Capítulo 3 -...

Capítulo 3Mudanças ambientais

É preocupação crescenteSecar podem os mananciaisSe o clima ficar tão quente.

Plantas, homens e animaisPoderão ficar doentes

E alguns biomas globaisTerão hostis seus ambientes.

Geovane Alves de Andrade

81

Savannas and Global Climate Change:source or sink of atmospheric CO2

Rattan Lal

AbstractSavanna biomes, occupying an area of 20 x 106 km2 in the tropics and of 9 x 106 km2 intemperate climates, cover ~ 20 % of the earth’s surface. With high biomass productivity,savannas play a major role in the global carbon (C) cycle. In the tropics, these biomes arefound in Africa, South America, Asia and the Pacific; in temperate climate regions theycomprise prairies of North America as well as derived savannas in Europe. Savannas arerapidly being used for agricultural, forestry and urban purposes. The total biomass C pool insavannas and grasslands may be 326 Pg out of a global total of 2137 Pg. The soil C pool ofthe savannas ranges between 10 % and 20 % (from 150 Pg to 300 Pg) of the global total poolof 1550 Pg. Fire is an important factor to determine variables such as the amount ofvegetation cover, species composition and elemental cycling. It also emits soot/black C (BC)and aerosols to the atmosphere with a correspondent impact on radiation average andprecipitation, thus contributing for the presence of BC as an important component of soil Cpool. Conversion of native ecosystems to agricultural ones decrease precipitation andevapotranspiration but increase air/soil temperatures. They also reduce the ecosystem Cpool, especially the biomass C and soil organic C (SOC) pools. With judicious land use andthe adoption of recommended management practices, savannas (grasslands and rangelands)have a potential C sink capacity of 0.5 Pg C yr -1 -1.7 Pg C yr -1. However, degraded pasturesand croplands are a source rather than a sink of atmospheric CO2. Conversion of plow tillageto no-till (NT) can increase the SOC pool in some soils, especially in the top 0 cm-30 cmlayer. The rate of SOC sequestration in NT systems is of 3 Mg C ha-1 yr -1 -1.1 Mg C ha-1 yr -1.Conversion to NT in degraded soils, however, can lead to a decline in the SOC pool duringthe first 2 to 5 years. Detailed assessment of an ecosystem C budget, with complete life cycleanalysis, is needed to determine whether savannas are a source or sink for atmospheric CO2.Research information is needed on NPP regarding the amount of above ground and belowground input to the SOC pool, the fate of the SOC once land use is changed, and the impactthat managerial interventions, including fire, may have. Such data are scanty but urgentlyneeded to identify policies and develop appropriate decision support systems.

Savanas: desafios e estratégias para o equilíbrio entre sociedade , agronegócio e recursos naturais82

Introduction

Atmospheric concentration of CO2 increased from 280 ppm, as measured in 1750,to its current 2008 level of 387 ppm, the cumulative increase being 37 %. It is currentlyincreasing at the rate of about 1.5 ppm or 0.5 % yr -1. Principal sources of CO2 and othergreenhouse gases (GHGs) are fossil fuel combustion, land use conversion and agriculturalactivities. Land use change, soil cultivation, and drainage of wetlands (peat soils) havebeen sources of GHG emissions ever since the dawn of settled agriculture. However,restoration of degraded soils and ecosystems and the adoption of best managementpractices (BMPs) are potentially able to off-set anthropogenic emissions by about 50 ppmover 40 to 50 years. It is in this connection that judicious management of the savannascan play an important role in moderating the global C cycle and mitigating global warmingby off-setting fossil fuel emissions.

The savanna biomes cover a global area of 29 x 106 km2, including 20 x 106 km2 inthe tropics and 9 x 106 km2 in the temperate regions (SCURLOCK; HALL, 1998; CHEN et al.,2003). These biomes account for 20 % to 30 % of primary production (IPCC, 2000; GRACE etal., 2006). Savannas are a highly diverse ecosystem comprising tropical savannas (TS)and temperate prairies and grasslands (TPG). TS biomes include large areas in Africa,South America and the Pacific. TPG biomes comprise prairies and steppes of NorthAmerica and Russia and the derived savannas in Europe. TS biomes are caracterized by aclimate with distinct wet and dry seasons leading to strong patterns of physiological andecophysiological processes. TS are among the most seasonal of the world’s major biomeswith strong and contrasting climatic conditions within a year, as well as high variabilitybetween years (VARELLA et al., 2004).

There are three global regions with predominance of TS biomes: (1) Africa with anarea under TS of 15.1 x 106 km2 or 50 % of its continental land area (30.1 x 106 km2), and(2) South America with an area under TS of 2.1 x 106 km2 or 11.7 % of its continental landarea (17.8 x 106 km2), and (3) Asia and the Pacific with distinct TS biomes. The AustralianTS biomes cover an area of about 2 x 106 km2 or about 12 % of the world’s savanna.

The Cerrado, the main TS in South America, refers to the common savanna-likevegetation of low trees, scrub brush and grasses. It occurs entirely within Brazil, andcovers approximately 2 x 106 km2 (204 Mha) or 23 % of Brazil’s land area (BUSTAMANTEet al., 2006). In the Cerrado, about 127 million hectares (Mha) out of 204 Mha (62 %) issuitable for agriculture (LILIENFEIN; WILCKE, 2003). The annual precipitation in the Cerrados

Savannas and Global Climate Change: source or sink... 83

ranges from 600 mm to 2200 mm. It is characterized by a dry season that lasts from 4 to 7months. The mean annual temperature varies from 22 °C to 27 °C (BUSTAMANTE et al., 2006).Cultivated pastures in the Cerrado region cover about 66 Mha (SANO et al., 2000). Pasturesare prone to degradation by excessive grazing (DA SILVA et al., 2004). Total area under arableland use, mostly soybean, is estimated at 18.0 Mha (JANTALIA et al., 2007).

Land area, net primary productivity (NPP), total C pool, C sink capacity and therate of C sequestration for global biomes are shown in Table 1. TS and TPG biomes have abiomass C pool of 326 Pg out of the global biomass C pool of 2137 Pg (~15 %). With NPPat about 20 Pg C yr -1, TS and TPG both have a C sink capacity of about 0.4 Pg C yr -1 out ofa global C sink capacity of 2.6 Pg C yr -1 (Table 1). Therefore, understanding components ofthe ecosystem C pool is essential to identify management options to harness the C sinkcapacity. This information is not available for site-specific soil, land use and other

physiographic characteristics.

Table 1. Land area and total net primary productivity of tropical savannas and other ecosystems.

Ecosystem Area Total C Pool NPP C Sink Capacity C Sequestration

(106 km2) (Pg C) (Pg C yr -1) (Pg C yr -1) (Mg C ha-1 yr -1)

Tropical savannas & 27.6 326 19.9 0.39 0.14

grasslands

Temperate grasslands 15.0 182 5.6 0.21 0.14

Tropical forests 17.5 553 21.9 0.66 0.37

Temperate forests 10.4 292 8.1 0.35 0.34

Boreal forests 13.7 395 2.6 0.47 0.34

Crops 13.5 15 4.1 0.02 0.01

World 149.1 2137 67.6 2.55 2-3

Source: Adapted from Grace et al., 2006.

The ecosystem C pool has three major components: (i) above ground biomass anddetritus material; (ii) below ground biomass; and (iii) soil organic carbon (SOC) pool. Mainfluxes consist of gross primary productivity (GPP), soil and plant respiration, erosion andleeching as well as humification (Fig. 1). The magnitude of pools and fluxes in naturalecosystems depends on soil, climate, physiography, and vegetation. An example of theecosystem C pool of the eucalyptus-dominated savanna in northern Australia is shown inFig. 2. Chen et al. (2003) reported that the total C pool of the natural savanna in northernAustralia is of 204 ± 53 Mg C ha-1, where approximately 84 % of it is below ground and 16 %


is made of above ground C pools. The SOC pool is 151 ± 33 Mg C ha-1 (74 % of the

ecosystem C pool). The biomass C pool is of 53 ± 20 Mg C ha-1 of which 39 % is in the root

and 61 % in the shoot (trees, shrubs, grasses). Annual gross primary productivity (GPP) is

20.8 Mg C ha-1, of which 5.6 Mg C ha-1 comprises above ground components and

15.2 Mg C ha-1 the below ground components. The NPP is 11 Mg C ha-1 yr -1 of which

8.0 Mg C ha-1 yr -1 is below ground and 3.0 Mg C ha-1 yr -1 is above ground. Annual soil C

efflux is 14.3 Mg C ha-1 yr -1 of which about 75 % occurs during the wet season. The natural

ecosystem is a net C sink during the wet season and a weak source during the dry

season. The residence time of C, calculated as a ratio of total biomass C to NPP, differs

according to the different biomes: 3.4 to 5 yr in the natural savanna (CHEN et al., 2003;

SCHOLES; HALL, 1996), 8.6 yr in the woodlands (WHITTAKER; LIKENS, 1973) and of 10-16 yr in

the tropical rainforest (TRF) (MALHI et al., 1999). Along with concentration and radioactive

forcing, residence time is an important determinant of global warming potential (GWP) of

GHGs (1 for CO2, 21 for CH4 and 310 for N2O).

Fig. 1. Ecosystem carbon pools and fluxes in natural and managed land use in thesavanna ecosystem.

SOC

(A) Natural sources (B) Agricultural ecosystem

SOC

Emis

sion

s of

Co

, CH

, NO

24

2

Emis

sion

s of

Co

, CH

, NO

24

2

GPP

GPP

Eros

ion

Leec

hing&

Eros

ion

Leec

hing&

Hum

ifica

tion

Hidd

en C

cos

ts

Hum

ifica

tion

Above groundBiomass

Below groundBiomass

Above groundBiomass

Below groundBiomass


Similar to what has been done in the Australian savannas, the ecosystem C pool in

the Cerrado has also been measured. The average pool in the Cerrado is estimated at

29 Mg ha-1 in vegetation and 117 Mg ha-1 in soil (1-m depth), which translates into a total of

5.9 Pg in the entire vegetation and 23.8 Pg in all soils (IPCC, 2000). Because of a large

variability, the site-specific pool varies widely among soils and local conditions. The SOC pool

ranges from 87 Mg C ha-1 to 210 Mg C ha-1 (BUSTAMANTE et al., 2006). Abdala (1993)

estimated the total C pool of a Cerrado in central Brazil at 265 Mg ha-1. Vegetation coverage is

arboreal (28.5 Mg ha-1), herbaceous (4 Mg ha-1) and litter (5 Mg ha-1). Roots and detritus

comprise 42.5 Mg ha-1. The SOC pool to 1-m depth is estimated at 185 Mg ha-1.

Fig. 2. Ecosystem C pool and flux in the eucalyptus savanna of northernAustralia The GPP is 20.8 Mg C ha-1 yr-1 comprising of 5.6 Mg C ha-1 yr -1

in the above ground and 15.2 Mg C ha-1 yr -1 in the below ground. The NPPis 11 Mg C ha-1 yr -1 of which 73 % (8 Mg) is below ground and 27 % (3 Mg)is above ground.Source: Redrawn from Chen et al., 2003.

Hum

ifica

tion

Above groundBiomass

32.3 ± 12.2 Mg C ha-1

Below groundBiomass

20.7 ± 6.0 Mg C ha-1

SOC Poll151 ± 33 Mg C ha-1

Resp

iraat

ion GP

P 20

.8 M

g C

hayr

-1-1

14.3

Mg

Cha

yr-1

-1

5.6 Mg C ha ha

yr-1

-1

15.2 Mg C ha

yr-1

-1


This article collates and synthesizes available information to determine whether

savannas, and more specifically TS biomes, are sink or source of GHGs. The specific

objective is to identify land use and soil/vegetation management options that may

enhance the ecosystem C pool in TS biomes.

Fire and Emission of Greenhouse Gases

In view of the interest in sources and sinks of GHGs, it is important to understand

both the magnitude and the determinants of gaseous fluxes caused by natural and

managed fires in savanna ecosystems. Biomass burning is a major source of GHG

emission (CRUTZEN; ANDRAE, 1990). Fires affect climate change by influencing the

emission of soot and aerosols (KAUFMAN; FRASER, 1997); altering vegetation cover

(which includes re-growth of grasses and trees); changing albedo, soil moisture and

temperature regimes; and disturbing cycling of elements and water. Mortality of trees

and seedlings due to fires can reduce the presence of trees and woody species while

promoting dominance of grasses and herbaceous vegetation (CARDOSO et al., 2008).

These processes create man-made savannas (HOFFMANN et al., 2000), and such

environmental degradation on a large scale may weaken the hydrological and C cycles

with large scale changes in the environment, temperature and precipitation patterns

(HOFFMANN; JACKSON, 2000).

TS biomes are fire-prone or fire-dependent ecosystems. Frequency and intensity of

a fire, depending upon the quantity of biomass available for burning, have an effect on:

species composition; soil properties and processes; sediment and elemental transport in

water runoff; and emission into the atmosphere not only of GHGs, but also of particulate

organic matter (POM) and soot/black C. Mouillot and Field (2005) estimated that an

average of 608 million hectares (Mha) yr -1 were burned every year throughout the 20th

century, 86 % of this having occurred in TS biomes. In comparison, fire in forest biomes

consumed 70.7 Mha yr -1 in the beginning of the 20th century, mostly in boreal and

temperate forests of the northern hemisphere. Occurrence of fire in the northern

hemisphere decreased to a 15.2 Mha yr -1 level in the 1960s and to a 11.2 Mha yr -1 level by

the end of the 20th century. During the same period, burnt areas increased to 54 Mha yr -1 in

the TRF biome (MOUILLOT; FIELD, 2005).


Fires, natural and anthropogenic ones, consume ~ 3 Pg C yr-1. This has an impact on

the gaseous composition of the atmosphere and its air quality (GRACE et al., 2006; FREITAS

et al., 2005). Biomass burning in South America emits 30 Tg yr -1 of aerosol particles to the

atmosphere (ANDREAE, 1991). Because of their small size, aerosol particles and black C

(soot) have a long residence time in the atmosphere (KAUFMAN, 1995). Smoke plumes in

South America cover an area of about 4-5 x 106 km2 during the fire season (PRINS et al.,

1998). Persistence of aerosol particles can affect radiation budget and regional climate due

to a resulting high concentration of black C in the atmosphere (ANDREAE, 2001). Biomass

burning is also a source of CH4 and NOx (N2O, NO, NO2). Annually, as a result of biomass

combustion, the emission of NOx is 6.7 Tg N yr -1 (DAVIDSON; KINGERLEE, 1997).

Fire-derived charcoal or black C (BC) contributes to recalcitrant pool formation

(CZIMCZIK; MASIELLE, 2007). Of the 3 Pg of biomass C burnt annually, 1.1 Pg is emitted

into the atmosphere (CO2, soot, POM, aerosol, etc.) and about 50 Tg is converted into

charcoal, of which 26-31 Tg is BC (FEARNSIDE, 2000). BC is an important component of

the global C cycle. Dai et al. (2005) estimated that the concentration of BC in temperate

mixed-grass savanna ranges from 50 g (BC) kg-1 to 130 g (BC) kg-1 of SOC (equivalent to

0.55 g BC kg-1 to 1.07 g BC kg-1 of soil). Contribution of BC to the SOC pool increases along

with an increase in soil depth. Ansley et al. (2006) observed that BC comprised from 13 %

to 17 % of the SOC pool in temperate-mixed grass savannas. The dynamics of BC in TS

ecosystems needs to be understood with regards to the processes involved.

Studies of natural fire dynamics are rare. Experiments conducted on grassland

fires under natural conditions in the Senegal National Park and in Tanzania showed that

volatilized proportions of biomass and N are substantially less than 100 %, combustion

and volatilization losses being strongly influenced by the amount mass burned and by the

intensity of fire. Relatively more of N than mass is volatilized as the intensity of fire

increases, and much less mass and N are volatilized in natural fires than indicated in

laboratory studies (McNANGHTON et al., 1998).

Conversion of Savannas to an Agricultural Ecosystem

TS soils and climates are suitable for grain crop production and pastures.

Therefore, TS ecosystems have been widely converted to agricultural production systems

(e.g., corn, soybean, sorghum, millet, pasture) since the second half of the 20th century.


Conversion to agricultural systems and susceptibility to fire have resulted in strong

changes in vegetation coverage and in soil properties and processes; also disrupting

elemental and water cycles (C, N, P, S) . Large scale conversion has an impact on local

and regional climate. It reduces precipitation by about 10 % and causes an increase the

frequency of dry periods during rainy seasons, albedo and evaporation rates and the mean

air temperature (by 0.5 °C), the reason for this being a reduction in the surface rough mass

length (HOFFMAN; JACKSON, 2000) (Table 2). There is also a rise in air and soil

temperatures as well as a reduction in the water infiltration rate with a correspondent

increase in runoff, and an increase in evaporation, all being factors leading to an effective

decrease in (green water) precipitation. In addition, conversion of natural TS to agricultural

ecosystems leads to a decrease in C pool both in the above ground and in the below

ground biomass. There is also loss of SOC pool due to a decrease in impact of the C

biomass. Losses caused by decomposition, erosion and leaching can be even more

severe (Fig. 1a, b). Dominant land use in managed TS ecosystems ranges from the beef

cattle production present in northern Australia and in Brazil (where large scale agriculture

is also carried out) to the mixed grazing and shifting/traditional agriculture still found in

Africa (WINTER, 1990; KLINK et al., 1995; HOFFMAN; JACKSON, 2000). About 40 % of the

Brazilian Cerrado had already been converted to agricultural land use by 1995, and the

remainder is being converted at the rate of 1.7 % yr-1 (Table 3; KLINK et al., 1995). Large

scale conversion of Cerrado vegetation of this important ecosystem is a threat not to be

ignored (SCARIOT et al., 2005). Harvesting of firewood is another important economic

factor of deforestation of TS in Africa.

Table 2. Change in precipitation and evapotranspiration due to conversion of tropical savannas tomanaged ecosystems.

Rainfall (mm yr-1) Evapotranspiration (mm yr-1)

Region Native Managed % Change Native Managed % Change

Cerrado 1590 1439 - 9.5 1080 987 - 8.6

Llanos 1790 1610 - 10.0 1142 1023 - 10.4

Southern Africa 1226 1125 - 8.2 896 804 - 10.3

Australia 928 807 - 13.0 789 719 - 8.9

Source: Adapted from Hoffman; Jackson, 2000.


Table 3. Main use of land in the Cerrado.

Land Use Area (Mha) % of Total Area

Native/undisturbed 116.1 56.9

Planted pastures 65.9 32.3

Cropland 18.0 8.8

Planted forests 0.12 0.06

Urban land/bare soil 3.0 1.5

Others 0.93 0.5

Total 204 100

Source: Recalculated from Klink; Machado, 2005.

Around 80 % of the ecosystem C pool in TS biomes is in the soil. Thus degradation

affects the ecosystem C pool both in biomass and soil. Removal of protective tree

coverage from TS also depletes the SOC pool. The main determinants of the ecosystem C

balance upon land use conversion are changes affecting GPP and NPP, rooting depth,

erosion, leaching, and temperature-induced decomposition of the SOC pool. The magnitude

of change in C pools depends on the specific land use to which the Cerrado is converted

to. There are four predominant land uses: (i) from native Cerrado to pasture; (ii) from

native Cerrado to cropland under conventional tillage; (iii) from plow tillage (PT) to no-till

(NT); and (iv) from native Cerrado to forestry (BUSTAMANTE et al., 2006).

From Native Cerrado to Pastures: Brazil has more than 167 million heads of cattle

(FAO, 2006), raised mostly on grazed pastures. Cultivated pastures, covering 50 Mha in

the Cerrado region, out of a total of 80 Mha under Brachiaria spp. in Brazil, are rapidly

being degraded. Immediately after their establishment, these pastures can support

1-2 animal units (AU) ha-1. With severe degradation and appearance of termite hills

(termitaria), in a short period of time, pastures can only support 0.5 AU ha-1 (DE OLIVEIRA

et al., 2004). In addition to compaction and decline in soil structure, nutrient depletion

and loss of the SOC pool are also important factors (BODDEY et al., 2004). Restoration of

degraded pastures can enhance the SOC pool. Da Silva et al. (2004) reported that an SOC

pool to 1-m depth in managed pastures was about 100 Mg C ha-1, compared to the

200 Mg C ha-1 found in the Colombian Llanos. Degraded pastures and poor management

are a source rather than a sink for atmospheric CO2 (DA SILVA et al., 2004).


Bustamante et al. (2006) summarized the rates of C sequestration upon conversion

of native Cerrado to other land uses. The average C sequestration rate upon conversion of

native Cerrado to pasture is 1.3 Mg C ha-1 yr-1 with a range of -0.87 Mg C ha-1 yr-1 to +

3.0 Mg C ha-1 yr-1 (DA SILVA et al., 2004). Conversion of native Cerrado to PT cropland

leads to depletion of the SOC pool, mostly due to accelerated soil erosion. The rate of loss

of the antecedent pool ranges from 40 % to 80 % in the 0 cm - 15 cm depth depending on

the clay content. Loss becomes more severe if the clay content decreases with the

increase in duration of cultivation.

Restoration of degraded pastures can enhance the SOC pool. The rate of SOC

sequestration in restoring degraded pastures is about 1.5 Mg ha-1 yr -1 (BUSTAMANTE et

al., 2006). Experiments conducted in the Llanos (Colombian savannas) show that the SOC

pool can be greatly enhanced through the introduction of deep-rooted African pasture

species and legumes into native savannas (FISHER et al., 1994; FISHER et al., 1995;

TRUJILLO et al., 2006; RONDON et al., 2006).

From Native Cerrado to Forest Plantations: Replacement of savannas by short

rotation woody perennials and other tree plantations can enhance the ecosystem C pool,

and allow TS biomes to become a net C sink (SCURLOCK; HALL, 1998; CORAZZA et al.,

1999; ZINN et al., 2002). Furthermore, an increase in the atmospheric CO2 concentration

can also enhance the terrestrial C pool of plantations through a CO2 fertilization effect

resulting from it . Establishment of tree plantations can increase the ecosystem C pool

from a mean value of 67 Mg C ha-1 under native savanna conditions to a mean value of

150 Mg C ha-1 under a tree plantation condition (SCURLOCK; HALL, 1998). Thus, conversion

of an area of 11.5 x 106 km2 of native savannas to such plantations has a potential of

sequestering 95.5 Pg C over 50 yr, with a mean sequestration rate of ~2 Pg C yr-1. San

Jose and Montes (2001) estimated the potential of the Orinoco Llanos of Colombia for

storing C at a rate of 8.3 Pg over a 50 yr period.

Soil Management and Emission of Greenhouse Gases

Soil management (tillage, residue management, nutrient management), cropping

systems (rotation, cover crops), weed and pest control also affect a soil C budget. Most

croplands in Brazil are rapidly being converted from PT to NT. The land area under NT in

Brazil increased from 1 Mha in 1992 to 24 Mha in 2005 (FEBRADPD, 2005), of which 8 Mha


are in the Cerrado region. Jantalia et al. (2007) reported that loss of SOC pool upon

conversion from native Cerrado to cropland over a 20 year period was of 10 Mg C ha-1 with

NT compared with 30 Mg C ha-1 with PT. Conversion of PT to NT can positively impact the

soil C pool, with a rate of SOC sequestration of 0.5-1.2 Mg C ha-1 a yr-1 depending on soil

depth (Table 4). Corbeels et al. (2006) estimated that conversion of 6 Mha of PT soybean

to a NT system of seedbed preparation can enhance soil C storage by 4.9 Tg C yr -1.

Positive impacts of NT systems on SOC sequestration in Cerrado soils have been widely

reported (CORRAZA et al., 1999; LEITE et al., 2004; DE OLIVEIRA et al., 2004). However, the

NT system adopted in degraded soils may cause decline in the SOC pool especially over a

short time period (Tables 4, 5; LILIENFEIN ; WILCKE, 2003). The rate of C sequestration in

NT management also depends on cropping systems, soil textures, and availability of N.

Adoption of NT making use of double cropping sequesters more C than in the case of a

single crop.

Table 4. Rate of soil carbon sequestration by no-till farming in the Brazilian Cerrados.

Cropping Duration Soil Depth C Sequestration ReferenceSystem (yrs) (cm) (Mg C ha-1 yr-1)

Soybean 12 20 0.83 Corbeels et al. (2006)

Soybean 12 40 0.7-1.15 Corbeels et al. (2006)

Corn-Soybean 2 30 - 1.5 San José and Montes (2001)

Rice (upland) 5 10 0.35 Lilienfein and Wilcke (2003)

Soybean-Maize 8 20 0.3-0.6 Metay et al. (2007a)

Table 5. Soil carbon pool in different land uses in the cerrado region of Minas Gerais.

Age Soil Organic Carbon Pool (Mg ha-1)

Land use (yrs) (0 m – 0.3 m) (0 m – 2 m)

Cerrado - 55 ± 2.3 ab 180 ± 6.8 a

Pinus 20 49 ± 2.9 b 170 ± 9.8 a

Degraded Pasture 14 60 ± 4.7 ab 180 ± 14.0 a

Productive Pasture 14 64 ± 8.1 a 190 ± 26.0 a

No-till 2 58 ± 5.3 ab 190 ± 5.8 a

Plow tillage 12 61 ± 3.2 ab 170 ± 12.0 a

Figures in the column followed by the same letters are statistically similar.Source: Recalculated from Lilienfein; Wilcke, 2003.


There are two other factors that determine whether the managed TS biomes are a

source or sink for atmospheric concentration of GHGs. The first factor is the flux of GHGs

(CO2, NOx, CH4), and the second factor is the hidden C cost of all input. Varella et al. (2004)

observed no significant differences in annual CO2 soil emissions between Cerrado and

pasture vegetations, but the temporal trends differed, with higher fluxes in pastures during

the transition from wet to dry season. Cropland soils, due to application of nitrogenous

fertilizers, have larger NOx emissions than the undisturbed TS soils (PEREZ et al., 2007). In

general, NT soils have a higher efflux of N2O than CT soils because of high soil moisture

content and lower gas diffusivity. In the Cerrado region, Metay et al. (2007b) observed that

the N2O emission was 30.7 g N ha-1 yr -1 for CT compared to 35.3 g C ha-1 yr -1 for CT.

Similarly, CH3 emission was 245 g C ha-1 yr -1 for NT vs. 403 g C ha-1 yr -1 for CT. With

regards to hidden C costs, fuel consumption in NT was estimated at 14 L ha-1, compared

with 34 L ha-1 to 42 L ha-1 in PT (SORRENSON; MONTOYA, 1989).

The net SOC sequestration in agro-ecosystems must be computed with due

consideration to the hidden C costs (Table 6). Among fertilizers, hidden C costs are the

highest for nitrogenous fertilizers. Pesticides have 4 to 5 times higher hidden C costs than

fertilizers. Lifting ground water for supplemental irrigation has additional costs which

increase along with the fall in water table levels, such as is the case of the Indo-Gangetic

Basin of South Asia. It is the high hidden C costs that demand a judicious use of the C-

based input through adoption strategies such as: (i) NT farming which reduces or

eliminates pre-planting seedbed preparation; (ii) integrated nutrient management (INM)

that reduces the rate of fertilizer application; (iii) integrated pest management (IPM) that

minimizes dependence on pesticides; and (iv) water harvesting, recycling and

conservation in the root zone for reduction of supplementary irrigation. Choice of soil and

crop management practices must be determined by the need to increase use-efficiency of

all C-based input by reducing losses caused by erosion, leaching, volatilization, etc. It is in

this connection that the importance of precision farming, sub-drip irrigation (fertigation),

nano-enhanced material, and GM plants (Bt and round up ready crops) cannot be over-

emphasized.


Table 6. Hidden carbon costs of farming practices.

Source/ Practice Equivalent carbon emission(kg C E)

I. Fuel (kg of fuel)1. Diesel 0.94

2. Gasoline 0.593. Oil 1.01

4. Natural gas 0.85

II. Tillage (per ha)

1. Moldboard plowing 15.22. Chisel plowing 7.9

3. Disking 8.34. Cultivation 4.0

III. Fertilizers (Per kg)

1. Nitrogen 1.32. Phosphorus 0.2

3. Potash 0.154. Lime 0.16

IV. Pesticides1. Herbicides 6.3

2. Insecticides 5.1

3. Fungicides 3.9

Source: Lal, 2004.

Savannas: source or sink of atmospheric CO2

Native savannas, undisturbed and under natural conditions, are small sinks of

atmospheric CO2. The data in Fig. 3 (GRACE et al., 2006) show that NPP of 20 Pg yr-1,

supports a SOC pool of about 480 Pg out of the global SOC pool (1-m depth) of 1550 Pg

(~31 %). Whether managed savannas are source or sink of CO2 and other GHGs needs a

careful appraisal of the ecosystem C budget under site-specific conditions. It is estimated

that savanna biomes sequester as much as 0.5 Pg C yr-1, which may contribute to the

so-called “missing sink” (SCURLOCK; HALL, 1998). The data in Table 7 also reveal that

grasslands and rangelands together have a potential SOC sink capacity of 0.5 Pg C yr-1 to

1.7 Pg C yr-1. The actual and attainable C sink capacity may be much lower.


Fig. 3. Carbon pools and fluxes in savanna ecosystems.All figs of pool are in Pg C, and of fluxes Pg C yr -1.Source: Modified from Grace et al., 2006.

Above-Ground C Pool26

Atmospheric C Pool

780 (+ 3.5)

Below-Ground C Pool52

Soil Organic Carbon Pool480

NPP

Litte

r Fal

l 15

Fire

4.5

20

Land

Use

Cha

nge

0.4

- 0.8

Soil

Resp

iratio

n?

Table 7. Potential carbon sink capacity of global ecosystems.

Ecosystem Potential Carbon Sink Capacity (Pg C yr-1)

Grasslands 0.5Rangelands 1.2Forests 1-3Urban forests and grasslands -Deserts and degraded lands 0.8 – 1.3Agricultural lands 0.85 – 0.9Biomass croplands 0.5 – 0.8Terrestrial sediments 0.7 – 1.7Boreal peatlands and other wetlands 0.1 – 0.7

Total 5.65 – 10.1

Source: US Department of Energy, 1999.


Realization of this vast potential, however, demands a detailed life cycle analysis

of pools and fluxes under main land use systems. It is widely recognized that the

ecosystem C pool declines as ecosystems shift from native to agricultural, the result of

that being a drastic loss of biomass C (both above and below ground) as well as of the

SOC pool. This ecosystem C pool can be restored through a conversion to planted forests

(Eucalyptus, Pinus, etc.). Soil and vegetation degradation, such as is the case with

degraded pastures, make these ecosystems a source of CO2 and other GHGs.

Conversion of PT to NT can lead to an increase in the SOC pool at the rate of

0.3 1.0 Mg C yr -1 to 1.0 Mg C yr -1. There is also a saving in fossil fuel through the

elimination of primary and secondary tillage operations. The adoption of a NT system on

the 18 Mha of croplands in the Brazilian Cerrados can lead to a sequestration level of 15 Tg

C yr -1 in the SOC pool. However, net C sequestration must be assessed with due

consideration of the hidden C costs and of the increase in N2O emission. In addition to

soybean, cultivation of upland rice (covering 2 Mha) is another option that needs a careful

evaluation (PINHEIRO et al., 2006). Aerobic rice has lower CH4 emission and lesser water

requirements than continuously flooded rice paddies.

As in the case of croplands, restoration of degraded pastures is an important

option for SOC sequestration. With a total land area of 66 Mha, the potential of SOC

sequestration in pastures ranges from 15 Tg C yr -1 to 30 Tg C yr -1.

Land use management systems that allow for a net sink of CO2 and CH4 include:

(i) afforestation and reforestation of degraded ecosystems; (ii) restoration of degraded

pastures and judicious management with controlled/rotational grazing; (iii) conversion of

plow tillage to NT farming with mulch, cover crops, integrated nutrients and pest

management; and (iv) increase of productivity per unit input of C-based input (e.g., diesel,

fertilizers, pesticides, irrigation) (Fig. 4). The strategy is to avoid further conversion of TS

and adopt land saving options for nature conservancy.


Fig. 4. Strategies to make TS biomes a net C sink.

Conclusions

TS biomes, similar to TRF biomes, are the frontline ecosystems in the struggle of

mankind to feed the growing world population. TS biomes are ecologically sensitive, have

high biodiversity, and are sources of numerous ecosystem services. Among these,

moderation of the global C cycle, strengthening of elemental cycling and maintenance of

renewable fresh water resources are particularly important.

TS biomes, by virtue of climate and soils, are more suited for intensive grain

cropping (soybean, corn, sorghum, millet, cowpeas) and livestock production than TRF

biomes. The latter are suited for tree crops, energy plantations, and growth of roots and

tubers along with other perennials.

••Biofuel plantations•Tree plantations•Short rotation woody

perennials•Grasses

•Species selection•Stand management•Fire management•GM crops/biotech•Deep root system

•Creating positive Cand nutrient budgets

•Reducing losses•Enhancing biodiversity

•NT farming•Cover cropping•Precision farming•INM/IPM•Nano-enhanced

materials

•Soil fertility management•Improved pasture species•Controlled grazing

•Above ground biomass•Below ground biomass•Soil C pool

Tropical Savannas

•Afforestation•Reforestation•Enhancing Soc pool

Restoring DegradedEcosystems

•Species•Soil Management•Fire management

Restoring Pastures

•Positive C budget•Positive nutrient budget•High use efficiency of

input

Restoring Cropland


Conversion of TS by fire and mechanical means leads to depletion of the

ecosystem C and nutrient pools and to the emission of GHGs the atmosphere. Biomass

burning leads to emission of 5 Pg C yr -1 - 8 Pg C yr -1, along with black C/soot aerosol and

GHGs. Among three ecosystem C pools (i.e., above ground biomass C, below ground

biomass C, and soil C pool), both above ground and below ground biomass C pools are

heavily depleted upon conversion of native to managed TS. Depletion of the SOC pool is

exacerbated by soil degradation and desertification. In erosion-prone PT soils, the SOC

pool can decline to < 50 % within 10 to 20 years after conversion from the native TS to

croplands.

Restoration of degraded pastures and conversion of a system from PT to NT can

enhance the SOC pool. Considering all components, managed TS biomes are likely a

source of GHGs compared to native TS biomes. The magnitude of source is more in PT

croplands and degraded pastures than in NT croplands and productive pastures.

Taking into account climate change and C sequestration, the following strategies

need special attention:

• Halt conversion of native TS to agricultural ecosystems (e.g., croplands, pastures).

• Encourage of conversion of degraded pastures to tree plantations or improvedpastures.

• Convert of croplands from the PT to the NT system with cover crops and residuemulch.

• Adopt of integrated nutrient management and integrated pest management practicesto reduce dependence on fertilizers and pesticides.

• Harvest and recycling runoff, erosion control and ground water conservation.

• Use slow release formulations of fertilizers with nano-enhanced materials, and growthof genetically modified plants to enhance use efficiency of input.

• Adopt land-saving practices of agricultural intensification for increasing productionfrom existing lands so that natural TS biomes can be preserved for natureconservancy.


• Create another income stream for farmers through development of payments forecosystem services (e.g., trading C credits).

• Establish biofuel and timber plantations.

• Regulate on fire or biomass burning, and evaluation of their impact on BC and on the

global C cycle.

Through the implementation of such measures, TS biomes have a potential C sink

capacity of ~ 0.5 Pg C yr-1.

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