Seasonal soil CO2 efflux along a toposequence in the ...

SEASONAL SOIL C02 EFFLUX ALONG ATOPOSEQUENCE IN THE

TRANSVOLCANIC MEXICAN BELT

Sara Covaleda1, Juan F. Gallardo'. Christian PraF y Jorge D. Etchevers-

IC. S. I. C., Aptado. 257, Salamanca 37071 (Espaiia). Tit.: +34 923219606. Fax:+34.923219609. E-mail:<[email protected]>.

2/.RD.lCENAPROS, Morelia 58090 (Michoacan, Mexico).3Colegio de Postgraduados, Campus Montecillo, Texcoco 56230 (Mexico).

Abstract:

Soil CO2 emissions have been scarcely studied in Mexico; and these data arevery important for constructing greenhouse gas inventory of Mexico. SoilCO2 emissions were seasonally determined in ten different systems foundalong an altitudinal gradient in a watershed of the Mexican transvolcanic belt(Atecuaro, Michoacan, 19°34'N; 101°J O'W). The dominant soils are Andosols(top half) and Acrisols (lower half), and the climate is sub-humid template(rain from June to October). The most representative systems along theclimo-toposecuence, in the Andosol arc: grasslands (P )-2615 m); pine-oakforest with increasing land degradation downslope (P2, P3 and P6:2500-2350m); fuelwood area (P4-2370 m); agricultural land (P5-2330 m); and in theAcrisols: forest patches (P7-2320 m). cultivated fields (P9-2280 m).grasslands (P10-2290 m), and eroded patches areas around gullies (P8-231Om). Vegetation was characterized at every point and soil samples collected.Soil CO2 fluxes and temperature were measured using a soil respirationsystem (PP Systems, five replicates at each point). five series ofmeasurement were conducted during one year. Clear seasonal variations ofsoil CO2 fluxes were observed. The lowest values were found in dry seasonand the highest during the rainy period. The Andosols (P I,P2,P3,P5) hadhigher CO2 fluxes (higher available water content, higher SOC contents andmicrobial activity), being on the average P5 the highest. The lowest soil CO2

flux was always in P8, a soil without A horizon and almost null SOC content.In spite that Acris ols presented lower CO2 fluxes, lower SOC content, higherbulk density, lower porosity, higher clay content than Andosols, thedifferences between the two were lower than expected. A linear positivecorrelation between SOC content and CO2 effluxes was established.

Key words: Soil respiration, catena, Mexico.

1. Introduction

The evident increase of atmospheric concentration of greenhouse gases have made theglobal C balance an important scientific and political topic. Consequently, scientists express adeep concern for the C02 exchange between the biosphere and the atmosphere. Theassessment of soil C02 emissions into the atmosphere allows visualizing of the dynamics of thisgas in the ecosystems, its variation in time and the effect of climatic conditions. Then, on thebasis of this information, it is possible to carry out evaluations of this gas and to establish thepossible impact of the changes of land use.

Soil respiration is a very large fraction of the gross primary productivity in terrestrialecosystems and its quantification is a high priority in attempts to establish ecosystem C budgets(Franzluebbers et at., 2002); data collected by Bremer et at. (1998) in a prairie ecosystem in

Kansas (USA) showed that soil respiration was a major component of the ecosystem C balance,Wu et ai, (2006) found that soil and ecosystem respiration in a temperate broad-leaved Koreanpine forest had similar seasonal patterns and they estimated that 67 % of the C gained byecosystem photosynthesis was lost to the atmosphere through soil respiration,

The current emphasis on ecosystem and soil management for increased Csequestrationneed a basis of quantifying studies of the different aspects of the C cycle, in particular, it isimportant to accurately estimate C budgets and dynamics. Improved monitoring of soil respirationis, then an essential feature to achieve this objective and to obtain reliable long-term predictionsof soil Cpool dynamics in models that use gas exchange measurements.

Soil C02 emissions have been scarcely studied in Mexico and those data are veryimportant for constructing the country greenhouse gas inventory, Soil C02 emissions wereseasonally determined in ten different systems found along an altitudinal gradient in a Watershedof the Mexican trans-volcanic belt.

2. Materials and methods

Sitedescription

The study site is located at the Atecuaro catchment, within the Cuitzeo Basin, 12 kmaway from Morelia (State of Michoacan, Mexico: 19° 34' N; 101°10' W); the weather is subhumidmild with an annual mean temperature ranging between 15.7 and 17.7 QC; and an annual meanprecipitation of750 to 1100 mm yr1 , with a summer rain system (from June to October),

Forests are located mainly in the South and Southeast part of the catchment, between2300 and 2500 m a.s.l. in moderate to steep slopes, These areas are very degraded as aconsequence of illegal timber and resin extraction activities, Most disturbed forests are located inthe areas near Morelia city; further places preserves higher densities of forest vegetation (Rubio,1998), The forest succession ispine-oak at the summit, with a growing degradation on the hillsideuntil reaching maize cultivations on foothills, Ten sites were selected along the toposequence(Table 1), The most representative systems along the climo-toposecuence in the Andosol areaare: grasslands (P1); pine-oak forest with increasing land degradation downslope (P2, P3 andP6); fuelwood area (P4); agricultural land (P5) with a fallow management; and in the Acrisol area:forest patches (P7), cultivated fields (P9) also managed as a fallow system, grasslands (P10),and eroded patches areas around gullies (P8),

Table 1. Situation and characteristic of the points studied along the toposequencePoints Situation Altitude (m) Soil Vegetation

P1P2P3P4PSPGP7PSpgP10

19°34'08.3"-N19°34'22T-N19°34'38"-N

19°34'53.4"-N19°34'43T-N19°34'55.4"-N19°35'02,9"-N19° 35'060"-N19° 35'22.1"-N19° 35'33T-N

101 °09'59.4··-W101°09'23T-W101°09'32T-W101°09'51.5"-W101°10'106"-W101 °1 O'30T-W101°10'39T-W101°10'45,O"-W101°12'19,5"-W101°12'20T-W

2

2615249624112370233023202300230822812290

AndosolAndosolAndosolLuvisol

AndosolAndosolAcrisolAcrisolAcrisolAcrisol

GrasslandPine-oak forestPine-oak forestFuelwood area

Agricultural landPine-oak forestPine-oak forest

Eroded areaAgricultural land

Grassland

The edaphologic map of the Atecuaro catchment (1 :60000) produced by Mendoza-Camu(2003) showed that points P1, P2, P3, P5 and P6 are located in the Andosol area; P7, P8, P9 andP10 are situated in the Acrisol area and P4 in an area where Luvisols dominate. This map followsthe World Reference Base for Soil Resources (WRB) classification (FAO, 1998). The dominantsoils in the catchment studied are Andosols (top half) and Acrisols (lower halt) comprising the 70%of the total surface area (Medina, 2002).

Soil sampling

In the forest and grassland sampling sites a micro-profile (A horizons) was opened andsoil samples from the A horizons were colleted. In the agricultural lands soil samples from the Aphorizon were taken with a cylindrical probe. Samples were taken at0-10 cm and 10-20 cm depth.

Soil analyses

For chemical and physicochemical analyses, soil samples were air-dried andhomogenized, the macroscopic organic matter (MOM) was manually extracted from the mineralsoil; later, soil samples were sieved through a 2 mm mesh. Soil organic C (SOC) was determinedthrough dry combustion (TOCA); Total I\J (Nt) was measured using the micro-Kjeldahl method;extractable Pwas determined by Bray 1 method, P-sorption according to Blakemore et al. (1987)and pH was analyzed in water (1:2 ratio). Physical analyses were conducted over field fresh soilsamples; bulk density (Da) was measured with the cylinder method and available water contentby the pressure chamber method.

Soil C02 measurements

Soil C02 efflux (SCE) and temperature were measured using a Soil Respiration System(PP Systems, Stotfold, UK) that consists in a portable infrared gas analyzer and a soil respirationchamber with a small pump. The principle of soil respiration measurements is a closed systemthat calculates increments ofC02 in the chamber atmosphere.

Six sets of measurement were conducted during one year. Five replicates ofsoil C02 nuxand soil temperature were taken at each point. Soil moisture was measured on four occasionscoinciding with SCE measurements; soil samples from the upper 10 cm of soil were collected anddried at 105 QC for 24 h.

3. Results and discussion

The results of soil analyses performed in A horizons at every point are presented in table2.

pH showed acid values in every case, Andosols had, on average, a pH of 5.5 in the Ahorizons and Acrisols 5.2, P4 showed the highest pH (6.1) in the upper A horizon. ExtractableBray P was, in most cases, in the order of trazes probably because of the high P-sorption found,specially in Andosols (97% average); the only exception was P9, an agricultural area wherephosphate fertilizers were used. Andosols are very rich in SOC and Nt (Nanzyo etal., 1993a), theAo epipedons found in the areas of forest over Andosols contained particularly highconcentrations and even the agricultural land (P5) had high levels of SOC and Nt; on the otherhand, the agricultural land (P9) and the bare eroded soil (P8) in the Acrisol area showed the

3

lowest values. The high CIN ratio in the areas of forest and grasslands indicates a slowmineralization process; in contrast this ratio is particularly low in the eroded site due to the almostinexistence of SOC.

A low value of bulk density is another characteristic of Andic soils (Da < 0.9 g/cm3 is adefinitory characteristic of Andic horizons in the WRB); however points P3 and P5 showed highvalues due to the compactation processes, as a consequence of anthropogenic activity.

The high water retention of Andosols is primarily due to their large volume of meso-poresand micro-pores (Nanzyo et al., 1993b); plant available water occurs primarily in the meso-pores.The available water of Andosols is on average 23.5 % and of Acrisols 7.5 %, the most preservedsite with a forest vegetation (P2) was the one with the highest quantity of available water, thebare soil (P8) presented the lowest.

The soil content of MOM is dependent on the productivity of the vegetation cover(Garcia-Oliva et al., 2006), being higher in forest areas and much lower in agricultural lands andgrasslands.

Table 2.Results of the edaphic parameters measured in the A horizons ateach point of the toposequence

Depth pH H2O P Brayp.

C N Da MOMAvailable

Sites Horizon sorption C/N water (%(cm) (1:2.5) (ppm)(%)

(mg/g) (mg/g) (g/cm3) (Mg/ha)m oisture)

P1 A1 0-30 5,6 3,7 98,2 58.0 4,0 14,5 0,8 2.5 14,2

A2 30-60 6,1 t 98,4 25.7 1,6 15,8 0,6 0,4 12,3

P2 Ao 0-7 5.2 1,1 95,1 320,4 15,5 20,7 0,5 58.2 45,2

A1 7-27 5,5 t 99,2 99.3 6,2 15,9 0,8 9,0 24,5

A2 27-37 5,4 t 98,0 69,4 4,4 15,7 1,0 4.6 22,7

P3 AD 0-2 4,7 0.6 97,0 233,6 9,2 25,4 1,1 58,4 21,3

A1 2-20 5,5 t 97.8 84.9 4,4 19.1 2,1 44,4 18,0

A2 20-33 5,4 t 97,1 42,9 2,4 18.1 2,0 18,3 17,8

P4 A1 0-7 6,1 1,1 60,5 84.0 3,6 23,4 0.9 19,6 15,1

A2 7-20 5.2 0,2 54,9 24.2 1.3 19,4 1,4 5,6 8,0

P5 Ap1 0-10 5.6 0.6 92,9 58,3 4,7 12,3 1,1 7,5 22,9

Ap2 10-20 5.5 0.6 93.5 56,1 4,4 12,7 1,2 5,2 24.0

P6 AD 0-1 5,3 0,7 94,1 187,9 9,4 20,1 * 36,8 34.5

A1 1-17 5.7 t 98,4 75,2 4,3 17,6 * 31,9 27,3

A2 17-40 5,8 t 98,4 47,6 2,8 16.8 * * 20.8

P7 A1 0-2 5.8 0.7 59,5 70.6 4,6 15,2 1,2 33,3 7,9

A2 2-32 5,1 t 51.7 11.2 * * 1,4 1.7 5.2

P8 A1 0-5 4,8 t 65,0 1.6 0,6 2,6 1,5 0,0 4,4

A2 5-20 4.8 1.2 56,3 1.2 0,2 6,1 1,6 0,1 3,8

P9 Ap1 0-10 5,4 3,4 61,8 1.8 0,2 12,0 1,3 1.6 *

Ap2 10-20 4,9 3.8 59,4 1,7 0,2 11,3 1,1 3.1 *

P10 A1 0-5 5,4 t 55,7 * 0,2 * * * 14.2

A2 5-15 5.7 t 57,5 * 0,2 * * * 9,6

Da: bulk density; MOM: macroscopic organic matter; t: trazes

4

The results of the SCE, soil temperature and moisture measured during 2005 in all thepoints studied along the toposequence are showed in Table 3.

Table 3. SCE, soil temperature and soil moisture measurements at6 different moments in 2005 along thetoposquence ofAtecuaro (Mexico).

P1 P2 P3 P4 P5 P6 P7 P8 pg P10

SCE (gC02 m-2 h-1) 0.23 0,77 0,21 030 0,57 0,40 0,41 0,03 0,10 ·22/04/2005 Os 0,10 0,11 0,14 0,22 0,22 0,04 0.23 0,02 0,02 ·

T (OC) 27,9 17,5 17,5 20,3 21,5 24,3 25,0 30,4 29,0 ·Moisture 1%1 53,4 49,0 50,1 15,8 15,0 29,2

. • . •

SCE (gC02 m-2 h-1) 0,62 1,82 1,45 1,38 3,86 1,27 1,35 0,02 0,65 ·06/07/2005 Os 0,08 0,57 0,33 0,12 0,99 0,19 0,58 0,02 0,08 ·

T (OC) 18,7 14,2 15,0 20,7 26,1 20,1 20,4 21.5 17,3 ·SCE (gC02 m-2 h-1) 1,33 2,40 2,44 199 3,77 1,34 1.28 0,04 0,68 2,47

18/08/2005 Os 0.33 0,32 0,54 0,39 1,32 0,45 0,67 0,01 0,38 1.01

T (OC) 13,8 10,9 11,7 13,0 16,4 13.7 13,4 16,1 17,0 17,4

Moisture 1%1 65,3 108,2 134,8 67,8 51,0 72,9 48,8 29,6 . 41,2

SCE (gC02 m-2 h-1) 0,82 2,46 2,17 1,55 2,26 1,28 0,99 0,03 0.63 0,88

06/10/2005 Os 0,47 0,72 0,75 0,23 0,16 0.31 0,25 0,04 0,06 0,57

T (OC) 15,0 14,0 14,4 17.5 14,2 12,4 11,5 12,0 12,4 15,0

Moisture f%l 59,0 132,8 132,9 66,6 48,8 60,6 42.5 24,1 34,6 33,6

SCE (gC02 m-2 h-1) 1.05 2,44 2,48 1,11 0.73 1,24 1,43 0,09 0,17 0,24

014/11/2005 Os 0,16 1.05 0,46 0,36 0,10 0,35 0,39 0,03 0.05 0,07

T (OC) 23,2 14,6 14,3 17.7 7,6 11,7 11,6 23,9 20,8 19,5

SCE (gC02 m-2 h-1) 0,62 1,50 2,28 0,72 0.82 1,16 1,11 0,03 0,10 0,17

06/12/2005 Os 0,15 0,16 0,51 0,19 0,16 0,42 0,29 0,02 0,06 0,08

T (OC) 16,1 12,7 12,3 15,4 18,6 11,7 11,5 24,5 23,3 22.2

Moisture 1%1 29,9 47,1 31,6 25,0 26,6 35,4 . . 14,0 17,2

SCE: Soil C02 efflux; Os: standard deviation; T: Temperature.

Clear seasonal variations of SCE were observed (Figure 1). The lowest values werefound in the dry season and the highest during the rainy period. During the rainy season (in 2005the rains started in July, one month latter than usual and in November there were still someprecipitation) the standard deviations (Os) of the measured respiration were higher than duringthe dry season (measurements ofApril and December); probably this was due to a more variablesoil moisture conditions during the rainy period. On average, P5 had the highest SCE during therainy season probably due to manure received from cows during the fallow period, that stimulatedthe microbial activity. P2 and P3 also showed high SCE, both of them with a Aa epipedon veryrich in SOC. The lowest SCE was always in P8, a soil without A horizon and almost null Ccontent.

5

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5,0

- 4,0<"IE

N0 3,0u$u.Ju

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Soil respiration, including root respiration and heterotrophic microbial respiration, isinfluenced by type of vegetation, physical and chemical properties of soil, season of the year, andclimate regimes of inter-annual (Wu et al., 2006). It is generally accepted that temperature is akey variable that controls soil respiration; however, the exact nature of this relationship is lessclear (Subke et al., 2003). The influence of temperature and moisture on soil respiration has beenstudied by different authors, but none of them was able to demonstrate a significant influence ofsoil moisture on soil respiration; the relation between soil respiration and soil temperature wasdescribed as exponential, linear 0 quadratic (Mathes and Schriefer, 1985). Points P2, P3 (forestover Andosol on the slope of the mountain) and P4 (a fuelwood area on the slope that suffered aforest fire in 2000), cou ld be grouped relating SCE and soil temperature (Figure 2a); theypresented a linear negative relationship between both factors (R2=67.7; n=15). The reason of thisunexpected negative correlation is probably due to the influence of another factor, in this case,soil moisture. This iscorroborated because a positive relationship between SCE and soil moisturewas found in these points (R2=70.0; n=11; Figure 2b). This means that soil moisture can be thetrue limiting factor for soil microbiology; obviously, a higher temperature means a higherevapotranspiration and then the soil water content decreases (Bonneau y Souchier, 1979). P6and P7, both of them forests at the piedmont (the first over Andosol and the second over AcrisoD,also showed a similar behaviour, explained by a quadratic correlation (R2=89.1; n=10), with amaximum around 16 °C (Figure 2c); again moisture controls SCE, showing a positive quadraticcorrelation (R2=83.2; n=5; Figure 2d). Grasslands (P1 over Andosol and P10 over AcrisoD andthe agricultural land over an Acrisol (P9) showed a negative linear correlation between SCE andsoil temperature (R2=68.3; n=13; Figure 2e), but no significant correlation was found betweenSCE and soil moisture. P5 and P8 showed specific behaviours; the first probably due to theabundant manure left by cows grazing the area and the second due to the scarcity of SOC.During the dry period the relationship between SCE and soil temperature in all points studiedfollowed a quadratic negative regression (R2= 77.1; n=18; Figure 20, but no significant correlationwas found between SCE and soil moisture.

6

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Figure 2.a) SCE vs soil temperature in P2, P3 and P4. b) SCE vs soil moisture in P2, P3 and P4. c) SCE vs soiltemperature in P6 and Pl. d) SCE vs soil moisture in P6 and Pl. e) SCE vs soil temperature in P1, pg and P10. 0SCE vs soil temperature considering all points in the dry season (measurements ofApril and December).

In general, sites over Andosols (P1, P2, P3, P5 and P6) had higher SCE (higher availablewater content, sac contents and microbial activity) than those over Acrisols (P7, P8, pg andP10) that presented lower SCE, C content and porosity and higher bulk density and clay contentthan Andosols; but the differences between the two SOil groups were lesser than expected, As anexample, it is found that there is a similar characteristic of P6 and P7 and the grassland areas;this means that other factors different of soil properties, as vegetation cover and topographicalsituation are determinant for defining the relationship between SCE, soil temperature and soilmoisture,

7

SOC content was related to SCE for each set of measurements and linear positivecorrelations were found (Table 4), except for the measurements of April where no significantcorrelation could be established. A higher SOC content implicates higher soil respiration rates, asit was expected.

Table 4. Equations, R-square, significance and n ofthe relationships found between SCE and sca for each set ofSCE measurements

Month Equation R2 Significance nJuly SCE = 0.0140*SOC + 0,2358 81.2 ** 8

August SCE = 0.0201*SOC + 0,2439 83.0 ** 8October SCE = 0.0185*SOC + 0.1348 75.8 ** 9

November SCE = 0.0219*SOC - 0.1629 74.0 * 10December SCE = 0.0136*SOC - 0.0505 80A ** 9

* level ofsignificance < 0,05, ** level ofsignificance < 0,01.

4.Conclusions

- SCE showed seasonal variations in all sites along the toposequence. The lowest valuesoccurred in the dry season and the highest during the rainy period. The areas of forests on theslope with a Aa epipedon, very rich in SOC, showed, in general, high SCE. The lowest SCE wasfound in a bare soil in the piedmont without A horizon and almost null SOC content.

- An unexpected inverse correlation between SCE and soil temperature was found, butthis is probably because another factor is dominantly controlling soil respiration, in his turn relatedwith soil temperature; in this sense, soil water content showed positive correlations with SCE forthe majority of sites.

- The SCE measurements done between July and December showed a narrow positivecorrelation between SCE and SOC.

- Attending to SCE, the sites along the toposequence could be grouped: a) Areas offorest on the slope; b) areas of forest at the piedmont; and c) grasslands together with theagricultural land on the Acrisals area. This implies that topographical situation and vegetationcover are more important to soil respiration than the soil group.

5. Bibliography

Bremer, D.J., J.M. Ham, A.K. Knapp and C.E. Owensby. 1998. Soil respiration responses toclipping and grazing in a tallgrass prairie. J. Environ. Qual. 27: 1539-1548.

Blakemore, L.C., P.L. Searle and B.K. Daly. 1987 Methods for chemical analysis of soils. N.Z.Soil Bureau. Sci. Rep. 80. Soil Bureau, Lower Hutt, New Zealand.

Bonneau, M. y B. Souchier. 1979. Pedoloqie 2. Constituants et proprietes du sol. Masson.Paris. pp: 459.

FAO. 1998. World referente base for soil resources. Web page:<http://www.fao.org/documents/shoO_cdr.asp?urUile=/docrep/08594E/08594EOO. htm>

Franzluebbers, K., A.J. Franzluebbers and M.D. Jawson. 2002. Environmental controls on soiland whole-ecosystem respiration from a tallgrass prairie. Soil Sci. Soc. Am. J. 66: 254­262.

8

Garcia-Oliva, F., J.F. Gallardo, N.M. Montano and P. Islas. 2006. Soil carbon and nitrogendynamics followed by a forest-to pasture conversion in western Mexico. AgroforestrySystems 66: 93-100.

Mathes, K. and T. Schriefer. 1985. Soil respiration during secondary succession: Influence oftemperature and moisture. Soil BioI. Biochem. 17: 205-211.

Medina, L.E. 2002. Erosion hidrica y transporte de sedimentos en la microcuenca de Atecuaro,Michoacan. Tesis de Licenciatura. Facultad de Biologia. 1I.M.S.t\I.H., Morelia, Mexico. 77pp.

Mendoza-Cantu, M. E. 2003. Implicaciones del cambio de cobertura vegetal y uso del suelo enel balance hidrico a nivel regional. El caso de la cuenca del lago de Cuitzeo. DoctoralThesis. Universidad Nacional Autonorna de Mexico. Instituto deEcologia. Morelia.

Nanzyo, M., R. Dahlgren y S. Shoji. 1993a. Chemical characteristics of volcanic ash soils. En:Shoji, S., M. Nanzyo, R. Dahlgren (eds.). Volcan ic ash soils, genesis, properties andutilization. Elsevier Science Publ. Amsterdam. pp: 145-187.

Nanzyo, M., S. Shoji YR. Dahlgren. 1993b. Physical characteristics of volcanic ash soils. En:Shoji, S., M. Nanzyo, R. Dahlgren (eds.). Volcan ic ash soils, genesis, properties andutilization. Elsevier Science Publ. Amsterdam. pp: 189-207

Rubio, M. 1998. Dinarnica del carnbio de la cobertura y del uso de los suelos en la microcuencade Atecuaro (Mlchoacan, Mexico). Una perspectiva desde las Ciencias Ambientales.Degree report. Universidad Michoacana de San Nicolas de Hidalgo/UniversidadAutonorna de Barcelona. Morelia, Michoacan, Mexico.

Subke, J.A., M. Reichstein, J.D. Tenhunen. 2003. Explaining temporal variation in soil C02efflux in a mature spruce forest in Southern Germany. Soil BioI. Biochem. 35: 1467-1483.

Wu, J., D. Guan, M. Wang, T. Pei, S. Han and Ch. Jin. 2006. Year-round soil and ecosystemrespiation in a temperate broad-leaved Korean pine forest. Forest Ecology andManagement 223: 35-44.

6.Acknowledgements

The authors thanks to the European Union the support of the REVOLSO Project (INCOProgram, ICA4-CT-2001-10052) and also Paola and Chano for their help in the soil samplingprocesses.

9

SEASONAL SOIL CO2 EFFLUX ALONG A TOPOSEQUENCE IN THE

TRANSVOLCANIC MEXICAN BELT

Sua ConI....1, .llUIII F. Oa11ar1lo1, "I.Iorge D. I!tchenrs2 Christian pratJ'ConseJo Superlor de Invest lgac lones Clentltlcas, Aptado. 257, Salamanca 37071 (Spain). E·maU; <jgall ar d@usa l.es >.

21. R. N., Coleglo de Postgraduado de Monteclllo, Texcoco 56230 (Mex ic o). E·mall; <j. t c b. y@cpl ppl .m x >.' Instlt ut de Recherche pour le Developpement, Montpelller (France). E·[email protected]>•

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