthe subsoil. Reference depths of 0–30 cm and 0–100 cm are
commonly used to facilitate the comparison of research soil organic matter may extend to depths in excess of
© 1997 Blackwell Science Ltd., Global Change Biology,3, 161–173
164 N.H. BATJES & W.G. SOMBROEK
Fig. 1 Distribution of organic carbon
with depth in different soil types of the
tropics and subtropics (based on data
from Sombroek et al. (1993)).
m
–2
for the Tropical Rainforest life-zone. Comparison of
these figures with those of other researchers (Post et al.
1982; Ojima et al. 1993; Houghton 1995) is complicated by
differences in definitions and procedures for aggregating
life-zones or ecosystems. Nonetheless, both Figs 2 and 3
illustrate the potentially large amounts of C that can
escape from the soil following deforestation, conversion
to grazing land, or draining of peatlands. Current carbon
emissions to the atmosphere from tropical deforestation
are estimated to range from 1.1–1.6 Pg C y
–1
, of which
loss of soil organic matter is estimated to account for up
to 30% with the remainder coming from trees (Houghton
1991). The percentage for SOM-loss may be even less if
the recorded increase in SOM in well-established pastures
after tropical deforestation or pasture improvement on
Fig. 2 Weighted soil carbon density by FAO-Unesco soil unit
tropical savannas (Fisher et al. 1994; de Moraes et al. 1996)
(kgCm
–2
, –1.0 m depth). Abbreviations: A 5Acrisols; T 5
proves to be of more than local occurrence.
Andosols; Q 5Arenosols; B 5Cambisols; C 5Chernozems;
F5Ferralsols; J 5Fluvisols; G 5Gleysols; M 5Greyzems;
O5Histosols; K 5Kastanozems; I 5Lithosols; L 5Luvisols;
Carbonate carbon
N5Nitosols; H 5Phaeozems; W 5Planosols; P 5Podzols;
Large accumulations of carbonate carbon in the soil occur
D5Podzoluvisols; U 5Rankers; R 5Regosols; E 5Rendzinas;
Z5Solonchaks; S 5Solonetz; V 5Vertisols; X 5Xerosols; Y 5
mainly in arid and semi-arid regions (Fig. 3), with global
Yermosols (based on data from Batjes 1996)
estimates for the upper 1 m ranging from about 700 Pg
C (Sombroek et al. 1993; Batjes 1996) to 1738 Pg C
(Eswaran et al. 1995). Carbonate carbon in the soil is8 m in some soils of the Brazilian Amazon. Ohta &
Effendi (1991) pointed at the important role of the subsoil relatively stable, unless soils are irrigated or become
acidified with increased nitrogen and sulphur inputs.in C, N and P cycling in a tropical rainforest in East
Kalimantan. Changes in secondary carbonate may have great effects on
both phosphorus (P) availability and the biogeochemicalEstimates by Holdridge life-zone. Mean soil organic carbon
contents (SOC-C) by Holdridge life-zone are shown in cycle of P, notably in dryland systems where most inor-
ganic P is present as soluble Ca-phosphates in the soil.Fig. 3. Weighted averages for SOC-C range from 3.2 kg
m
–2
for soils of the Hot Desert life-zone to 23.2 kg m
–2
Thus more data are needed on the rates of deposition
and weathering of carbonate-carbon in the soil underfor soils of the Boreal Forest life-zone, with estimates of
5.6 kg m
–2
for the Tropical Semi-Arid life-zone and 10.5 kg projected climatic and vegetational changes.
© 1997 Blackwell Science Ltd., Global Change Biology,3, 161–173
C SEQUESTRATION IN TROPICAL AND SUBTROPICAL SOILS 165
Fig. 3 Weighted soil carbon density by
Holdridge life-zone (kg C m
–2
–1.0 m
depth; SOC-C, soil organic carbon; CAC-
C , soil carbonate carbon; corrected for
coarse fragments—based on data from
Batjes 1996).
Soil nitrogen Possible effects of human-induced changes on
carbon sequestration
The nitrogen pool in the upper 1 m of the soil was
estimated at 96 Pg N based on a soil taxonomic approach
Land-use changes
(Eswaran et al. 1995), which is similar to the 92–117 Pg
of N obtained using an ecosystems approach (Zinke et al.A simple conceptual model of SOM accumulation, as
1984; Post et al. 1985). Batjes’s (1996) estimate of 133–140 developed by Johnson (1995), is presented in Fig. 4. At
Pg N is 13–32 % higher that the preceding values, possibly the left-hand side (I) it starts with steady-state conditions
due to the fact that the database used contains data for in which litter production (L) is equal to decomposition
a large number of agricultural soils where N levels may (D). The amount of SOM that corresponds with steady
have been increased by chemical fertilizer application. In state conditions in a certain natural ecosystem or agro-
comparison to about 10 Pg of N held in plant biomass ecosystem is determined by the soil forming factors, as
and about 2 Pg N in microbial biomass (Davidson 1994), defined by Jenny (1941). Since the average rate of C-
the total soil N pool is large. input is higher for natural tropical forest (µ10 Mg ha
–1
y
–1
) than for cultivated soils, the steady state level for
the former is high as compared to that of most cultivated
Sources of uncertainty
soils (see Greenland et al. 1992).
Upon a disturbance of the vegetation (II), Dwill firstSummarizing, recent estimates of total carbon and nitro-
gen in the upper 1 m of the soils of the world differ by exceed Ldue to increased mineralization and reduced
input of new organic materials. Mechanical clearing anda factor of about 1.6. The range is from 1940 Pg C
(Sombroek et al. 1993) to 3293 Pg C (Eswaran et al. 1995) cultivation may further cause the removal of topsoil
and disrupt soil aggregates, whereby the SOM presentfor total carbon, and from 92 Pg N (Zinke et al. 1984) to
140 Pg N (Batjes 1996) for soil nitrogen. The figures becomes more accessible to decomposing organisms. The
associated decrease in SOM tends to be greater in soilsillustrate that these global estimates are still not very
reliable. Accurate calculations of world soil C and N of the warm tropics compared to those of the temperate
regions, and it varies with the type of land-use changereserves are complicated by a number of factors, notably:
(a) the limited reliability of areas occupied by different (Buringh 1984; de Moraes et al. 1996). The effect of land-
use changes on soil carbon content generally becomeskind of soils; (b) the limited availability of reliable,
complete and uniform data for these soils; (c) the high negligible between a depth of 10–60 cm (Mann 1986;
Detwiler 1986), but other studies show evidence for lossesspatial variability in carbon and nitrogen content, stoni-
ness and bulk density of similarly classified soils; (d) the up to a depth of 100 cm (Brown & Lugo 1990). The wide
range in depth affected can be attributed to differenceslimited comparability and precision of analytical methods
used, and (e) the effects of climate, relief, parent material, in agro-ecological zone, soil type and land-use.
Bowman et al. (1990) observed that total soil organicvegetation, land degradation and land-use.
© 1997 Blackwell Science Ltd., Global Change Biology,3, 161–173
166 N.H. BATJES & W.G. SOMBROEK
old-agricultural soils in the Amazon Basin and ‘Plaggen’
soils in Western Europe (Sombroek et al. 1993), others are
not, once the beneficial land management practices are
stopped. This has been the case for one of the Rothamsted
Classical Experiments in which yearly applications of
farm-yard manure were stopped after about a century,
after which organic carbon levels dropped by 12–18% in
the first 5 y (Johnston 1991).
From the speed and magnitude of the changes
described earlier, it is clear that especially changes from
Fig. 4 Conceptual model of soil organic matter decomposition/
forest to agricultural use will have a marked effect on
accumulation following disturbance (a) stabilization at lower
soil C-pool size and CO
2
evolution to the atmosphere.
than original level; (b) stabilization at original level; (c)
Their nature, however, may change under predicted
stabilization at above-original level—after Johnson 1995, p. 358)
global warming and increased atmospheric CO
2
concen-
tration.
C, N, and P declined by 55–63 % in the surface 15 cm
after 60 years of cultivation, with about half of this loss
Climate change
occurring in the first 3 years of cultivation. In comparison,
the labile fractions of the organic C and N declined by Climate is a major factor in soil formation and it regulates
biodiversity and community changes. Brinkman & Som-67–75 % after 60 years, but over 80 % of the labile-C loss
and more than 60 % of the labile-N loss occurred during broek (1996) indicated that, in most cases, changes in the
soil by direct human intervention are likely to have athe first 3 years of cultivation. Half of the total P decline
came from the organic pool, representing about a 60 % greater impact (on the short-term) than predicted climate
change. The effect of global climate change on thedecrease in the organic P levels in the first 3 years.
The loss of carbon from the soil under shifting cultiva- dynamics of soil organic matter and nutrient flows
depends on the relative sensitivity of photosynthesis,tion generally is less than the loss under intensive cultiva-
tion, provided the recovery period is sufficiently long autotrophic and heterotrophic respiration to these
changes. According to Idso & Idso’s (1994) review of(.15–20 y). Data on C-dynamics under shifting cultiva-
tion from West Africa showed that the loss of C in 100 mainly short duration experiments, it appears that the
relative growth-enhancing effects of atmospheric CO
2
years ranged from 20% for a soil with a 12-y fallow cycle
to 45% for a soil with a 4-y fallow (Nye & Greenland 1960). enrichment is greatest when resource limitations and
environmental stresses are most severe. According toAfter each disturbance, a period of constant manage-
ment is required in order to reach a new steady state Sage (1995) elevated atmospheric CO
2
concentrations
increase the performance of C
3
plants relative to their C
4
(III). It may take from 10 to 50 y (Buringh 1984; Detwiler
1986; Brown & Lugo 1990) for soil organic C and between competitors, stimulate biological nitrogen fixation, and
enhance the capacity of plants toaccess limiting resources15 and 20 y for N (Brown & Lugo 1990), depending on
climate. Soil N levels seemingly recover faster in dry life- such as water and mineral nutrients. Idso & Idso (1994)
therefore argued that it could well be that the percentagezones than in humid life zones. In general, soils reverted
to natural ecosystems and under climax vegetation have growth response of natural ecosystems to atmospheric
CO
2
enrichment will be greater than that of managedhigher organic carbon than those of agro-ecosystems.
Upon the introduction of sound management practices agricultural systems. In how far these experimental trends
apply also to longer-term periods in ecosystems remainsor reforestation, whereby L.D(IV), the organic carbon
content in the soil will gradually increase towards a new to be assessed.
Ecosystem models discussed by Schimel (1995) suggeststeady state (V). This new equilibrium may be lower (line
A), similar (line B) or higher (line C) than the original that plant growth, under increased atmospheric CO
2
concentration, may become nutrient-limited in the longclimax level (stage I). The latter has been the case for old
agricultural soils to which large amounts of farm-refuse, term. In principle, this problem should not occur in
inherently fertile soils, while in chemically poor tropicalrich in phosphorus, have been added over prolonged
periods (Sombroek 1966; Bennema 1974; Sandor & Eash soils such as Ferralsols it can be addressed by larger
applications of chemical fertilizers and promotion of1995), for agricultural soils supplied with large amounts
of farmyard manure annually for over 100 y (Johnston integrated plant-nutrition measures in agro-ecosystems.
´az et al. (1993), however, have shown that there may1991), and for soils under corn supplied annually with
NPK for 30 y (Gregorich et al. 1996). While some of these be a feedback mechanism in which elevated CO
2
concen-
tration causes an increase in substrate release into theincreases are of a long-lasting nature, as is the case for
© 1997 Blackwell Science Ltd., Global Change Biology,3, 161–173
C SEQUESTRATION IN TROPICAL AND SUBTROPICAL SOILS 167
rhizosphere by non-mycorrhizal plants, leading to min- the world is also an important issue, notably for irrigated
agriculture (WRI 1994).eral nutrient sequestration by the expanded microflora
and a consequent nutritional limitation for plant growth. Climatic shifts may be accompanied by increases in air
temperature, changes in seasonal rainfall distribution,Lambers et al. (1995) suggested that effects of increased
temperature, under elevated atmospheric CO
2
concentra- increased atmospheric N and S deposition, burning and
so on. This makes prediction of changes in SOM dynamicstion, on the allocation pattern of C between roots and
leaves can be largely explained by the effect of root difficult when other factors such as biodiversity, including
the incidence of insect pests of crops and their predators,temperature on the root’s capacity to transport water.
Changes in plant communities associated with changes and community changes are considered. Since the Q
10
for litter decomposition is greater (1.3–4.0) than the Q
10
in climate will affect litter quality, which may lead to
either positive or negative feedbacks to the atmosphere for primary production (1.0–1.5), litter decomposition is
projected to proceed more rapidly under scenarios withdepending on whether lignaceous perennials or non-
lignaceous annuals take over. Jongen et al. (1995) found increased temperature (Kohlmaier et al. 1990), provided
the water and nutrient supply do remain adequate.evidence for higher carbon sequestration in soils of
grassland areas as the atmospheric CO
2
-concentration Several model studies suggest that the effects of temper-
ature and water availability on soil respiration are smallerincreases. Similarly, model simulations by Parton et al.
(1995) pointed at tropical savannas becoming small sinks than those associated with the CO
2
fertilization effect
(Klein Goldewijk et al. 1994), which would result in afor soil carbon. This supports observations by Fisher et al.
(1994) for managed savannas in South America that lack net terrestrial sink for atmospheric CO
2
. Thus it seems
worthwhile to further explore the possibility to maintainsignificant woody vegetation. Since the overall quality
and degradability of plant organic matter may decrease or improve soil carbon and nutrient status by manipulat-
ing the activities of soil fauna and flora.under doubled CO
2
concentration in the atmosphere
(Bradbury & Powlson 1994; Cou
ˆteaux et al. 1995), this
would lead ultimately to an accumulation of organic
Management options for carbon sequestration
matter in soil (Lekkerkerk et al. 1990; Sombroek 1995).
on tropical and subtropical lands
Brinkman & Sombroek (1996) argue that a transient
scenario of global change, i.e. a gradual doubling of CO
2
Management techniques for enhanced C-sequestration
must consider differences in soil type, their suitabilityconcentration over the next 100 years or so, would have
a largely positive effect on the soil, as agro-ecosystems for different uses and the factors of soil formation, notably
the local biological activity in which man is often awould have time to adapt to increased photosynthesis
and water use efficiency. The latter would apply if the predominating factor. Basically, there are three manage-
ment strategies for increasing SOM content in the soilincidence of extreme weather events does not increase
drastically, the amount of weatherable minerals in the (Kern & Johnson 1993): (i) maintain current levels of
SOM; (ii) restore depleted SOM levels; and (iii) enlargesoil remain adequate, the soil moisture regime remains
favourable for plant growth, and the land resources are and maintain SOM above the natural carrying capacity
or steady state (see Fig. 4).properly managed. These prerequisites, however, may
not apply in increasingly large sections of the world.
About 305 310
6
ha of the terrestrial globe is so severely
Reforestation and afforestation
degraded by human activities (Oldeman et al. 1991) as to
greatly reduce possibilities for increased plant growth McFee & Kelly (1995) published a state-of-the-art review
on carbon forms, functions, sequestration and C-fluxesand enhanced carbon sequestration through improved
management in the ‘short-term’; natural restoration after between the terrestrial biosphere and the atmosphere,
with emphasis on forest soils. Recent work by Grace et al.serious degradation may require from 50 to 100 y
depending on the soil’s resilience. Many cultivated soils (1995) suggests that tropical forests, in total, are net
absorbers of about 0.9 Pg C y
–1
, but this subject shouldare becoming increasingly deficient in essential plant
nutrients from cropping for decades without replenish- be researched further (Houghton 1995).
Reforestation or afforestation of bare land or redundantment (Hartemink & Bridges 1995) or are getting over-
loaded with chemicals (Stigliani 1991), and their physical arable land has been proposed as a good measure for
(temporarily) sequestering atmospheric CO
2
in growingcondition and biodiversity has deteriorated. According
to Greenland et al. (1994) only 10% of tropical soils trees and for reducing the greenhouse effect (Kimball
et al. 1990; Houghton 1990; Bekkering 1992). Goudriaanhave the natural fertility to be suitable for continuous
cultivation without enhancement, pointing at the need for (1990) showed that in order to produce the amount of
fuelwood equivalent to the world use of fossil fuel-energyexternal nutrient inputs to sustain yields. The declining
quantity and quality of water resources in many parts of anno 1980 (15 Pg stemwood) would require from 10 to
© 1997 Blackwell Science Ltd., Global Change Biology,3, 161–173
168 N.H. BATJES & W.G. SOMBROEK
37 310
6
km
2
of managed forests annually. Many re- and pool (see Woomer & Swift 1994). Most management
efforts to increase carbon sequestration in the soil willafforestation studies, however, do not consider where
these vast number of trees could be planted, nor whether result first in a build-up of the more labile C-fractions.
In agro-ecosystems, as compared with native grasslandthe available land is suited to grow these trees, nor
whether this land is needed for other purposes than C- ecosystems, increased aeration due to tillage and cultiva-
tion causes higher mineralization at the litter level andsequestration (food, fibre, timber, housing) (see Ciesla
1995). less transformation of plant-accumulated C into stable
SOM (no more than 20% of annual production). InShukla et al. (1990) coupled numerical models of global
atmosphere and biosphere to assess the effects of Amazo- contrast, more than 40% of plant-C passes through stable
SOM in native prairie (Buyanovski & Wagner 1995). Thusnian deforestation on regional and global climate. When
tropical forests were replaced by degraded pastures, the wide-spread implementation of tillage practices that limit
depth and intensity of disturbances – minimum tillage –model indicated there would be a significant increase in
surface temperature, a decrease in evapotranspiration could enlarge the soil carbon pool significantly, the ulti-
mate aim being to increase the fraction of soil C storedand precipitation over the Amazon basin. The simulation
further showed that the length of the dry season would in stable micro-aggregates, reducing its accessibility to
micro-organisms. Response to minimum tillage will varyincrease, which would make it difficult to re-establish
forests as potential captors of atmospheric CO
2
after with soil type (Frede et al. 1994; Lal 1995); dry regions
with high temperatures are considered more suited forextensive deforestation. Since uncontrolled clearing of
primary forest land for other uses is often associated reduced tillage than cool and rainy locations, which are
considered more suited for conventional tillage. Somewith a significant loss of nutrients and soil degradation,
this may form an additional limitation for widespread tropical soils, such as Vertisols, must be tilled regularly
to maintain a good seedbed and topsoil structure. In suchC-sequestration through reforestation. This would hold
especially in areas where the inherent fertility of the soil cases, mixed- and inter-cropping have the advantage that
the soil is very rarely left completely bare, reducing theis low and chemical fertilizers are difficult to obtain.
risk of erosion by water and wind. In the humid tropics,
minimum tillage methods may need to be combined
Modifications in agro-ecosystems
with herbicide use for weed control, possibly leading to
environmental pollution and a reduction in biodiversity.The role of agriculture in sequestering organic carbon
remains ambiguous. The overall picture is confounded Some measures that enhance C-sequestration in the
biomass and soil, such as N-fertilizer application, mayby technological, social, economic and cultural factors
which must be addressed specifically in management of negatively affect emissions of greenhouse gases such as
N
2
O. Wetland rice soils are a special case with respect tofragile and ecologically sensitive ecosystems. Important
soil management practices to sequester carbon and reduce carbon fluxes. When fresh organic materials are incorpor-
ated in these soils when flooded, fermentation processesthe risk of soil degradation (both on-site and off-site)
include: (i) conservation tillage in combination with lead to methane formation and emission. Thus wetland
rice soils are not appropriate to use as sites to sequesterplanting of cover crops, green manure and hedgerows;
(ii) organic residue management; (iii) mulch farming, carbon. While flooded, predominantly anaerobic soils are
sources of CH
4
, aerobic soils can be a sink for atmosphericespecially in dry areas; (iv) water management, including
in-situ water conservation in the root-zone, irrigation, CH
4
(see IPCC 1992).
and drainage to avoid potential risk of salinization and
water-logging; (v) soil fertility management, including
Evidence of enhanced soil carbon sequestration
use of chemical fertilizers and organic wastes, rhizobium
inoculation, liming and acidity management in order to Commonly reported historical losses of soil carbon upon
cultivation, such as in the ‘US dust bowl’, were oftentake full advantage of the CO
2
-fertilization effect; (vi)
introduction of agroecologically and physiologically associated to low production levels, inadequate fertilizer
application, removal of crop residues and intensive till-adapted crop/plant species, including agroforestry; (vii)
adapting crop rotations and cropping/farming systems, age. Sombroek et al. (1993) drew attention to historical
examples of sustainable doubling of the SOM componentwith avoidance of bare fallow; (viii) controlling of grazing
to sustainable levels; and, (ix) stabilization of slopes of soils under century-long human occupation. Encour-
aging reports have become available also on enhancedand terraces.
Management practices should be aimed at optimizing C-sequestration under improved grasslands both for the
tropics and subtropics (Liegel 1992; Fisher et al. 1994), asCO
2
-utilization from soil and plant in photosynthesis to
increase crop productivity and yields, and at increasing well as on the positive effects of agroforestry in the
Brazilian Amazon (Smith et al. 1995; de Moraes et al. 1996).especially the passive fraction of the soil organic matter
© 1997 Blackwell Science Ltd., Global Change Biology,3, 161–173
C SEQUESTRATION IN TROPICAL AND SUBTROPICAL SOILS 169
Stable increases in SOM in deeply weathered tropical processes (Mellilo 1994; Klein Goldewijk et al. 1994;
Schimel et al. 1995; Parton et al. 1995; Powlson et al. 1996).soils occur especially with additions of phosphate and
lime (as slowly soluble rock phosphate). A possible One of the greatest queries to projections of global change
and ecosystem models may well come from the realm ofexplanation for this phenomenon is that complexation of
SOM with kaolinite and minerals having surface Fe-OH biology in that these models still account little for the
complex functioning of the living world, and the soil inand Al-OH groups, phosphate and calcium may have
altered clay mineralogy; a result of this is that the organic particular. In addition, most of these models do not
consider the critical effects of human-induced landcarbon present is better protected against degradation, a
hypothesis worthy of further laboratory research. degradation on crop/ecosystem productivity nor the
effects of the increasing demand for food production onApplication of lime to these ‘low activity clay’ (LAC)
soils will further increase their nutrient retention capacity the environment.
Confidence in assessments of land-use and globaland raise the pH to a level above which exchangeable
aluminum is no longer toxic to most crops. In addition, change impacts on SOM dynamics atregional and contin-
ental scales will be improved greatly by reducing uncer-increased soil P levels will allow many crops to root
deeper, making them less vulnerable to drought. Organic tainty in current biophysical data sets (Leemans & Van
den Born 1994; Pan et al. 1995; Turner et al. 1995), as wellcompounds, probably by competing for sorption sites,
can substantially reduce P-sorption of tropical LAC-soils as by model evaluation. While significant progress has
been made in developing geographically explicit soil(Mesquita Filho & Torrent 1993), thereby rendering P-
application and P-cycling through organic residue decom- databases, the available information needs to be updated
(Oldeman & Van Engelen 1993; Batjes & Bridges 1994;position more effective.
The root-system of tropical pastures can be used effec- Arnold 1995; Madsen & Jones 1995). On a timescale of
10–15 year, ISSS/FAO/UNEP and ISRIC will be updatingtively to sequester and redistribute carbon deeper in the
soil profile (Nepstad et al. 1991) where it tends to be the information on world soil resources in SOTER (Van
Engelen & Wen 1995). In order to be most useful to thebetter protected and less susceptible to decomposition.
Carbon sequestration in many grasslands in semi-arid international modelling community, such georeferenced
databases must be linked to updated databases on theareas can be increased by reduction of biomass burning,
by raising the nutrient status of the soil and by introducing main driving factors of land-use and climate change (see
Turner et al. 1995).improved grasses and legumes in combination with
controlled stocking rates (Fisher et al. 1994). Large-scale Global monitoring systems are needed that register
changes in soil quality with time in conjunction withfield programmes to stimulate organic-carbon storage in
tropical soils, in combination with application of locally changes in the driving biophysical and socio-economic
forces.A Global TerrestrialObserving System (GTOS),con-available rock phosphates, thus would serve two pur-
poses: (i) increased productive capacity of the soil neces- ceived by FAO, UNEP, UNESCO, WMO and ICSU/IGBP,
is about to start. With respect to carbon turn-over andsary to feed the growing population, and (ii) a reduction
in the current rise in atmospheric CO
2
concentration sequestration, monitoring systems should consider SOM
fractionation schemes that produce repeatable, interpret-through enhanced terrestrial carbon sequestration. Lal
et al. (1995) estimated that with improved land-use, able and meaningful data on carbon pools, whereby the
impact of different land-uses and climate change on thecultivated and (resilient) degraded soils can annually
sequester 0.1–1 Pg of carbon depending on management. dynamics and size of soil carbon pool can be studied in
space and time. Remote sensing techniques, while provid-According to Sampson et al. (1993), globally, agro-eco-
systems may be converted to a net carbon sink of up to ing a powerful tool for assessing the dynamics of the bio-
sphere and climate, cannot yet be used to monitor the7 Pg C during the next 50 y by use of appropriate
soil management practices; this would require increased chemical and physical properties of soils. Although soil
organic carbon status can be related to normalized differ-production and major improvements in management on
much of the world’s cultivated areas, and notably in the ence vegetation index (NDVI) data, additional collection
and manipulation of site information remains necessaryless-developed regions.
for local-, regional- and global-scale prediction (Merry &
Levine 1995). Operational and scientific problems associ-
Conclusions
ated with the accuracy and regional representativeness of
the various spatial and attribute data in global databasesThe study of past, present and future global exchange of
atmospheric CO
2
associated with growth of terrestrial are well known, yet difficult to remedy in studies based on
available data. Right now there is an unfortunate tendencyplants and soil respiration remains difficult in view of
the ubiquitous, and spatially and temporarily diverse to neglect ground surveys of soil and terrain conditions
and the supporting laboratory analyses of soils at repres-nature ofthe underlying physical, chemical and biological
© 1997 Blackwell Science Ltd., Global Change Biology,3, 161–173
170 N.H. BATJES & W.G. SOMBROEK
entative sites. Compensation and recognition to so-called
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