Voi. 511996): 421-430. Carbon dioxide evolution from snow-covered agricultural ecosystems in Finland Hiroshi Koizumi Division ofPlant Ecology, National Institute ofAgro-Environmental Sciences, Tsukuba, Ibaraki, 305 Japan Markku Kontturi Agricultural Research Centre ofFinland, Institute ofCrop and Soil Science, FIN-31600 Jokioinen, Finland Shigeru Mariko Sugadaira Montane Research Center, University ofTsukuba, Sanada, Nagano, 386-22 Japan Timo Mela Agricultural Research Centre ofFinland, Institute ofCrop and Soil Science, FIN-31600 Jokioinen, Finland The release of C0 2 from the snow surface in winter and the soil surface in summer was directly or indirectly measured in three different soil types (peat, sand and clay) in agricultural ecosystems in Finland. The closed chamber (CC) method was used for the direct and Pick’s diffusion model (DM) method for the indirect measurements. The winter soil temperatures at 2-cm depth were between 0 and I°C for each soil type. The concentration of C0 2 within the snowpack increased linearly with snow depth. The average fluxes of C02 calculated from the gradients of C0 2 concentration in the snow using the DM method ranged from 10 to 27 mg C0 2 nr 2 h and with the CC method from 18 to 27 mg C0 2 m 2 h'. These results suggest that the snow insulates the soil thermally, allowing C0 2 production to continue at soil temperatures slightly above freezing in the winter. Carbon dioxide formed in the soil can move across the snowpack up to the atmosphere. The winter/summer ratio of C0 2 evolution was estimated to exceed 4%. Therefore, the snow-covered crop soil served as a source of C02 in winter, and C0 2 evolution constitutes an important part of the annual C0 2 budget in snowy regions. Key words: closed chamber method, C0 2 flux, C0 2 profile, heavy clay soil, peat soil, sand soil, snowpack, soil respiration, subarctic climate region ntroduction and the soil. Budgets assume that microorgan- isms and plant roots in snow-covered soils stop respiring and so there is no evolution of C02 from the snow surface when soil temperatures drop to around O°C (Steudler et al. 1989, Bouw- Carbon dioxide is the primary gas involved in the exchange of carbon between the atmosphere © Agricultural and Food Science in Finland Manuscript received June 1996 421 AGRICULTURAL AND FOOD SCIENCE IN FINLAND https://www.c-info.fi/en/info/?token=x4X3QP-MKCkL--gw.yATl03h3s2X2siiDkFCztw.qX1ziFPiDQ0KQPHG7MBF8COcniN4e_sqYgcGQwSLg-kGdl_JwXrmuEXMrv933MywdDB0kmO7WHnP2WSp5WFi0JgKmw512k10W5NZndV-XTui0y0DhPs4re4SIFRmN_3zx_idLoEsF-JNhGShVFmdy4lyuwEg3Q_7o7-3XIQNUzDDac6v7TgYYlyGZyS1LKpb3nKpdKvz0YSPDXoCVnd_XMizgSnf5IE0w8O6ayodWufXItmRXGbEvwL3QcXXL1Ss6wXRWrjQCCVqP-llHHbD9P6uhZYBc1J66-0aLEwb2pZT-_mKRY4OEiMEIvCVLeUc6IA Koizumi, H. et al. Carbon dioxide evolution from agricultural ecosystems man 1990). Thus the contribution of CO, from alpine or arctic regions in winter has not been considered important in calculations of global carbon balances. Some reports have, neverthe- less, shown that C0 2 concentrations are high at the base of snowpacks in arctic and temperate regions (Kelley et al. 1968, Solomon and Cer- ling 1987). In addition, there is some evidence that microorganisms in soils beneath the snow continue to respire at temperatures close to O°C (Taylor and Parkinson 1988, Sommerfeld et al. 1991). It is currently thought that the soils un- der alpine and sub-alpine snowpacks evolve CO, to the atmosphere throughout the snow-covered period (Sommerfeld et al. 1993, Mariko et al. 1994). Snow can cover between 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 some C0 2 evolution in winter, the C0 2 re- leased can strongly influence the annual global carbon budget. This flux needs to be measured urgently for a more complete understanding of the global carbon balance. However, not only C02 fluxes from the snow 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 the present study is to measure directly or indirectly 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 et al. 1995, Bekku et al. 1996) was used for the direct measurement. For methodological comparison, the C02 fluxes were measured with Pick’s diffusion model (DM), which so far has been used as 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 data at the ARCF site showed that annual precipitation averaged 581 mm, the mean annual temperature was 3.9°C and the av- erage snow depth in March was 39 cm during the period from 1961 to 1990. The monthly mean temperature in July 1992 was 16.0°C and in March 1994 -3.2°C. The warmth index at this site was 38.1 degree-months, indicating that the site is in the subarctic climate range. Three barley fields consisting of three dif- ferent soil types were investigated. Classified on the Finnish and FAO system, the soils were peat (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 field was fallowed from September to the following April. After the measurements of CO, flux in win- ter, snow pits were dug beneath five flux-meas- urement points in each soil type to assess snow depth and porosity (Sommerfeld et al. 1993). At the same time, air temperatures were measured 50 cm above the snow or at the soil surface, snow temperatures at 10-cm depth and/or soil temper- atures at 2-cm depth in winter and summer. The temperatures were measured with a digital-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 summer and from the snow surface in winter was 422 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Vol. 5 (1996): 421^130. Table 1.Cultivation methods of spring barley in three soil types in agricultural ecosystems. Soil type Growth period Variety Seeds Row space N fertilizer Fallowing (g rrr2) (cm) (g rrr 2 ) period Peat soil 15May~25Aug. Jo 1545 19.0 12.5 6.0 Sept. ~ followingApril Sand soil 15May~20Aug. J01545 19.0 12.5 9.2 Sept. - followingApril Clay soil 12May~20Aug. J01545 19.0 12.5 9.2 Sept. ~ followingApril measured on 14 July 1992 and 18 March 1994, respectively, for the peat soil, on 14 July 1992 and 18 March 1994 for the sand soil, and on 16 July 1992 and 18 March 1994 for the clay soil. The flux was measured at five points in each soil type during a diurnal period lasting from 10:00 to 14:00. The C0 2 flux was determined directly in situ with the CC method, because this method can be used at sites where electricity or special equip- ment is not available. In this method, a closed chamber is placed over the soil or snow surface and the increase in the concentration of C02 within the chamber is measured as a function of time. The CO, flux is calculated from Eq. (1). F = (V/A)(AC/At), (1) where Fis the C02 flux (mg C02 nr2 h 1), V is the volume of air within the chamber (m 3 ), A is the area of the soil or snow within the cham- ber (m 2 ), and AC/At is the rate of change in the C02 concentration in the air within the chamber (mg CO, m 3 h ')• A polyvinyl-chloride cylinder (15-cm high, 21-cm internal diameter) was placed on the soil or snow surface. About an hour later, the cylin- der was closed with a polyvinyl-chloride lid, and a rubber-capped (blood collection) needle was fitted onto an air sample port on the top of the lid. Air in the chamber was aspirated through the needle into an evacuated vial (5 ml) three times at equal intervals (2 or 5 minutes), and the C02 concentration in the vial air was measured as described in the previous paper (Mariko et al. 1994). The concentration of C0 2 was plotted against the time (Fig. 1). Fig. 1.Typical time courses of C02 concentration in a cham- ber of the CC method in (a) summer and (b) winter. Results were selected to illustrate the lowest and highest rates of C02 increase on summer and winter days for three soil types. Regression equations and correlation coefficients: (a) Peat (•); Y=52.281X+329.24, R 2=0.998 Sand (■); Y=21.910X+325.34, R 2=0.997 Clay (A); Y=33.472X+341.39, R 2=0.999 (b) Peat (•); Y=1.6892X+340.46. R-0.999 Sand (■); Y=1.2912X+336.20, R 2=0.996 Clay (A); Y=1.3334X+333.96, R 2=0.997 423 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Koizumi, H. et al. Carbon dioxide evolutionfrom agricultural ecosystems Profile of C0 2 concentration in snowpack The concentration gradient of C02 in the snow- pack and atmosphere was determined on the same day as the 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 sampling ports were in- stalled at different snow depths. Snow air was drawn through the needle with an evacuated sam- pling vial (5 ml). Some air (about 2 ml) in the gas collector was previously removed with a gas- tight syringe connected to the needle at the end of the gas collector. The air samples were taken twice at two lateral locations at the same snow depth to obtain a more representative concen- tration at a particular depth. Calculation of C0 2 evolution from snow surface using Fick’s DM The evolutionof C02 from the snowpack in win- ter was calculated assuming simple diffusion, average C02 gradients, average snow depth and average porosities (Sommerfeld et al. 1993, Mariko et al. 1994). The diffusion of gases in the snowpack can be described by Fick’s first law, which states that the steady state transport of gas by diffusion through a unit area is pro- portional to the concentration gradient measured along the line normal to the area: F = -aDp (dC/dz), (2) where Fis the C02 flux (mg C02 nr2 h '), a is a constant for unit conversion (36 as a value), Dp is the diffusion coefficient (cm 2 s ') and dC/ dz is the vertical concentration gradient of C02 (mg C02 m 3 cm 1). The term dC/dz can be de- termined directly by drawing a straight line through the average concentrations from the snow surface to 20-cm depth (C0 2 profile) and by calculating the slope. The diffusioncoefficient Dp is influenced by the atmospheric pressure and temperature in 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 (cm 2 s '), 0 is the porosity of the snow measured (cm 3 cm 3 ), and Ais the empirical tor- tuosity factor (cm cm ') (van Bavel 1951). D 0 was corrected for the temperature ( T, °K) and the at- 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 the meteoro- logical data of the Meteorological Observatory of the Agricultural Research Centre ofFinland. Dg(S) is the diffusion coefficient at the normal state (the empirical value 0.135 cm 2 s' 1 for C02 ) and an exponent n is 1.71 for C02 (Osozawa 1987). The tortuosity factor for snow was not measured here. However, the constant 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 were almost constant over the snow depths meas- ured, and similar to those (c. 0.58) of Sommer- feld et al. (1993) and Solomon and Cerling (1987). The average porosity of snow was be- tween 0.53 and 0.59 at all measurement points. Measurement of C0 2 concentration of air stored in the sampling vial The concentration of C02 in the air stored in the sampling vial was determined with an infrared gas analyser, IRGA (Model ZRC, Fuji Elec. To- kyo, Japan). A 1- or 2-ml aliquot of air in the vial was removed with a gas-tight syringe (2-ml volume). During the withdrawal, the air removed was replaced with distilled water, adjusted at pH4.O with phosphoric acid (H,PO 4 ). The with- drawn air was injected into a gas line purged with 424 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Vol. 5 (1996): 421^30. pure N 2 gas at a flow rate 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 steep peak, like a spike on a pen recorder (Servocorder 5R6312, Graphtec, Tokyo, Japan). The spike heights (X) were used for quantitative analyses ofCO, concentration. A standard curve was pre- liminarily prepared by injecting the same vol- ume of C02 gas (1 or 2 ml) in various known concentrations (T), which gave a linearresponse curve to the spike heights (T=ll.OBX + 20.15, r=0.99, pcO.001). Carbon and nitrogen contents in soil After cultivation of the fields, soil samples from each field were collected at five flux-measure- ment points from seven 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 a 0.2-mm mesh sieve, care being taken to sieve the entire sample. This portion of the sample was analysed for total carbon and nitro- gen using an 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 nitrogen contents in three soil types. The soil carbon and nitrogen contents were 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 depth were approxi- mately 20% for peat, 1.7% for sand and 3.5% for clay soil, and those of nitrogen were approx- imately 1.3% for peat, 0.13% for sand and 0.25% for clay soil. Storage ofcarbon within the upper 60-cm layers of soil amounted to 54.4 kgC m 2 in the peat, 16.9 kgC m 2 in the clay and 8.4 kgC nr2 in the sandy soil. Fig 2. Vertical distribution of soil carbon and nitrogen contents (%, dry weight basis) in peat, sand and clay soil fields, •, carbon content; O, nitrogen content 425 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Koizumi, H. et al. Carbon dioxide evolutionfrom agricultural ecosystems Table 2. Temperature (°C) of air, snow and soil on the measurement days in three 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 average snow depth on the measurement days was approximately 20 cm for the peat, 15cm for the sand, and 25 cm for the clay soil. Table 2 shows the temperatures of air, snow and soil on the measurement days in each soil type. The air temperatures in winter were between -1 and +2°C in the three soil types, and those in the summer between 17 and 19°C. The summer air temperatures were slightly higher than the aver- age maximum air temperature of the warmest month (July, 15.8°C) as is normal for these soil types. The winter soil temperatures at 2-cm depth were between 0 and I°C in each soil type, and the summer soil temperatures between 18 and 19°C. The snow temperatures below 10-cmdepth were almost O°C in all soil type fields (Table 2). Concentrations of CO, in the snowpack (av- erage of five measurement points) increased with snow depth and its gradients differed from one soil to another (Fig. 3). The concentration de- creased linearly between the snow surface and 15-cm or 20-cm depth. The gradient was small- est in the sand soil, but largest in the peat soil; it was intermediate in the clay soil. The winter fluxes of C02 from the snowpack (average of five measurement points) differed depending on the soil types, ranging from 18 to 27 mg CO, nr2 h 1 for the CC method and from 10 to 27 mg CO, m 2 h 1 for the DM method (Ta- ble 3). Both methods showed a similar differen- tial pattern between soil types. The snow-cov- ered fields in the peat and sand soils produced the largest and smallest amounts ofC0 2 , respec- tively. The clay soil had intermediate winter flux- es. The variance in the winter fluxes within the same soil types was relatively larger in the DM than in the CC method (Table 3). The results obtained with the CC method indicated that the average summer C02 evolution was 14-24 times higher than the winter evolution. Discussion Carbon dioxide in soils derives primarily from root respiration and microbial oxidation of or- ganic matter (Witkamp and Frank 1969,Wildung et al. 1975, Kowalenko et al. 1978, Heinemeyer et al. 1989, Koizumi et al. 1993, Nakadai et al. 1996). Therefore, low temperatures during the snow-covered period retard C0 2 production in soils. Based on some empirical or theoretical studies (Steudler et al. 1989, Bouwman 1990), most C02 budgets have been calculated by as- suming that C02 exchange stops when the soil is covered with snow or soil temperatures drop to around O°C. The present study, however, sug- gests that the assumption for the winter CO, budget is incorrect. In sub-alpine and cool-tem- perate regions, soils at 5- to 10- or 20-cm depth, or the active layer of soil respiration (Crill 1991), never freeze during the winter because of the thermal insulation effect of the snow cover (Som- merfeld et al. 1993). This allows soil microor- ganisms to produce enough CO, to form gradi- ents within the snowpack (Fig. 3). It is suggest- ed that each soil type serves as a source of CO, in winter. Table 3 also indicates that the CO, evolution rates in winter were higher in the peat than in 426 AGRICULTURAL AND FOOD SCIENCE IN FINLAND the sand or clay soil fields. The C02 evolution rates in the peat soil were about twice as high as those in the sand soil. These results suggest that the higher availability of organic matter in the peat soil results in higher rates of C0 2 produc- tion in that soil (cf. Fig. 2). Using the DM method it was estimated that the snow-covered fields in southernFinland can evolve 10-27 mg of CO, per m 2 per hour on some winter days (Table 3). The winter fluxes show somewhat lower values than those calcu- lated by Sommerfeld et al. (1993) from the CO, profiles in the snowpacks of sub-alpine mead- ow, southeasternWyoming. Nakane (1978) esti- mated C02 evolution rates from Japanese cool- temperate beech/fir forest soils beneath the snowpack to be between 50 and 70 mg per m 2 per hour. Mariko et al. (1994) also demonstrat- ed that the C02 evolution rates from snow-cov- ered soils in four Japanese cool-temperate de- ciduous broadleaved and evergreen needle for- ests ranged from 20 to 75 mg per m 2 per hour. Here, direct measurement 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 important part of the annual Table 3. Average and standard deviation of C02 evolution rate (mgC0 2 m 2 h') from snow surface (winter) and soil surface (summer) in three soil types. The flux was meas- ured directly using the closed chamber method (CC meth- od) and calculated using Pick’s law from C02 profiles over a 0-20 cm layer within the snowpack (DM method). Sand ClayPeat 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 *: CE refers to C0 2 evolution rate. Numerals in parentheses indicate the percentage ofCE in summer. Average C02 flux was calculated from five measurment points. Fig. 3. Profiles ofC02 concentration in air and snowpack in peat, sand and clay soil. Results are averages of five measure ment points. Horizontal solid lines refer to snow surface. 427 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Koizumi, H. et al. Carbon dioxide evolutionfrom agricultural ecosystems CO, budget in snowy regions (Coyne and Kel- ley 1974, Nakane 1978, Mariko et al. 1994). Still, it has not been established whether C02 evolution through the snow surface changes dur- ing the winter period. Sommerfeld et al. (1993) stated that, as soil microorganisms remain ac- tive without change during the winter because there is no fluctuation in soil temperature (Solo- mon and Cerling 1987), the results obtained dur- ing limited study periods are representative of the entire winter.The actual winter fluxes, how- ever, seem to change seasonally depending on snowfall. This is suggested by the findings of the previous study (Mariko et al. 1994) showing that the C02 gradient within a thick snowpack is small compared with that within a thin snow- pack, even in areas with almost the same vege- tation (Kelley et al. 1968, Solomon and Cerling 1987). To understand the contribution of winter to the annual carbon budget, the flux must be determined throughout an entire winter period. Seasonal comparison of C0 2 evolution from each soil type can be made using data obtained with the CC method in the subarctic climate range. On the basis of these data, winter C02 evolution in subarctic climate ranges is estimat- ed to be between 4.2% and 7.2% of evolution in summer, when it is almost totally due to soil res- piration. These percentages represent the ex- treme minimum fluxes on winter days in rela- tion to the maximum fluxes on summer days with the highest annual temperature. Therefore, the winter/summer ratio of the C02 evolution rate calculated from seasonally-integrated fluxes is estimated to exceed 4%. The snowpack influences winter CO, evolu- tion in both positive and negative ways. First, the snow insulates the soil thermally, allowing CO, production to continue throughout the win- ter at temperatures above freezing. Second, the snow supplies water that participates in weath- ering reactions in the soil, allowing CO, produc- tion to increase (Solomon and Cerling 1987). Third, the snow causes the CO, concentration in snow and soil to rise by partially capping the snow-soil column and by lowering the soil dif- fusion coefficient as the soil pores are filled with water from the melting snowpack. Moreover, there is the interesting suggestion of Sommer- feld et al. (1993), who pointed out that the snow feeds fungi and bacteria, making them able to respire there. Direct measurement (CC method) showed the same trend in the difference in winter fluxes be- tween sites as the indirect measurement (DM method). The CC method tended to show larger absolute average fluxes than the DM method even when the measurements were made at the same points and on the same day. The variance in the fluxes within each soil type tended to be slightly larger with the DM than with the CC method. These differences may be due to the qualitative difference in their measurement prin- ciples, which result in different kinds of meas- urement errors (Roiston 1986). Of great practi- cal interest is to know which of the methods is more reliable in situ. This will be determined in a further study to be conducted by the authors. The present study does at least suggest, howev- er, that the CC method is acceptable for the prac- tical measurement of C02 flux through the snow surface. Acknowledgements. The authors thank MattiMatilainen and Arto Timonen, Agricultural Research Centre of Finland, and Harri Jalli, Helsinki University, for their kind assist- ance during the field work in Jokioinen, and Dr Masayuki Yokosawa, National Institute of Agro-Environmental Sci- ences, forhis technical advice on the calculation of gas flux. The authors also thank Hiroko Nemoto for her help in this study. This research was partially supported by a grant from the Science and Technology Agency of Japan. 428 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Vol. 5 (1996): 421^130. References Barry, R.G. 1992. Climate-ice interactions. In: Encyclo- pedia of Earth System Science Vol. 1. Academic Press, Sandiego. p. 517-524. Bekku, Y., Koizumi, H., Nakadai, T. & Iwaki, H. 1995. Measurement of soil respiration using closed chamber method: An IRGA technique. Ecological Research 10: 369-373. - , Koizumi, H., Oikawa.T. & Iwaki, H. 1996. Examina- tion of four methods for measuring soil respiration. 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Suorissa mittauksissa käytettiin suljetun kammion menetelmää ja epäsuorissa Fickin diffuu- siomallia. Kaikkien em. maalajien talvilämpötila vaihteli 2 cm:n syvyydessä Oja +1 °C välillä. Hiilidioksidi- pitoisuus lumikerroksen sisällä oli sitä korkeampi mitä paksumpi lumikerros oli. Lumen hiilidioksidi- pitoisuudesta mitattu keskimääräinen hiilidioksidivir- taus vaihteli 10-27mg C0 2 nr 2h'‘ diffuusiomallilla ja 18-27 mg C0 2m 2 h ‘ suljetun kammion menetelmäl- lä mitattuna. Tämän tutkimuksen mukaan lumi on hyvä maan lämpöeriste, ja C02:n tuotanto jatkuu maassa lämpö- tilojen pysyessä lumipeitteen alla jäätymispisteen ylä- puolella. Maaperästä vapautuu ilmakehään hiilidiok- sidia lumipeitteen läpi. Hiilidioksidin muodostumis- ta talvella ja kesällä kuvaavan suhdeluvun arvioitiin olevan noin 4 %. Täten merkittävä osa vuotuisista hiilidioksidipäästöistä syntyy talvella. 430 AGRICULTURAL AND FOOD SCIENCE IN FINLAND