jurnal alkilhalida

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letters to nature

NATURE | VOL 403 | 20 JANUARY 2000 |42

6. Rasmussen, R. A. et al. Concentration distribution of methyl chloride in the atmosphere. J.Geophys.

Res. 85, 7350 7356 (1980).

7. Singh, H. B., Salas, L. J. & Stiles, R. E. Methyl halides in and over the eastern Pacic (408N 328S).

J. Geophys. Res. 88, 36843690 (1983).

8. Cicerone, R. J. Halogens in the atmosphere. Rev. Geophys. Space Phys. 19, 123139 (1981).

9. Blake, N. J. et al. Biomass burning emissions and vertical distribution of atmospheric methyl

halides and other reduced carbon gases in the south Atlantic region. J. Geophys. Res. 101(D19),

2415124164 (1996).

10. Moore, R. M., Groszko, W. & Niven, S. J. Ocean-atmosphereexchange ofmethyl chloride: resultsfrom

NW Atlantic and Pacic Ocean studies. J. Geophys. Res. 101, 2852928539 (1996).11. Yokouchi, Y. et al. Isoprene in the marine boundary layer(Southeast Asian Sea, Eastern Indian

Ocean, Southern Ocean): comparison with DMS and bromoform. J. Geophys. Res. 101(D7),

80678076 (1999).

12. Li, H.-J., Yokouchi, Y. & Akimoto, H. Measurements of methyl halides in the marine atmosphere.

Atmos. Environ. 33, 18811887 (1999).

13. Zimmerman, P. R. Testing of hydrocarbon emissions from vegetation, leaflitter and aquatic

surfaces and development of a methodology for compilingbiogenic emission inventories. (Report

EPA-450/4-

79-004, US Environmental Protection Agency, Research Triangle Park, North Carolina, 1979).

14. Yokouchi, Y. et al. Determination of monoterpene hydrocarbons in the atmosphere. J.Chromatogr.

209, 293298 (1981).

15. Harper, D. B. Halomethane from halide iona highly efcient fungal conversion of

environmental signicance. Nature 315, 5557 (1985).

16. Attieh, J. M., Hanson, A. D. & Saini, H. S. Purication and characterization of a novel methyl-

transferase responsible forbiosynthesis of halomethanes and methanethiol in rassica oleracea. J.

Biol. Chem. 270, 9250 9257 (1995).

17. Saini, H. S., Attieh, J. M. & Hanson, D. Biosynthesis of halomethanes and methanethiol by

higher plants via a novel methyl transferase reaction. Plant Cell Environ. 18, 10271033 (1995).

18. Atlas, E. et al. Alkyl nitrates, nonmethane hydrocarbons, and halocarbon gases over the equatorial

Pacic Ocean during saga 3. J. Geophys. Res. 98, 1693316947 (1993).

Acknowledgements

We thank staff at the Atmospheric Environment Service station for assistance in sample

collection at Alert. We are grateful to the National Institute ofPolarResearch, Japan forthe opportunity to participate in the 39th Japanese Antarctic Research Expedition. We

also thank the staff of the Global Environmental Forum Foundation for theirassistance

with sample collection at Hateruma and with the Skaugran cruise. In addition, we alsothankthe staffof the Environmental Management Center (Indonesia) for theirassistancein theeld study at Jakarta and Bandung. This work was supported in part by the Science and

Technology Agency of Japan, by the Global Environment Fund (Environmental Agencyof Japan), and by the Atmospheric Environment Service of Canada.

Correspondence and requests for materials should be addressed to Y.Y.(e-mail:yo k ouchi@nie s.g o .jp).

.................................................................Halocarbons producedby naturaloxidationprocesses duringdegradation of organic matterF. Keppler, R. Eiden, V.Niedan, J. Pracht & H. F. Scholer

Institute of Environmental Geochemistry, Heidelberg

University, ImNeuenheimer Feld 236, D-69120 Heidelberg,

Germany

..............................................................................................................................................

Volatile halogenated organic compounds (VHOC) play an impor-

tant role in atmospheric chemical processescontributing, forexample, to stratospheric ozone depletion

1 4. For anthropogenic

during the oxidation of organic matter by an electron acceptorsuch as Fe(III): sunlight or microbial mediation are not requiredfor these reactions. When the available halide ion is chloride, thereaction products are CH3Cl, C2H5Cl, C3H7Cl and C4H9Cl. (The

corresponding alkyl bromides or alkyl iodides are produced whenbromide or iodide are present.) Such abiotic processes could makea signicant contribution to the budget of the important atmos-pheric compounds CH3Cl, CH3Br and CH3I.

In a study of VHOC in surface peat-bog waters containing

suspended organic matter, we found that besides the well-knowntrichloromethane which is attributed to an enzymatic halogenationof organic compounds18, signicant amounts of other halocarbonssuch as iodomethane, iodoethane, iodopropane and iodobutanewere released from organic-rich waters. The water samples dis-

played conspicuous concentrations of Fe(III) (up to 0.1 mmol l-1

),ahigh iodide content and a pH value of 4.2. Investigations of threedifferent soil samples from anthropogenic unpolluted areas inWestern Patagonia/Chile and Hawaii (Table 1) showed partiallyhigher concentrations of CH3Cl, C2H5Cl and CH3Br in addition tothe alkyl iodides. Isolation and identication of VHOC wereperformed by a headspace gas chromatography technique withelectron capture and mass spectrometry detection.

To differentiate between biological or geochemical processes, a

part of each sample was dried at 105 8C in order to destroybiological active material, such as microorganisms or enzymes,while the other part was freeze-dried. Both samples, whenresuspended in distilled water, showed a similar methyl halidedistribution to those we found in the natural untreated samples.

We considered that in soils there should exist an abiotic reactionmechanism that forms alkyl halides. The thermodynamically labileorganic matter is oxidized and the redox partner (for example,Fe(III)) is reduced (to Fe(II)). Phenolic moieties of the naturalorganic matter containing methoxy groups might be oxidizedwhile Fe(III) is reduced. During this process halides are methylated,and the methyl halides formed represent degradation products ofoxidized organic matter (Fig. 1). Ethoxylated and propoxylatedphenolic structures could be responsible for the occurrence ofethyland propyl halides.

To verify this hypothesis the following experiments wereinitiated: (1) three different organic-rich soils (Table 1) fromthe natural reserve Rotwasser (Rotw.) in the Hessian Odenwald(Germany) were collected and tested for VHOC emission inrelation to oxidant concentration and halide ion content; (2) forcomparison, the same parameters were measured using 2-methyoxyphenol (guaiacol), which represents a naturalmonomeric constituent ofhumus.

We suspended dried soil samples in sterile distilled watercontain-ing Fe(III) and halide ions. Fe(III) was readily reduced to Fe(II)depending on the organic carbon content of the soil samples(Fig. 2a). The higher the organic carbon content of the soilsample the more Fe(III) was reduced during the same period.

AtmosphereAlkylhalides

(MeX, EtX,PrX, BuX

VHOC whose sources are well known5, the global atmospheric

input can be estimated from industrial production data. Halogen-ated compounds of natural origin can also contributesignicantly to the levels of VHOC in the atmosphere6. Theoceans have been implicated as one of the main natural

sources7 10

, where organ- isms such as macroalgae andmicroalgae can release large quan- tities of VHOC to the

atmos

p

her

e11,12.Someterrestrialsourceshave also

beenidentied,

such as wood-rottingfungi13, biomass

burning14 and volcanicemissions15. Here wereport the identica-
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ww w .natu re.com43

Soil

ox

idi

ze

d

Org

anic matter

reduced

X (Cl, Br, I)

Fe2+

Fe3+

tion of a different terrestrial source of naturally occurring VHOC.We nd that, in soils and sediments, halide ions can be alkylated

Figure 1 Model for alkyl halide formation by the reaction of Fe(III)and organic matter inthepresence of halide ions.
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.............................................

Alkyliodideproduced(pMp

erflask)

Fe(II)(M)

Ha

lomethaneproduced(pMp

erflask)

Alkyliodideproduced(pMp

erflask)

Table 1 Organic carbon, pH, Fe, halogens and halogen ratio in different soil samples

Soil Corg pH Fe Cl Br I Cl:Br:I (mol)......................................................................................... ...

(% dry wt) (% dry wt) (mg kg-1dry wt)...................................................................................................................................................................................................................................................................................................................................................................

Grassland soil Ah (Rotw.) 6.6 4.96 0.70 187 ,5.7 ,3.7Coniferous forest soil Ah (Rotw.) 14.8 3.01 0.39 233 15.8 6.6 123:2.3:1Suspended matter from peat water (Rotw.) 28.6 4.18 2.95 676 45.9 23.3 100:3:1Forest soil Oh/Ah (Hawaii I) 26.3 5.6 1.7 195 33.3 37.3 15:1.4:1Forest soil Ah (Hawaii II) 13.5 4.2 15.4 236 101 236 2.8:0.6:1Grassland soil Oh/Ah (Patagonia) 46.9 4.10 0.26 2,900 260 32.9 320:12.5:1...................................................................................................................................................................................................................................................................................................................................................................

Subscript h indicates soil horizon; Corg

is organic carbon content; Rotw. indicates Rotwasser, Germany (see text); wt, weight.

VHOC release rates were measured during the reduction of Fe(III) inthe presence of halide ions. We identied four different alkylhalides with decreasing release rates in

the following order: halomethane .haloethane . 1-halopropane > 1-halobutane. To quantify thealkyl halides we used iodide as the halide source. As a result,iodomethane, iodoethane, 1-iodopropane and 1-iodobutane

were formed in the proportion 19:5:1.5:1 (Fig. 2b). An almostsimilarproportion (17:4.5:0.5:1) was obtained from untreated peat-bog water.

The inuence of Fe(III) concentration on the alkyl halide forma-tion can be inferred from Fig. 3a. When higher amounts ofFe(III)were added to the soil, the alkyl halide emission increased. Therewas a little production of alkyl halides and Fe(II) even if no Fe(III)was added to the soil. This is probably caused by the presence ofFe(III) as a mineral phase in the form of iron oxides and oxyhydr-

10a oxides in the soil samples (0.7 3% of dry weight) which could be

reductively dissolved by natural organic compounds.

a 10,000

11,000

Suspended matter from peat water, 10 M Fe(III) Suspended matter from peat water, no Fe(III)added

Coniferous forest soil, 10 M Fe(III) Grassland soil, 10 M Fe(III) Grassland soil, no Fe(III) added Coniferous forest soil, no Fe(III) added

100

10

0.10 20 40 60 80 100 120

Time (min) 1Iodomethane Iodoethane

1-lodopropane 1-lodobutane

1,000

100

b 0.1

b

10,000

0 20 40 60 80 100

Fe (II) conc. in medium (M)

1,000

10

Iodomethane

Iodoethane

1-lodopropane

1-lodobutane1

0 20 40 60 80 100 120

Time (min)

100

10

Iodomethane

Bromomethane

Chloromethane

0 2 4 6 8 10

Figure 2 Abiotic reduction of Fe(III)insoils and theproduction of alkyl iodides in thepresence of iodide. a, Abiotic reduction of Fe(III) to Fe(II)by different organic-richsoils.

100 mg soil sample (dried at 105 8C) was suspended in 10 ml bidistilled water containing

10 mM dissolved Fe(III) as electron acceptor. The pH in the medium was 3.0 andtemperature was 30 8C. Subsamples were ltered (0.45 mm pore lter) and Fe(II) wasphotometrically analysed as ferrous iron-phenanthroline complex. Three to six replicates

were measured for each sample (n 36). The standard deviation (s.d.) was in therange

731% error(j1). b, Alkyl iodide formation ofa grassland soil (Rotw.) in the presence ofFe(III)and iodide. 100 mg grassland soil (dried at 105 8C) was added to 10 mlbidistilled water in a 20-ml headspace ask containing 10 mM dissolved Fe(III).Before

the askwas sealed the medium (pH 3.0) was supplemented with 100 mM Kl. After

shaking at a temperature of 30 8C the volatile halogenated organic compounds (VHOC)

in the headspace were determined by gas chromatography (GC) with an electron

capture detector(ECD) (n 46; s.d. 425% error).


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Halide ion conc. in medium (mM)

Figure 3 Inuence ofFe(III)and halide ions on the formation of alkyl halides. a, Alkyl iodideproduction from a coniferous forest soil (Rotw.) by using Fe(III) as electron acceptor.Coniferous forest soil (500 mg, dried at 105 8C) was suspended in 10 ml bidistilled water.

In 20-ml headspace asks. The medium (pH 3.0) was supplemented with Kl (100 mM)

and 5100 mM Fe(III).The asks were sealed and shaken for 1 h at a temperature of

30 8C. VHOC concentrations in the headspace were measured by GCECD (n 45;s.d. 544% error).b, Effect of halide ion concentration on yield ofcorresponding

halomethane production with a grassland soil (Rotw., from Rotwasser, Germany, see text)

as organic matter source. 1,000 mg grassland soil (freeze-dried) was added to 10 ml

bidistilled water in a 20-ml headspace ask. The medium (pH 5.0) was supplemented with110 mM KCl, KBr or Klbefore the ask was sealed. After shaking for 60 min at a

temperature of 30 8C, halomethane concentration in the headspace was monitored by

GCECD (n 46; s.d. 725% error).


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Iodomethaneproduced(pMp

er

flask)Fe(II)produced(nMp

er

flask)

Halomethaneproduced(nMp

er

flask)

Molarratioiodomethane/Fe(II)

Figure 3b illustrates the effect of different halide concentrationson the yield of volatile halomethanes with a grassland soil sample(Rotw.) as the organic matter source. Halomethane productionincreased with increasing halide ion concentration. When equimo-

iron oxyhydroxides in nature, was used to oxidize guaiacol. Methylhalide, Fe(II) and o-quinone have been identied as reactionproducts.

lar concentrations (1 mM) of chloride, bromide and iodide werepresent, the preferred halide ion was iodide, the respective halo-methanes being formed in the proportion of 1:1.5:10. We appliedthis experimentally determined proportion to the natural molarhalogen ratios of the Patagonian soil (Cl:Br:I; 320:12.5:1), the

Hawaiian soils I (15:1.4:1) and II (2.8:0.6:1), to obtain

OH O

OCH3

+ 2 Fe3+

(Ferrihydrite) + X-

(X = Cl, Br, I)

O

+ 2 Fe2+ + CH3X

CH3Cl:CH3Br:CH3I ratios of32:1.8:1, 7:1:5 and 3:1:11, respectively.

Very similar ratios of 48:1.9:1, 5.5:1:2.9 and 3.5:1:8.7 weremeasured from these natural untreated samples. Our resultssuggest that a fraction of the soil halogen content ismethylated by natural oxidation processes and that this fractionincreases in the sequence Cl, Br, I.

The formation of methyl halides by a model reaction ofguaiacolwith ferrihydrite (5Fe2O39H2O) and halide is presented in Fig. 4.

There was noproduction of halomethanes ifferrihydrite orhalidewas absent. We assume that methyl halides are produced in analmost synchronous reaction scheme: (1) the oxidation ofguaiacolby ferrihydrite and (2) nucleophilic substitution of the methylgroup by halide.

O

O

These ndings are in agreement with the data we presented fromnatural soils. Ferrihydrite, the most common initial precipitate of

2 Fe3+

HO (1)

(1)+ 2 Fe

2+

a10,000 0.1

O CH3

(2)

X-

(2)

CH3X (X = Cl, Br, I)

1,000

100

10

1

b10

1

0.1

0.01

Iodomethane (pM)

Fe(II) (nM)

Ratio Iodomethane/Fe(II)

0 5 10 15 20

Time (h)

Iodomethane

Bromomethane

Chloromethane

0.01

0.001

0.0001

The low molar ratio of halomethane to Fe(II) (Fig. 4a) indicatesthat further oxidation products of guaiacol were produced. Theformation of a methoxyphenol dimer as a result of oxidative

oligomerization ofclays has been reported17. However, our reactionmodel describes a chemical pathway leading from guaiacol tomethyl halide. This reaction is specic only for the methyl halidesbut in additional experiments with ethoxyphenol andpropoxyphe-nol the corresponding ethyl halides and propyl halides were found.

Soil is a highly dynamic and complex system where a multitude ofbiological, bio-mediated and abiotic processes interact. In this studywe have shown that in soils and sediments there exists an abiotic

process, hitherto unknown, forming alkyl halides, which dependson organic-matter content, halide-ion and oxidant concentrations.These ndings may have important implications for the globalproduction of VHOC. For example, it is known that: (1) thereduction of insoluble Fe(III) oxides and oxyhydroxides is one ofthe most signicant geochemical processes that takes place in the

sedimentary environment18; (2) many terrestrial soils exhibit highsalinities (up to 1% of dry weight); (3) worldwide 1,5002,200 Gt

of organic carbon is stored as humus in the soil layers19, andalkoxy- lated phenolic structures, especially methoxylated

phenolic structures, are monomeric constituents ofhumic substances.Nevertheless, the magnitude of VHOC

production in soil is difcult to estimate. In our modelexperiments, a maximum of 1 of30,000 carbon atoms was

liberated from the soil as VHOC within one hour. We cannot usethis relationship to estimate a global emission rate,

but our results do show that the terrestrial ecosystem has an0 50 100 150 200 250 300

Ferrihydrite (mg)

Figure 4 Data supporting the abiotic production of methyl halides by the interaction of

guaiacol with ferrihydrite and halide. a, Reductive dissolution of ferrihydrite by guaiacol

(2-methoxyphenol)and the related iodomethane formation. 20 mg ferrihydrite and 10 mMKl were suspended in 10 ml bidistilled water in a 20-ml headspace ask underargonatmosphere. The pH of the medium was adjusted with 1 M HNO3 to a value of4.4.

Guaiacol (2 mM) was added and theask was sealed. Subsamples were taken forFe(II)and iodomethane detection (n 36; s.d. 922% error).b, Inuence of ferrihydriteon yield of halomethane production. 10 mM KCl, KBr or Kl and 5275 mg ferrihydrite

were added to 10 ml bidistilled water in a 20-ml headspace ask. ThepH of the

medium was adjusted with 1 M HNO3 to a value of4.4. Before the ask was sealed the

medium was supplemented with 2 mM guaiacol. Aftera reaction time of 60 min methylhalide was monitored (n 36; s.d. 432% error).

enormous potential to release CH3Cl, CH3Br and CH3I into theatmosphere.

The described processes will depend on many factors, includingthe ambient environmental conditions (such as moisture, tempera-ture and soil acidity) and the type of organic material within thesoil. Additional studies and associated eld measurements arerequired in order to calculate the release rates of VHOC fromsoils and sediments, and to distinguish further between biogenicand abiotic production. M

Received 2 September; accepted 14 December1999.

1. Crutzen, P. J. & Arnold, F. Nitric acid clouds formation in the cold Antarctic stratosphere: a

majorcause for the springtime `ozone hole'. Nature 324, 651655 (1986).

2. Solomon, S. Progress towards a quantitative understanding ofAntarctic ozone depletion. Nature347,


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3. Anderson, J. G., Toohey, D. W. & Brune, W. H. Free radicals within the Antarctic vortex: The roleof

CFCs in the Antarctic ozone loss. Science 251, 3946 (1991).

4. Wever, R. Ozone destruction by algae in the Arctic atmosphere. Science 355, 501 (1988).

5. Butler, J. H. et al. A record of atmospheric halocarbons during the twentieth century from polar

rn air. Nature 399, 749755 (1999).

6. Gribble, G. W. Naturally occurring organohalogen compoundsasurvey. J.Natural Prod. 55,1353

1395 (1992).

7. Lovelock, J. E. Natural halocarbons in the air and in the sea. Nature 256, 193194 (1975).

8. Singh, H. B., Salas, L. J. & Stiles, R. E. Methylhalides in and over the Eastern Pacic (408N-328 S).J.

Geophys. Res. 88, 36843690 (1983).9. Class, T. & Ballschmiter , K. Sources and distribution of bromo- and bromochloromethanes in

marine air and surface water of the Atlantic Ocean. J. Atmos. Chem. 6, 35 46 (1988).

10. Moore, R. M., Groszko, W. & Niven, S. J. Ocean-atmosphere exchange of methyl chloride:

Results from NW Atlantic and Pacic Ocean studies. J. Geophys. Res. 101, 2852928538 (1988).

11. Sturges, W. T., Sullivan, C. W., Schnell, R. C., Heidt, L. E. & Pollack, W. H. Bromoalkaneproductionby

Antarctic ice algae. Tellus B 45, 120126 (1993).

12. Laturnus, F. & Adams, F. C. Methylhalides from Antarctic macroalgae. Geophys. Res. Lett. 25,

773776 (1998).

13. Harper, D. B. Halomethane from halide iona highly efcient fungal conversion of

environmental signicance. Nature 315, 5557 (1985).

14. Andreae, M. O. et al. Methylhalide emissions from savanna res in southern Africa. J. Geophys.Res.

101, 2360323613 (1996).

15. Rasmussen, R. A., Rasmussen, L. E., Khalil, M. A. K. & Dalluge, R. W. Concentration of

methyl chloride in the atmosphere. J. Geophys. Res. 85, 73507356 (1980).

16. Hoekstra, E. J., Lassen, P., van Leeuwen, J. G. E., De Leer, E. W. B. &Carlsen, L. in NaturallyProduced

Organohalogens(eds Grimvall, A. & De Leer, E. W. B.) 149158 (Kluwer Academic, Dordrecht,1995).

17. Desjardins, S., Landry, J. A. & Farant, J. P. Effects of water and pH on the oxidative oligomerization

of chloro and methoxyphenol by a montmorillonite clay. J. Soil Contamin. 8, 175 195 (1999).

18. Lovley, D. R. Dissimilatory Fe(III) and Mn(IV) reduction. Microbiol. Rev. 55, 259 287 (1991).

19. Post, W. M., Emanuel, W. R., Zinke, P. J. & Stangenberger, A. L. Soil carbon pools and world lifezones.

Nature 298, 156159 (1982).

Acknowledgements

We thank D. Schlo sser and K. Kratz for the instrumental neutron activation analysis(INAA) measurements and I. Fahimi and L. Warr forreviewing the manuscript.

Correspondence and requests for materials should be addressed toF.K. (e-mail:[email protected] elber g .de).

.................................................................Annualuxes of carbon fromdeforestation and regrowthin the Brazilian AmazonR. A. Houghton*, D. L.Skole, Carlos A.Nobre, J. L. Hackler*,K. T. Lawrence* & W H. Chomentowski

* Woods Hole Research Center, PO Box 296, Woods Hole, Massachusetts

02543, USA Department of Geography, Michigan State University, East Lansing,

Michigan 48824, USA InstitutoNacional de Pesquisas Espaciais, Caixa Postal 515,

nearly balanced with respect to carbon, but has an interannualvariability of6 0.2 PgC yr

-1.

We determined the annual ux of carbon with a `bookkeeping'model7,8 that tracks the annual emission and uptake of carbon thatfollow the clearing of forest for agriculture and the regrowth ofsecondary forests on abandoned agricultural land. Changes incarbon include (1) the immediate loss of carbon to the atmospherefrom plant material burned at the time of clearing, (2) the slowerrelease of carbon from decay of dead plant material left on site

(slash) and removed for wood products, and (3) the accumulationof carbon during forest growth. Changes in soil carbon were notincluded in this analysis, as they are small relative to the changes in

biomass and are inconsistent in direction912.We used two estimates of deforestation, three estimates of

biomass and two estimates of the rate of decay of organic matterto calculate a range of net carbon emissions attributable to land-usechange. The rst estimate of deforestation was obtained from theBrazilian Space Agency (INPE), where data from the Landsatsatellite are delineated manually for each state to determine bothannual rates of deforestation and cumulative areas deforested foreach year between 1988 and 1998 (except 1993). The annual andcumulative data are not entirely consistent, and we used thecumulative areas deforested to calculate annual rates of change

(Table 1). INPE also determined the area deforested in 1978;before1960 rates of deforestation were negligible13.The second estimate of deforestation was based on a map of land

cover derived from classication of 1986 Landsat multi-spectralscanner data (Fig. 1). Areas classied as deforested in 1986 wereconsistently lower than INPE's 1988 estimate of deforested area.Because the dates were different, we interpolated a rate for 1988based on maps of land cover derived from 1986 and 1992 Landsatdata. The interpolated area deforested in 1988 was still about 25%lower than INPE's estimate, although the actual percentage variedamong states (Fig. 2). We used this lower estimate for a secondestimate of deforestation, varying it annually in proportion to therates from INPE.

According to the Landsat-derived land-coverclassication, about30% of the deforested area was in secondary forest in 1986

presumably as a result of the abandonment of agricultural land14

17. The percentage varied from 5% in Gois to 65% in Maranhao. Aswe lacked data to suggest temporal trends in abandonment,we assumed that cleared lands were abandoned annually at therate dened by the ratio of secondary forest to deforested area in1986.

SaoJos

dos Campos, SP, CEP 12201-970, Brazil

..............................................................................................................................................

The distribution of sources and sinks of carbon among the world's

ecosystems is uncertain. Some analyses show northern mid-latitude lands to be a large sink, whereas the tropics are a netsource1; other analyses show the tropics to be nearly neutral,whereas northern mid-latitudes are a small sink2,3. Here we showthat the annual ux of carbon from deforestation and abandon-ment ofagricultural lands in the Brazilian Amazon was a source of

about 0.2 Pg C yr-1

over the period 1989 1998 (1 Pg is 1015

g).This estimate is based on annual rates of deforestation andspatially detailed estimates of deforestation, regrowing forests

andbio-mass.Logging mayaddanother5 10%to this

estimate4, and

res

Forest Deforested

Regrowing Cerrado
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Water

Cloud &shadow

500 km

may double the magnitude of the source in years following adrought4. The annual source of carbon from land-use change andre approximately offsets the sink calculated for natural eco-

systems in the region5,6

. Thus this large area of tropical forest is

Figure 1 Land cover in Brazilian Amazonia as of

1986, based on a classication ofLandsat MSS

data. The classication identies seven classes ofland cover: forest, deforested land, regrowing

forest, water, clouds, cloud shadow and cerrado

(savanna). Here data for cloud and cloud shadow

are grouped together.


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