View of Carbon dioxide evolution from snow-covered agricultural ecosystems in Finland

Carbon dioxide evolution from snow-covered

agricultural ecosystems in Finland

Hiroshi Koizumi

Divisionof^{PlantEcology,} National InstituteofAgro-EnvironmentalSciences, Tsukuba, Ibaraki, 305Japan Markku Kontturi

AgriculturalResearch Centreof^{Finland, Institute}of^Cropand Soil Science, FIN-31600 Jokioinen,Finland ShigeruMariko

SugadairaMontane Research Center, Universityof^{Tsukuba, Sanada,} Nagano, 386-22^Japan Timo Mela

AgriculturalResearch Centreof^{Finland, Institute}of^Cropand Soil Science, FIN-31600 Jokioinen,Finland

The release of C0₂from thesnow surfaceinwinter and the soil surface insummer wasdirectly or indirectlymeasured inthree different soil types(peat, sand and clay) in agriculturalecosystemsin Finland. The closed chamber(CC) methodwasused for thedirect and Pick’s diffusion model (DM) methodfor theindirect measurements. The winter soil temperatures at 2-cm depthwere ^{between 0} and I°Cfor each soil type. The concentration of C0₂ within the snowpack increasedlinearly with snow depth. The average fluxes of C0₂ calculated from^the gradients of C0₂concentration inthe snowusingtheDMmethodrangedfrom 10to27 mg C0₂nr^2h and with the CC method from 18to 27 mg C0₂

m 2 h'.

These results suggest that the snow insulates the soilthermally, allowing ^C0₂ production tocontinue at soil temperatures slightly above freezing inthe winter. Carbon dioxide formedinthe soilcan move acrossthesnowpackup to theatmosphere.The winter/summer ratio of C0₂ evolution wasestimated to exceed 4%.Therefore, the snow-covered crop soil servedas a sourceof C02 in winter, and C0

2evolution constitutesan importantpart of the annual C0₂budget in snowy regions.

Key words: closed chamber method, C0

2 flux, C0₂ profile, heavy clay soil,peat soil, sand soil, snowpack,soilrespiration, subarctic climateregion

ntroduction

and the soil. Budgets assume that microorgan- isms and plant rootsin snow-covered soils _stop respiring and sothere is no evolution of C0₂ from the _snow surface when soil temperatures drop toaround O°C(Steudleret

al.

1989, Bouw- Carbon dioxide is the primary gas involved in

the exchange of carbon between the atmosphere

©Agricultural and Food ScienceinFinland Manuscriptreceived June1996

man 1990).Thus the contribution ofCO,^from alpine orarctic regions in winter has not been considered important in calculations of global carbon balances. Some reports have, neverthe- less, shown that C0₂concentrationsare high_at the base of snowpacks in arctic and _temperate regions (Kelley etal. 1968, Solomon and Cer- ling 1987). Inaddition, there is someevidence that microorganisms in soils beneath the snow continuetorespire attemperatures closetoO°C (Taylor and Parkinson 1988, Sommerfeldetal.

1991). It is currently thought that the soils un- der alpine and sub-alpine snowpacks evolveCO,

tothe atmosphere throughout the snow-covered period (Sommerfeld _etal. 1993, Mariko _etal.

1994).

Snow can coverbetween 44% and 53% of the land area of the northern hemisphere, and in alpine and sub-alpine regions it may be several meters deep for more than half the year (Barry 1992). Sommerfeld et al. (1993) reported that C0₂ fluxes through snowpacks in these regions ranged from 31 to 84 mg C0₂ nv² h^', as calcu- lated from Pick’s law. If snow-covered soils show _someC0

2evolution inwinter,the C0

2re- leased canstrongly influence the annual global carbon budget. This flux needs tobe measured urgently for a more complete understanding of the global carbon balance. However, not only C0₂ fluxes from thesnow surface but also C0₂ concentrations in the snowpack have rarely been measured (Coyne and Kelley 1974, Solomon and Cerling 1987). In particular, no researcher has measured the fluxes directly with chamber meth- ods.

The aim of thepresent study is to measure directly orindirectly the CO,released from the snow surface in winter and the soil surface in summer in three different soil types in agricul- tural_ecosystems in Finland. The closed cham- ber (CC) method(Bekku etal. 1995, Bekku et

al. 1996)was used for the directmeasurement.

For methodological comparison, the C02fluxes were measured with Pick’s diffusion model (DM), whichsofar has been usedas an indirect method(Roiston 1986).

Material ^and methods

Study site

The investigations were carried ^out in experi- mental fields of the Agricultural Research Cen-

tre of Finland (ARCF) in July 1992 and March 1994. The fields _were located at Jokioinen (60°9’N, 23°0'E, 104 m asl) in southern Finland.

Meteorological dataat the ARCF site showed that annual precipitation averaged 581 mm, the mean annualtemperature was 3.9°C and theav- eragesnow depth in March was 39 cm during the period from 1961to 1990. The monthlymean temperature in July 1992 was 16.0°C and in March 1994 -3.2°C. The warmth index ^at this sitewas 38.1 degree-months, indicating that the site is in the subarctic climate range.

Three barley fields consisting of three dif- ferent soil types wereinvestigated. Classifiedon the Finnish and FAOsystem, the soils_werepeat

(histosol), sand (cambisol) and heavy clay(cam- bisol). Table 1 lists the cultivation methods for each soil _type in the agricultural ecosystems.

Spring barley _was cultivated in one field from the middle of May ^to the end of August, after which the fieldwasfallowed from Septemberto the following April.

After themeasurementsofCO,flux in win- ter, snowpits weredug beneath five flux-meas- urement points in each soil type toassess snow depth and porosity (Sommerfeld etal. 1993). At the same time,air temperatures were measured 50cmabove thesnow oratthe soilsurface, snow temperatures^at 10-cm depth and/or soil^temper- atures at2-cm depth in winter andsummer. The temperatures were measured withadigital-read- ing, spike stem,dual metallic thermometer.

Measurement of C0

2

flux using the CC method

The evolution of CO, from the soil surface in summerand from thesnowsurface in winterwas

Table 1.Cultivationmethods ofspring barley inthree soil typesin agriculturalecosystems.

Soil type Growthperiod Variety ^Seeds Row space Nfertilizer Fallowing (g rrr²⁾ (cm) (g rrr²⁾ period

Peat soil 15May~25Aug. Jo1545 19.0 12.5 6.0 Sept.^~ following April

Sandsoil 15May~20Aug. J01545 19.0 12.5 9.2 Sept.^- following April

Claysoil 12May~20Aug. J01545 19.0 12.5 9.2 Sept.^~following April

measuredon 14 July 1992 and 18 March 1994, respectively, for thepeat soil, on 14 July 1992 and 18 March 1994 for the sandsoil,and on 16 July 1992 and 18 March 1994 for the clay soil.

The fluxwasmeasuredatfive points in each soil typeduring a diurnal period lasting from 10:00 to 14:00.

The C0

2fluxwasdetermined directly in situ with the CC method, because this method can be usedatsites where electricityorspecial equip-

mentis not available. In this method, a closed chamber is placed over the soilor snow surface and the increase in the concentration of C02

within the chamber is measured _{as a}function of time. TheCO, flux is calculated from Eq. ^(1).

F ⁼ (V/A)(AC/At), (1)

where Fis the C0₂flux (mg C0₂ nr²h 1),V is the volume of air within the chamber(m^3),A is theareaof the soil or snowwithin the cham- ber(m^2), and AC/At is the^rateof change in the C0₂concentration in the air within the chamber (mgCO, m³ h^')•

A polyvinyl-chloride cylinder (15-cm high, 21-cm internal diameter) wasplaced on the soil or snow surface. Aboutan hourlater,the cylin- derwasclosed with apolyvinyl-chloride lid,and arubber-capped (blood collection) needlewas fittedonto an air sample port on the top of the lid. Air in the chamberwasaspirated through the needle intoanevacuated vial (5 ml) three times atequal intervals (2or5 minutes), and the C0₂ concentration ⁱⁿ the vial air was measured as described in the previous paper (Mariko etal.

1994). The concentration of C0₂ was plotted against the time (Fig. 1).

Fig.

1.

^{Typicaltimecoursesof C02}concentrationinacham- ber of the CC methodin^(a)summerand (b) winter. Results wereselected to illustrate the lowest and highestrates of C0₂increaseonsummer^{and winter}daysfor three soil types.

Regression equationsand correlation coefficients:

(a) Peat ^(•);Y=52.281X+329.24, R²=0.998 Sand (■);Y=21.910X+325.34, R²=0.997 Clay (A);Y=33.472X+341.39, R²=0.999 (b) Peat ^(•);Y=1.6892X+340.46. R-0.999

Sand (■);Y=1.2912X+336.20,R²=0.996 Clay (A); Y=1.3334X+333.96, R²=0.997

Profile of C0

₂

concentration in snowpack

The concentration gradient of C0₂ in the snow- pack and atmosphere was determined on the

samedayasthe direct determination of the C0₂ flux. Air was sampled, using the gas collector described in a previous paper (Mariko et al.

1994), at various depths below ^or above the closed chamber. The air samplingports werein- stalled atdifferentsnow depths. Snow air was drawn through the needle withanevacuatedsam- pling vial (5 ml). Some air (about2 ml) in the gas collectorwaspreviously removed withagas- tight syringe connected tothe needleatthe end of the gas collector. The air samples weretaken twice at twolateral locationsatthe same snow depth to obtain a morerepresentative concen- trationat aparticular depth.

Calculation of C0

₂

evolution from ^snow

surface using Fick’s ^DM

The evolution of C02from the snowpack in win- ter was calculated assuming simple diffusion, average C0₂ gradients, average _snowdepth and average porosities (Sommerfeld et al. 1993, Marikoetal. 1994). The diffusion of gases in the snowpack can be described by Fick’s first law, which states that the steadystate transport of gas by diffusion through a unit areais pro- portionaltothe concentration gradient measured along the line normal^to thearea:

F ⁼ -aDp (dC/dz), (2)

where Fis the C0₂flux (mg C0₂nr² h'),a isaconstantfor unit conversion (36as avalue), Dp is the diffusion coefficient(cm² s^')and dC/

dzis the vertical concentration gradient of C0₂ (mg C0₂ m³cm

1

^{). The term dC/dz canbe de-}

termined directly by drawing a straight line through the average concentrations from the snow surface to20-cm depth^(C0₂ profile) and by calculating the slope.

The diffusion coefficient Dp is influenced by

the atmospheric pressure andtemperaturein situ.

Dp for C0₂ in _{snow can} be estimated from Eq.

(3):

Dp^- D()Q A, (3)

where

D 0 is

the diffusion coefficient of C0₂ in air (cm² s^'), 0 is the porosity of the snow measured(cm³cm^3),and Ais the empirical tor- tuosity factor (cmcm^')(van Bavel 1951).

D 0 was

corrected for thetemperature⁽T, °K) and theat- mospheric pressure ^(P, hPa) at each study area from Eq. (4):

D 0

^{(T, P) =DJS)} (T/273f 1013/P (4) T and P were estimated from themeteoro- logical data of the Meteorological Observatory of the Agricultural Research Centre of Finland.

Dg(S) is the diffusion coefficient_at the normal state (theempirical value 0.135cm²s'

1

^{for C02)}

and an exponent n is 1.71 for C0₂ ^(Osozawa 1987). The tortuosity factor for snow was not measured here. However, theconstant value of 0.69 estimated from the data of Sommerfeld ^et al. (1993) was adopted. This value is appropri-

ate since the porosities influencing tortuosity werealmost^constantoverthesnow depthsmeas- ured,and similartothose(c.0.58) of Sommer- feld ^et al. (1993) and Solomon and Cerling (1987). The average porosity of snow wasbe- tween0.53 and 0.59atall measurementpoints.

Measurement of C0

₂

concentration of air

stored in the sampling vial

The concentration of C0₂in the air stored in the sampling vialwas determined withan infrared gasanalyser, IRGA(Model ZRC,Fuji Elec. To- kyo, Japan). A 1- or2-ml aliquot of air in the vialwasremoved with_agas-tight syringe (2-ml volume).During thewithdrawal,the air removed was replaced with distilled water, adjusted _at pH4.O withphosphoric acid (H,PO₄^). The with- drawn air_wasinjected into_agas linepurged with

pure

N 2 ^gas

^{at aflowrate} of 0.5 ml min

1

^.This

gas line allowed the injected air to reach the IRGA in several seconds. The electric output from the IRGA was recorded as a steeppeak, like a spike on a pen recorder (Servocorder 5R6312, Graphtec, Tokyo, Japan). The spike heights (X) wereused for quantitative analyses ofCO,concentration. A standardcurve waspre- liminarily prepared by injecting the same vol- ume of C0₂ gas (1 or 2 ml) in various known concentrations (T), which gavealinear response curve to the spike heights ^(T=ll.OBX+ 20.15, r=0.99,pcO.001).

Carbon and nitrogen contents in soil

After cultivation of thefields,soil samples from each fieldwere collectedatfive flux-measure- mentpoints fromseven depths, 2.5, 10, 20, 30, 40,50 and 60_cm. Samples of dried soil ^(105°C) were ground to determine the carbon and nitro- gen contents. Each dried sample was sieved through _a0.2-mm meshsieve, carebeing taken

to sieve the entire sample. This portion of the sample _wasanalysed for total carbon and nitro- genusingan automatic carbon and nitrogen an- alyser (C-N Corder MT-600, Vanako, Kyoto, Japan)

Results

Figure 2 shows the vertical distribution of the carbon and nitrogencontentsin three soiltypes.

The soil carbon and nitrogencontentswere con- stant to a depth of 20 cm, but below that de- creased with depth in each soil. The contents differed, however, between the soil types. The values of carbon_to 20-cm depthwere approxi- mately 20% forpeat, 1.7% for sand and 3.5%

for claysoil,and those of nitrogenwereapprox- imately 1.3% forpeat, 0.13% for sand and 0.25%

for clay soil. Storage ofcarbon within the upper 60-cm layers of soil amounted ^to54.4 kgC m

2

in thepeat, 16.9 kgC

m 2 in

the clay and 8.4 kgC nr² in the sandy soil.

Fig 2.Vertical distribution of soil carbon andnitrogen contents^(%,dry weight ^basis)inpeat, sand andclaysoil ^fields,

•,carbon content; O, nitrogen content

Table2. Temperature (°C)ofair,snow^{and soil}onthe measurementdays inthree soil types.

Peat Sand Clay

March July March July March July

Air(+socm) -1.0 +17.5 +l.B +18.7 _+l.B +16.6

Snow (-10 cm) -1.4 -0.3 +O.O

Soil (Ocm) -0.5 +20.1 +O.O +20.3 +0.2 +18.4

Soil (-2cm) +0.2 +19.0 +0.6 +18.9 +0.5 +17.5

The averagesnow depthonthemeasurement dayswasapproximately 20cmfor thepeat, 15cm for thesand,and25cmfor the clay soil. Table2 shows the temperatures of air, snow and soil onthe measurementdays in each soiltype.The air temperatures in winter were between -1 and +2°C in the three soiltypes,and those in the summerbetween 17 and 19°C. Thesummer air temperatures were slightly higher than theaver- age maximum air temperature of the warmest month (July, ^15.8°C)as is normal for these soil types.The winter soiltemperatures at2-cm depth were between 0 and I°C in each soil type,and the summer soil temperatures between 18 and 19°C. Thesnow temperaturesbelow 10-cmdepth werealmost O°C in all soiltypefields(Table 2).

Concentrations ofCO, in the snowpack(av- erage of fivemeasurementpoints) increased with snow depth and its gradients differed from one soil toanother (Fig. 3). The concentration de- creased linearly between thesnow surface and

15-cmor20-cm depth. The gradientwas small- estin the sand soil,but largest in thepeat soil; it was intermediate in the clay soil.

The winter fluxes of C0₂from the snowpack (average of five measurement points) differed depending on the soiltypes,ranging from 18to 27 mgCO, nr²h

1

for the CC method and from 10^to 27 mgCO,

m 2 h 1

for the DM method^(Ta- ble3).Both methods showedasimilar differen- tial pattern between soiltypes. The snow-cov- ered fields in thepeat and sand soils produced the largest and smallestamountsofC0₂^,respec- tively. The clay soil had intermediate winter flux- es. The variance in the winter fluxes within the same soil types wasrelatively larger in the DM

than in the CC method ^{(Table 3).} The results obtained with the CC method indicated that the average summerC0₂evolutionwas 14-24 times higher than the winter evolution.

Discussion

Carbon dioxide in soils derives primarily from root respiration and microbial oxidation ofor- ganic matter(Witkamp and Frank 1969,Wildung etal. 1975, Kowalenko etal. 1978, Heinemeyer

etal. 1989, Koizumietal. 1993, Nakadai etal.

1996).Therefore, low temperatures during the snow-covered period retard C02 production in soils. Based _{on some} empirical _or theoretical studies (Steudleretal. 1989, Bouwman 1990), most C0₂budgets have been calculated by as- suming that C0₂ exchange stops when the soil is covered withsnow orsoil temperatures drop toaroundO°C.The present study,however, sug- gests that the assumption for the winter CO, budget is incorrect. In sub-alpine and cool-tem-

perateregions, soilsat5-to 10-or20-cm depth, orthe active layer of soil respiration ^(Crill 1991), never freeze during the winter because of the thermal insulation effect of thesnow cover^(Som- merfeld etal. 1993).This allows soil microor- ganisms toproduce enoughCO,^toform gradi- entswithin the snowpack (Fig. 3).It issuggest- ed that each soiltype serves as a source ofCO, in winter.

Table 3 also indicates that theCO, evolution rates in winterwere higher in thepeat than in

the sand or clay soil fields. The C0₂evolution rates in thepeatsoil_wereabout twice_ashigh_as those in the sand soil. These results suggestthat the higher availability of organic matter in the peat soil results in higher rates of C0₂ produc- tion in that soil(cf.Fig. 2).

Using the DM method it wasestimated that the snow-covered fields in southernFinland can evolve 10-27 mg ofCO, per

m 2 ^per

^{hour on}

some winter days (Table 3).The winter fluxes show somewhat lower values than those calcu- lated by Sommerfeldetal. (1993) from theCO, profiles in the snowpacks of sub-alpine mead- ow,southeasternWyoming. Nakane (1978) esti- mated C0₂evolutionrates from Japanese cool- temperate beech/fir forest soils beneath the snowpack tobe between 50 and 70 mg per m

2

per hour. Mariko etal. (1994) also demonstrat- ed that the C0₂evolutionrates from snow-cov- ered soils in four Japanese cool-temperate de- ciduous broadleaved and evergreen needle for- estsranged from 20to 75 mg per

m 2 ^per

^hour.

Here, directmeasurement with the CC method also demonstrated remarkable fluxes of 18-27 mg per

m 2 ^per

^{hour(Table 3),} indicating the

validity of the results obtained with indirect measurements or estimation. Thus the winter fluxes represent an importantpartof the annual

Table3.Averageand standard deviation of C0₂evolution rate(mgC0₂

m 2

^h')fromsnowsurface (winter) and soil surface (summer)inthree soil types. The fluxwas meas- ureddirectly usingthe closed chamber method (CC meth- od) and calculatedusingPick’s law from C0₂profilesover

a0-20cmlayerwithin the snowpack (DM method).

Sand Clay

Peat Winter

CE* (CC method) 27.4+16.9 18.3±10.7 21.0±13.2 (4.2%) (7.2%) (4.6%) CE* (DM method) 26.8+21.5 10.0±6.7 16.1±11.3

(4.0%) (3.9%) (3.6%) Summer

CE* (CC method) 645.9±145.7 253.9±84.8 452.2±29.2

*:CErefers to C0₂evolution rate.

Numeralsinparenthesesindicate the percentage of CE in summer.

Average ^C0₂ flux was calculated from five measurment points.

Fig. 3.Profiles of C0₂concentrationinair andsnowpack inpeat, sand andclaysoil. Resultsareaverages of fivemeasure mentpoints.Horizontal solid lines refer tosnowsurface.

CO, budget in snowy regions (Coyne and Kel- ley 1974, Nakane 1978, Marikoetal. 1994).

Still,it hasnotbeen established whether C0₂ evolution through thesnow surface changes dur- ing the winter period. Sommerfeld etal. (1993) stated that, as soil microorganisms remain ac- tive without change during the winter because there is _nofluctuation in soiltemperature^(Solo- mon and Cerling 1987),the results obtained dur- ing limited study periods arerepresentative of the entire winter. The actual winterfluxes,how- ever, seem to change seasonally depending on snowfall. This is suggested by the findings of the previous study (Mariko^etal. 1994) showing that the C0₂ gradient within _a thick snowpack is small compared with that within a thin snow- pack, evenin areaswith almost the same vege- tation (Kelley etal. 1968, Solomon and Cerling

1987).To understand the contribution of winter to the annual carbon budget, the flux mustbe determined throughout anentire winter period.

Seasonal comparison of C0₂evolution from each soiltype can be made using data obtained with the CC method in the subarctic climate range. On the basis of thesedata, winter C0₂ evolution in subarctic climate ranges is estimat- ed^tobe between4.2% and 7.2% of evolution in summer,when it is almost totally duetosoilres- piration. These percentages represent the ex- treme minimum fluxes on winter days in rela- tiontothe maximum fluxeson summerdays with the highest annual temperature. Therefore, the winter/summer ratio of the C0₂evolution rate

calculated from seasonally-integrated fluxes is estimatedtoexceed4%.

The snowpack influences winterCO,evolu- tion in both positive and negative ^{ways. First,} the snow insulates the soil thermally, allowing CO,^{production to}continue throughout the win-

ter attemperatures above freezing. Second,the snow supplies water that participates in weath-

ering reactions in thesoil, allowing CO,produc- tion to increase (Solomon and Cerling ^1987).

Third,thesnow causes theCO,concentration in snow and soil torise by partially capping the snow-soil column and by lowering the soil dif- fusion coefficient asthe soil poresarefilled with water from the melting snowpack. Moreover, there is the interesting suggestion of Sommer- feldetal. (1993),who pointed outthat thesnow feeds fungi and bacteria, making them able to respire there.

Directmeasurement (CC method) showed the sametrend in the difference in winter fluxes be- tween sites as the indirectmeasurement (DM method). The CC method tended_toshow larger absolute average fluxes than the DM method even when themeasurements were made atthe samepoints and _on the same day. The variance in the fluxes within each soiltype tended^to be slightly larger with the ^DM than with the CC method. These differences may be due to the qualitative difference in theirmeasurementprin- ciples, which result in different kinds ofmeas- urementerrors(Roiston 1986). Ofgreatpracti- cal interest is toknow which of the methods is morereliable in situ. This will be determined in afurther study ^to be conducted by the authors.

Thepresent study does atleast suggest,howev- er,that the CC method is acceptable for the prac- ticalmeasurementof C02flux through thesnow surface.

Acknowledgements.The authors thank Matti Matilainen and ArtoTimonen,Agricultural Research Centre ofFinland, and HarriJalli,HelsinkiUniversity, for their kind assist- anceduringthe field workin Jokioinen,andDrMasayuki Yokosawa,National Institute ofAgro-Environmental ^Sci- ences, for his technical adviceonthe calculation of gas flux.

The authors also thank Hiroko Nemoto for herhelp inthis study.This researchwaspartially supported byagrant from the Science andTechnology AgencyofJapan.

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SELOSTUS

Hiilidioksidin kulku lumipeitteisessä ja paljaassa maassa

HiroshiKoizumi, MarkkuKontturi, Shigeru MarikojaTimo Mela

National InstituteofAgro-EnvironmentalSciences, Japani,Maatalouden tutkimuskeskus

jaSugadairaMontane Research Center,Japani

Hiilidioksidin vapautumista lumipeitteisestä ja pal- jaastamaasta mitattiin suorasti ja epäsuorastiSuomen maaperästä.Mittaukset tehtiin turve-, hiekka- jasa- vimaasta. Suorissa mittauksissakäytettiin suljetun kammion menetelmääja epäsuorissa Fickin diffuu- siomallia.

Kaikkien em. maalajien talvilämpötila vaihteli 2 cm:nsyvyydessä Oja +1 ^°Cvälillä. Hiilidioksidi- pitoisuus lumikerroksen sisällä oli sitäkorkeampi mitäpaksumpi lumikerros oli. Lumen hiilidioksidi- pitoisuudesta mitattu keskimääräinen hiilidioksidivir-

tausvaihteli 10-27mg C0₂nr²h'‘ diffuusiomallillaja 18-27mg C0

2m^2h‘suljetun kammion menetelmäl-

lä mitattuna.

Tämän tutkimuksenmukaan lumi onhyvä maan lämpöeriste, jaC0₂:n tuotantojatkuumaassa lämpö- tilojen pysyessä lumipeitteen allajäätymispisteen ylä- puolella. Maaperästävapautuu ilmakehään^hiilidiok- sidialumipeitteen läpi. Hiilidioksidin muodostumis- tatalvella jakesällä kuvaavan suhdeluvun arvioitiin olevan noin 4^%. Täten merkittävä osa vuotuisista hiilidioksidipäästöistä syntyy talvella.