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Seasonality of vertically partitioned soil CO2 production in temperate and tropical forest HASHIMOTO, SHOJI

Soil Resources Laboratory, Department of Forest Site Environment, Forestry & Forest Products Research Institute, 1 Matsunosato, Tsukuba, Ibaraki, 305-8687, Japan

shojih@ffpri.affrc.go.jp

TANAKA, NOBUAKI

KUME, TOMONORI

YOSHIFUJI, NATSUKO

HOTTA, NORIFUMI

TANAKA, KATSUNORI

SUZUKI, MASAKAZU

This is the peer reviewed version of the following article: [Hashimoto, S., N. Tanaka,

T. Kume, N. Yoshifuji, N. Hotta, K. Tanaka and M. Suzuki (2007) Seasonality of

vertically partitioned soil CO2 production in temperate and tropical forest. Journal

of Forest Research, 12: 209-221. doi:10.1007/s10310-007-0009-9].

The final publication is available

at Springer via [ http://dx.doi.org/10.1007/s10310-007-0009-9 ].

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The seasonality of soil CO2 production at a number of depths was investigated in a temperate forest in Japan and in a tropical montane forest in Thailand. The CO2 production rates were evaluated by examining differences in the estimated soil CO2 flux at adjacent depths. The temperate forest had clear seasonality of temperature and only slight seasonality of rainfall, while the tropical montane forest showed clear seasonality of rainfall and only slight seasonality of temperature. In the temperate forest, the pattern of seasonal variation in soil respiration was similar at all depths, except the deepest (0.65 m-), and respiration was greater in summer and less in winter. The contribution of the shallowest depth (around 0.1 m) was more than 50 % of total soil surface CO2 flux whole year round, and the annual mean contribution was about 75 %. CO2 production mostly appeared to increase with temperature at shallower depths. In contrast, in the tropical forest, the seasonality of soil CO2 production appeared to differ with depth. The CO2 production rate at the shallowest depth was high during the rainy season and low during the dry season. The seasonality of soil CO2 production at greater depths (0.4 and 0.5 m-) showed the opposite seasonality to that at the shallower depth (around 0.1 m). As a result, the contribution from the shallow depth was greatest in the tropical forest during the rainy season (more than 90 %), while it decreased during the dry season (about 50 %).CO2 production appeared to be controlled by soil water at all depths, and the different ranges of water saturation seemed to cause the difference in seasonality at each depth. Our results suggest the importance of considering the vertical distribution of soil processes, particularly in areas where soil water is a dominant controller of soil respiration.

Key words: soil respiration; soil carbon; soil CO2; soil organic matter; belowground processes

Introduction Carbon stock in soil is estimated to be more than 1500 PgC (petagrams of carbon). The carbon pool size is significantly greater than that in living plants (500 PgC) or in the atmosphere (730 PgC in 1980s)(IPCC 2001). Decomposition of SOM (soil organic matter), usually referred as “heterotrophic respiration", release 55 PgC/year of CO2 into the atmosphere. This amount corresponds with several tens of times of annual net CO2 uptake by land (0.2±0.7 PgC/year and 1.4±0.7 PgC/year for 1980’s and 1990’s, respectively) (IPCC 2001). Soil temperature and soil water contents are dominant controlling factors of decomposition rate of SOM. Climate-change induced-perturbation in those environmental factors may cause great fluctuation in net CO2 uptake by terrestrial vegetation and consequently reflect in growth rate of atmospheric CO2. Hence we consider that investigating the spatio-temporal variation in CO2 originated from decomposition of SOM and its relationship to the controlling factors is one of the key issues for understanding the feedback mechanism of climate change and global carbon cycle. Soil respiration, i.e., the emission of CO2 produced by decomposition of soil organic matter and root respiration, is one of the most important mechanisms by which soil organic carbon moves from the soil to the atmosphere. Global warming induced by greenhouse gases has become a serious concern, and many researchers have investigated soil respiration. Global warming may increase soil respiration, while drought may decrease soil respiration. These processes may affect the atmospheric CO2 concentration. However, most studies have considered only soil surface efflux and soil temperature and water content at one depth; few have studied the vertical distribution of soil respiration. However, the source of soil respiration varies with depth, as do the environments that stimulate it. Therefore, it is important to evaluate soil respiration at several depths and investigate the response of deep-soil respiration to different environmental conditions. The

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impact of global warming may differ with depth, and even opposite effects might occur. Studying the vertical processes involving soil CO2, based on sampling at different depths, has the advantage of controlling environmental conditions and allowing accurate measurements under different circumstances. Ino and Monsi (1969) collected soil samples from various depths at seven sites and measured soil respiration after incubation. Hashimoto and Suzuki (2002) investigated the vertical profile of soil CO2 concentration and described the vertical distribution of soil CO2 production rates and gas diffusivity. Some studies have highlighted the importance of processes related to vertical distribution CO2 production rates and gas diffusivity on soil CO2 efflux through field observations. de Jong and Schappert (1972) measured soil respiration at several depths by measuring soil CO2 concentrations and gas diffusion coefficients. Campbell and Frascarelli (1981) measured the soil CO2 production rates at several depths by absorbing CO2 at each depth directly, using an absorbent solution. Hendry et al. (1999) simulated soil CO2 concentration and soil surface CO2 flux and quantified the CO2 production rate at each depth using parameterization and sensitivity analysis. Osozawa and Hasegawa (1995) investigated daily and seasonal changes in the CO2 flux in an Andisol. The method used was similar to that presented by de Jong and Schappert (1972). Some studies investigated relative contribution from different depth layer to total CO2 production in soil interior. Davidson and Trumbore (1995) measured the production of CO2 in deep soils of the eastern Amazon, and found that about 70-80 % of CO2 production in forest and pasture in the Amazon basin occur within the top 1 m of soil. Gaudinski et al. (2000) calculated the CO2 flux at each depth using Fick's first law and a radiocarbon inventory. They found that 63 % of soil respiration took place in the top 15 cm of the soil in a temperate forest. Davidson et al. (2006) reported that a conservative estimate of the O horizon's contribution was 40-48 % of the total annual soil CO2 efflux in a mixed hardwood stand of Massachusetts. Relationships between deep soil CO2 production and temperature and soil water were also reported. Hirsch et al. (2002) investigated deep soil respiration in a boreal forest and found that soil respiration varied linearly with temperature at a depth of 50 cm. Risk et al. (2002) showed that subsurface CO2 production was controlled by temperature across four study sites of contrasting vegetation cover and land use. Recently, Fierer et al. (2005) revealed that the subsurface (soil below 40 cm in depth) contribution to whole CO2 production was relatively less during the wet season at a California grassland site. Difference in a residence time of soil organic carbon between tropical and temperate forest was also reported; Trumbore (1993) compared the carbon dynamics in tropical and temperate soils using radiocarbon measurements and showed that the majority of the organic carbon in the upper 22 cm of tropical soil has a residence time of 10 years or less; conversely, residence time of soil organic carbon in temperate forest was found to vary significantly with range from 10 to 1000 years. Therefore, the characteristics of deep-soil carbon differs in different ecosystems, which must be considered when assessing the response of soil carbon to climate change. Soil respiration responds to temperature and soil water; however, the controlling factors differ from site to site depending on the climate. In this study, we investigated two watersheds with contrasting climates: the Fukuroyamasawa watershed is in a temperate coniferous forest in Japan, and the Kog-Ma watershed is a tropical montane forest in Thailand. The former experiences clear seasonality

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of temperature, but little seasonality of rainfall. In contrast, the latter shows little seasonality of temperature, but clear seasonality of rainfall. Our objectives were (1) to evaluate the vertical profile of CO2 production rates for different field conditions, (2) to examine sources and seasonality, and (3) to quantify the relationships between soil CO2 production rate, soil temperature, and water content. We calculated the soil gas diffusivity, evaluated the soil CO2 flux at each depth with Fick's first law, and obtained the CO2 production rate at each depth.

Methods

Sites This study compared two different types of forest: the Fukuroyamasawa watershed, a temperate coniferous forest in Japan, and the Kog-Ma watershed, a tropical montane forest in Thailand. The forest in the Kog-Ma watershed has been classified as a montane forest or a monsoon forest and has also been referred to as a hill evergreen forest (Chunkao et al. 1981; Bullock et al. 1995; Torreta and Takeda 1999; Tanaka et al. 2003). Details of the Fukuroyamasawa watershed are provided by Hashimoto and Suzuki (2004), and the Kog-Ma Watershed was described by Hashimoto et al. (2004) and Tanaka et al. (2003). The Fukuroyamasawa watershed is located in the University Forest of the University of Tokyo, Chiba Prefecture, Japan (35 ∘ 12'N, 140 ∘ 06'E, ca. 180 m alt.). Figure 1 shows the seasonality of air temperature and rainfall in the Fukuroyamasawa watershed. Air temperature shows clear seasonality, and the site averages over 100 mm of rainfall per month almost year round, except in August, December, and February. Even in those three months, the amount of rainfall exceeds 70 mm. Mean annual precipitation is about 2000 mm, and mean annual air temperature is about 14 °C. The forest is artificial forest. The trees were planted in 1929. This stand has been managed for wood production. Dominant species is Cryptomeria japonica, Chamaecyparis obtusa. The thickness of A layer was about 0.1 m and that of B layer was about 0.9 m (Kumagai et al. 1997). The soil type is Brown forest or Inceptisols (USDA Soil Taxonomy). The Kog-Ma watershed is located in the forest of Kasetsart University, near Chiang-Mai, northern Thailand (18 °48'N, 98 °54'E, ca. 1300 m alt.). The seasonality of air temperature and rainfall in this watershed is shown in Figure 1. In contrast to the Fukuroyamasawa watershed, the Kog-Ma watershed experiences little seasonality of temperature, but clear seasonality of rainfall, with a 6-month dry season (November-April) followed by a 6-month rainy season (May-October). Mean annual precipitation is about 2000 mm, and mean annual air temperature is about 20 °C. Dominant species is Castanopsis, Lithocarpus, and Quercus spp.. Monthly litterfall was highest in February and smallest in August (Boonyawat and Ngampongsai 1974). Udomchock et al (1983) reported soil properties. The thickness of surface litter was reported to be 0.05-0.08 m, and the thicknesses of A, B1 and B2 layer were 0.24, 0.25 and 0.46 m, respectively. The texture of A, B1, and B2 layer were sandy clay loam, clay loam and clay loam, respectively. The soil type is Reddish brown lateritic or Ustults (USDA Soil Taxonomy).

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Field measurements

Figure 1 We used similar methods to measure the soil respiration, soil CO2 concentration, and environmental variables at both sites. The observation of soil CO2 started in 1997 in Fukuroyamasawa watershed, and in 1998 in Kog-Ma watershed. Measurements of soil respiration and soil CO2 concentration in the Kog-Ma watershed were described by Hashimoto et al. (2004).

Soil respiration The closed-chamber method was used in both watersheds. When measuring soil respiration, we pressed the chamber firmly onto the soil surface, but avoided disturbing the soil surface structure; the edge of the chamber was not pushed through the soil surface (Hanson et al. 1993; Striegl and Wickland 1998). We monitored the CO2 concentration in the chamber with IRGA (Model LI6252, 6262, LI-COR, USA) and calculated soil respiration. The chamber volume of each site was 0.012 (0.37 m diameter and 0.15 height) and 0.00155 (0.135 m diameter and 0.105 height) m3, respectively. The flow rate at each site was 0.017 (or 0.02) and 0.17 l/sec, respectively; the log interval of concentration was 30 and 10 sec, respectively. Soil respiration was measured once a week at Fukuroyamasawa, and at 3-month intervals at Kog-Ma. Three fixed plots were set in both sites. We used the soil respiration data of CO2 gas measurement plot.

Soil CO2 gas concentration Sampling tubes were installed at depths of 0.1, 0.2, 0.3, 0.5, and 0.8 m at Fukuroyamasawa, and at depths of 0.1, 0.2, 0.4, and 0.6 m at Kog-Ma. Gas sampling tubes, which consisted of an 18-mm outer diameter PVC pipe with a sampling tube (ca. 1-mm inner diameter, 2-mm outer diameter) inside, were installed in auger holes that were dug to each depth. The PVC pipe provided physical support for the sampling tube, and the lower end of the PVC tube was open to the soil. About 3 cm above the bottom end, the inside of the PVC pipe was blocked with silicon bond to create an air space and to prevent the leakage of soil air near the lower end of the gas-sampling tube. The space was stuffed with polyester filter fiber to prevent soil from occupying the space. The upper end of the gas-sampling tube was capped with a rubber stopper, except during measurements, to prevent the leakage of soil air. At both sites, soil CO2 concentration was measured using gas-detector tubes (2LL, 2L and 2H, Gastec, Japan) with a gas-sampling pump (GV-100S, Gastec, Japan). The glass gas-detector tubes were 14 cm long and 0.5 cm in diameter and contained a gas-detecting agent. Several types of gas-detector tube are available depending on the range. The ranges of 2LL, 2L and 2H are 300-5000 ppm, 0.25-3 % and 1-10 %, respectively, and the detecting limits are 30 ppm, 0.025 %, 0.1 %, respectively. The coefficient of variation is 5-10 % (Gastec, Japan). During measurements, we broke off both tips of a detector tube, joined the tube between the gas-sampling tube and the hand pump, and withdrew an air volume of 100 ml through the detector tube. The CO2 concentration in the air was indicated after several minutes by the length of the zone of color-change in the gas detector tubes. This method is widely used in Japan (Hamada and Tanaka 2001), and is of lower precision than other gas detector methods (e.g., gas chromatography or

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IRGA methods); however, it is simple and of sufficient precision for evaluating the soil CO2 concentration (Hamada et al. 1996). The soil CO2 gas concentration was measured once a week at both sites. Soil CO2 gas concentration was measured at three plots at Fukuroyamasawa (one on ridge, two near valley), at one plot at Kog-Ma. Data used at Fukuroyamasawa were of the ridge plot.

Soil water In the Fukuroyamasawa watershed, tensiometers (DIK-3000-1, Daiki Rika Kogyou Co., Ltd., Japan) were installed at 0.2 and 0.5 m, and soil matric potentials were measured once a week. Soil matric potentials were converted into soil water content using previously determined relationships between the soil matric potential and water content (Kumagai et al. 1997). The total porosity or saturated water content previously reported (Kumagai et al. 1997) was used to calculate the gas diffusion coefficient (0.66 and 0.62 at depths of 0.25 and 0.5 m, respectively). Time domain reflectometers (TDR, CS615 Campbell Scientific) and tensiometers (DIK-3000-1, Daiki Rika Kogyou Co., Ltd., Japan) were installed at depths of 0.1, 0.2, 0.3, 0.4, and 0.5 m in the Kog-Ma watershed to measure the soil water content. The saturated soil water content in the Kog-Ma watershed was determined by analyzing the relationship between the soil water content and soil matric potential at each depth; the value of the soil water content at 0 cm H2O tension was used (0.53, 0.48, 0.45, 0.45, and 0.43 at depths of 0.1, 0.2, 0.3, 0.4, and 0.5 m, respectively).

Soil temperature At Fukuroyamasawa, the temperature of shallow soil (0.1 and 0.3 m) was measured using a thermo recorder (TR71S, T & D corp.) at 15-minute intervals; deep soil temperature (0.5 and 1.0 m) was measured manually at weekly intervals using an armored thermometer (Type S251, Yoshino). The thermometers were suspended in the well from a cap over the top of the access hole at each measuring depth using a chain. When measuring soil temperature, we opened the cap, quickly pulled up the chain, and took the reading. Details are described by Hashimoto and Suzuki (2004). At Kog-Ma, the soil temperature (0.02, 0.1, and 0.3 m) was measured using a thermistor (CT-UU-A10-2A, Grant Ltd.) with a logger (SQ1259, Grant Ltd.) at 15-minute intervals.

Theory for calculating CO2 flux and CO2 production at each depth

CO2 flux and CO2 production in soil We calculated the CO2 flux in the soil and CO2 production rate at each depth using a method similar to that presented by Hirsch et al. (2002) and Davidson et al. (2006). The soil CO2 flux (f) can be easily derived using Fick's first law:

f=-Ds dC/dz

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where Ds is the CO2 gas diffusion coefficient in the soil, C is the soil CO2 concentration, and z is the distance. dC/dz is the gradient of soil CO2 concentration. The difference form of (1) is as follows.

f=-Ds ∆C/∆ z

In this study, the CO2 flux at each depth was calculated using this equation. The values of Ds at each depth were calculated using the relationship between the soil water content and Ds as shown below. The gradient of the CO2 concentration, ∆C/∆ z, was obtained from observed data. Assuming that the CO2 concentration profiles had stabilized at close to a steady state, the gradient of flux at each depth indicates the CO2 production rate at that depth.

αi =fi-fi+1

Figure 2 whereαi is the CO2 production at the depth. Note that the CO2 production rate obtained in this study has the unit of flux. Figure 2 shows this calculation in diagrammatic form. The assumption of a steady state was not strictly met. Nevertheless, the method is suitable for approximating the amounts and trends of CO2 production in the soil (e.g., Goulden et al. 1998; Hirsch et al. 2002; Risk et al. 2002; Davidson et al. 2004; Fierer et al. 2005), which were our objectives. Ideally, concentrations at all depths should be used; however, calculations using the data for all depths resulted in a strongly negative and scattered CO2 production rate at some depths, which is unrealistic. We attributed this to the low precision of the CO2 concentration and gas diffusivity. For example, if a flux at a certain depth was underestimated, the CO2 production rate at the upper end of the flux was overestimated. Conversely, the CO2 production rate at the lower end of the flux was then underestimated, which probably resulted in a negative CO2 production rate at that depth. Hence, the concentrations at 0.2 m depths for the Fukuroyamasawa and Kog-Ma watersheds were not used. Figure 2 shows the partitioning of the CO2 production rate. The fluxes at 0-0.1, 0.1-0.3, 0.3-0.5, and 0.5-0.8 m were calculated for the Fukuroyamasawa watershed, and the CO2 production rate, which was defined as the difference between the fluxes, was calculated for 0.1, 0.3, 0.5, and 0.65 m for the Fukuroyamasawa watershed. The soil water of 0.2 m depth was used for 0.1-0.3 m flux, and the soil water content of 0.2 and 0.5 were used for 0.3-0.5 m flux. For the Kog-Ma watershed, the fluxes at 0-0.1, 0.1-0.4, and 0.4-0.6 m depths were calculated, and the CO2 production rates were calculated for depths of 0.1, 0.4, and 0.5 m. The geometric average of soil gas diffusion coefficient of 0.1, 0.2, 0.3 and 0.4 m depth was used for calculating a flux of 0.1-0.4 m depth, and the gas diffusion coefficient calculated with soil water content of 0.5 m depth was used for flux of 0.4-0.6 m depth. Note that the flux at 0-0.1 m was assumed to equal the soil surface CO2 flux, and was modeled as shown below (see soil CO2 efflux). Moreover, note that the CO2 production at the deepest depths was defined by the flux at the bottom. We referred to the CO2 production at each depth (or the difference between the fluxes) as α0.1 m, α0.3 m, α0.5 m, α0.65 m- for the Fukuroyamasawa watershed, and as α0.1 m, α0.4 m, α0.5 m- for the Kog-Ma watershed. The subscripts indicate that the CO2 production was the difference between adjacent fluxes, i.e., the efflux and influx, at that depth. We referred to the CO2 production rates at the deepest depths as α0.65 m- and α0.5 m-, because these were

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defined as including the total CO2 production below that depth. The depth of the subscripts was the average of 0.5 and 0.8 m for the Fukuroyamasawa watershed, and 0.4 and 0.6 m for the Kog-Ma watershed.

Soil CO2 efflux

Figure 3 Although the soil CO2 flux at each depth was calculated using equation (2), this approach was not used for the shallowest layer because the soil water content (i.e., the gas diffusion coefficient) was not measured at this depth. It might have been possible to use the soil water content of adjacent deeper layers (e.g., calculate the gas diffusion coefficient at 0.05 m using the air-filled porosity for 0.1 m); however, because soil properties and environmental conditions generally change rapidly near the surface, we instead estimated the soil CO2 effluxes using relationships between soil water, temperature, and surface fluxes measured with the chamber method. Correlations between the soil surface CO2 flux and soil temperature and the soil water content were observed. These relationships are shown in Fig. 3. In Fukuroyamasawa, level of soil water content remained above the level that was preferable enough for microbial activity throughout the observation period. In this condition, limitation of the microbial SOM decomposition due to water insufficiency was unlikely. We conclude that soil temperature was dominant controlling factor in this site. Therefore, for this site, we estimated soil CO2 efflux from soil temperature by using an empirical relationship between soil temperature and soil CO2 efflux observed by chamber-based measurements. In Kog-Ma site, soil water content showed distinct seasonal change between dry season and wet season while soil-temperature was almost constant throughout the year. In this site, the low soil water content in dry season likely affected the microbial activities in the soil. We supposed that soil water content was dominant controlling factor for the variation in soil CO2 efflux in Kog-Ma site. Then we estimate the soil CO2 efflux from observed soil water content by using an empirical relationship between soil water content and the flux observed by chamber-based measurements.

Gas diffusion coefficient The relationship between the soil water content and the gas diffusion coefficient was used to estimate the gas diffusion coefficient. Ideally, the relationship each site should be measured at each site. An empirical model of the soil water content and gas diffusion coefficient was used to analyze the Fukuroyamasawa data. Although several models have been used to describe the relationship between the relative gas diffusion coefficient in soil and porosity (Campbell 1985; Jin and Jury 1996; Moldrup et al. 1996, 2000), the three-porosity model (TPM: Moldrup et al. 2004) is the most reliable, and was therefore used in this study. The TPM model is as follows:

Ds/D0 = Φ2(ε / Φ)X

X = log[(2ε1003 +0.04 ε 100)/ Φ 2]/log(ε 100/ Φ),

where D0 is the gas diffusion coefficient in free air, Ds/D0 is the relative gas diffusion coefficient, Φ is the total porosity (m3m-3), ε is the air-filled porosity

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(m3m-3), X is a tortuosity-connectivity parameter (X=2.4 was obtained from the relationships between volumetric soil water content and matric potential of Kumagai et al. (1997)), and ε 100 is the air-filled porosity at -100 cm H2O of matric potential. In the Kog-Ma watershed, the relationship between air-filled porosity and the relative gas diffusion coefficient was determined experimentally (Hashimoto 2004); an undisturbed large sample (0.2 m diameter, 0.4 m long) was collected at 7 m distance away from the CO2 flux measurement point. The gas diffusion coefficients at 0.1, 0.2, 0.3 m depth were determined using a method by Hashimoto and Suzuki (2002), which evaluates CO2 production rates and gas diffusivity in a soil sample by measuring CO2 fluxes from top and bottom of the sample and CO2 concentration in the soil sample. The relationship is as follows: Ds/D0=2.03ε2.78 (R2=0.99, n=3) The gas diffusion coefficient in free air is affected by temperature and pressure and can be estimated as follows (Campbell 1985):

D0=Dstand ((T+273)/273 )1.75 (1013/P)

where Dstand is the CO2 gas diffusion coefficient in free air under standard conditions, T is the temperature (°C), and P is the pressure (hPa), respectively. Using the topographic elevations, P=1013 was assumed for Fukuroyamasawa, and P=870 for Kog-Ma.

Results

The Fukuroyamasawa watershed

Field observation data

Figure 4 We used data obtained from 1999 to 2001 (Fig. 4 ). Soil respiration was high in the summer and low in the winter. Soil temperature showed clear seasonality; it was high in summer, and low in winter. Soil temperature at 0.1 m showed maximum of about 24 °C in August and minimum of about 3 °C in February. Although a small phase shift was found at deeper depths (the difference between 0.1 and 1.0 m was about 1 month), the seasonality of soil temperature was similar for all depths. The soil respiration rate increased exponentially with soil temperature (Fig. 3). Conversely, the soil water content did not change in a clearly defined way, except for wet conditions after rainfall (Fig. 4). No clear correlation between soil respiration and soil water content was observed. The soil CO2 concentration at each depth was high in the summer and low in winter, and increased with depth. The soil CO2 concentration at the deepest depth, 0.8 m, ranged from 22,000 to 83,000 mgCO2m-3.

Seasonality of CO2 production at each depth

Figure 5

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The CO2 production rates at each depth are shown in Fig. 5. The CO2 production rate was greatest at the shallowest depth all year long. The CO2 production rate at 0.1 m was high in summer, low in winter. The CO2 production rates at greater depths were high in the summer and low in the winter, as observed in shallower soil, although the fluctuations were larger, especially at deeper depths (0.65 m-). The peaks in the CO2 production rate at 0.3 and 0.5 m depths appeared to be earlier than those at 0.1 m, although they were not clear.

Contribution of CO2 production at each depth

Figure 6 The contribution of CO2 production at each depth to the total soil respiration was calculated (Fig. 6). The contribution of the shallowest soil layer (0.1 m) was about 75 %; it was the greatest year round, although the scatter was large. The contributions of deeper depths were below 30 % year round.

Relationships between CO2 production rate and soil temperature

Figure 7 The relationship between soil temperature and the estimated CO2 production rate was investigated (Fig. 7), because the soil temperature changed considerably in this watershed, while the soil water content changed little. At shallower depths, CO2 production rates increased with soil temperature, although the deviations at deeper depths were very large, and the relationships at deeper depths were very weak. However, the CO2 production rate generally increases exponentially with temperature; therefore, an exponential function was fitted to the data. The Q10 coefficient, the factor by which CO2 production increases with a rise of 10 °C in temperature, ranged from 1.8 to 2.9; the correlation was very weak at the deepest depth (R2 =0.031, Fig. 6).

The Kog-Ma watershed

Field observation data

Figure 8 Data from 1998 to 2001 were used (Fig. 8). Soil respiration was relatively high in the rainy season, and low in the dry season; however, the interannual differences were large. Because soil temperature did not change markedly, we observed no clear relationship between soil respiration and soil temperature. Conversely, the soil water content changed dramatically and controlled the soil respiration rate (Fig. 3); soil respiration increased with the water content (Hashimoto et al. 2004). The soil CO2 concentrations tended to be high in the rainy season and low in the dry season, and they increased with soil depth. The soil CO2 concentration at the greatest depth, 0.6 m, ranged from 10,000 to 71,000 mgCO2m-3.

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Seasonality of CO2 production at each depth

Figure 9 The CO2 production rate at the shallowest depth showed distinct seasonality; it was high in the rainy season and low in the dry season (Fig. 9). The rate was significantly higher than at depth, especially in the rainy season. However, in the dry season, the CO2 production rate at the shallowest depth decreased significantly. As a result, the amplitude of the CO2 production rate at the shallowest depth was very large (0.026-0.35 mgCO2m-2s-1). CO2 production rates at greater depths (0.4 m and 0.5 m-) increased in the dry season and decreased in the rainy season, and had the opposite seasonality to that at the shallowest depth.

Contribution of CO2 production at each depth

Figure 10 Fig. 10 shows the calculated contributions from each depth. The contributions from each depth changed with season. The contribution from the shallowest depth was high in the rainy season (around 95 %), when it rose to about 100 % of the total; it decreased to less than half in the dry season. The contributions from greater depths were high in the dry season and low in the rainy season. In the dry season, these contributions rose to about 40 % of the total.

Relationships between CO2 production rate and soil water content

Figure 11 In general, CO2 production decreases under both excessively wet and dry conditions (e.g., Howard and Howard 1993; Bowden et al. 1998) ; subsequently, we investigated the relationship between the soil CO2 production rate and the degree of saturation (soil water content/saturated soil water content). This relationship differed at different depths. The CO2 production rate at the shallowest level increased with the degree of saturation. This tendency was very similar to the relationship to soil respiration measured at the soil surface. The relationship at a depth of 0.4 m was parabolic and seemed to peak at around 0.5 to 0.6, although the data were somewhat scattered. The degree of saturation ranged from 0.3 to 0.9. The CO2 production rate at the greatest depth decreased with increasing degree of saturation. The overall degree of saturation ranged from 0.4 to 1.0.

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Discussion

Interpretation

Temperate forest in Japan We showed that the seasonality of the CO2 production rate was similar at different depths (Fig. 5), with the contribution from the shallowest depth being the greatest year around (Fig. 6). This may be explained as follows. Similar to the soil surface (Fig. 3), the CO2 production rates mostly increased with soil temperature throughout the profile in the temperate forest (Fig. 7), although scattering was pronounced at deeper depths. Up to a depth of 1.0 m, the time lag in the temperature transfer was small, and the soil temperature at each depth was high in summer and low in winter (Fig. 4). As a result, the seasonal variation in the soil CO2 production at each depth was similar, and the relative contribution at each depth, as a result, did not change over the year (Fig. 6). The contribution from the shallowest depth was significantly greater than that from other depths and supplied around 75 % of the total, probably because of the presence of rich organic matter and the high root density (Jackson et al. 1996; Jobbágy and Jackson 2000). The reason for the very small scattering in the shallowest layer was the result of modeling the soil CO2 efflux using an exponential model. It is also the reason that the soil flux between 0 and 0.1 m or the soil CO2 efflux was significantly larger than that at 0.1-0.3 m, resulting in the strong influence of the soil CO2 efflux model on the CO2 production at 0.1 m. The relationship between the CO2 production rate at the bottom layer and soil temperature showed strong scattering (Fig. 7), which may have been caused by the low precision of the flux estimation. Moreover, temperature may also have had an influence; the CO2 production at the bottom layer was defined by the flux between 0.5 and 0.8 m, which was the total CO2 production rate below that depth. Figure 7 (d) compares the production rate to the temperature at 1.0 m, which could be inadequate. Although shallow soil contains much carbon, a significant amount of organic carbon is also present in the deeper soil. (Jobbágy and Jackson 2000). For example, more than half of the organic carbon present in the topmost 1 m of soil is found below 0.2 m, (Jobbágy and Jackson 2000). In the top 3 m of soil, 44 % of the soil organic carbon is found from 1 to 3 m, which amounts to a global total of about 842 petagrams of C. Therefore, the temperature response of soil organic carbon decomposition in deep soil is a critical issue for our understanding of global warming and its impact. The decomposition of organic matter is very sensitive to temperature. Goulden et al. (1998) found that deep-soil respiration was very sensitive to deep-soil temperatures in a black spruce forest in Canada. Hirsch et al. (2002) measured deep-soil respiration directly at the BOREAS northern old-growth black spruce site in Canada using a method similar to the one used here. They found that deep-soil respiration increased linearly with temperature at the 0.5-m level, with a slope of 0.2 kg C ha-1d-1 °C -1. Risk et al. (2002) also found positive relationships between soil temperature and subsurface CO2 production at four sites in eastern Nova Scotia, Canada. Davidson et al. (2006) reported that CO2 production

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estimates of O horizon to B horizon by a method similar to that in this study were correlated to temperature (Q10=2.3-4.4) although the scatters were large, and that within C horizon, the production estimate was too weak; however, Q10 value was 1.5 (R2=0.01). Our results agree with these four studies. In the Fukuroyamasawa watershed, although the scatter at deeper depths was very strong (e.g., R2=0.031 at deepest, Fig. 7), we found that the soil CO2 production rate tended to increase with temperature at all depths (Q10 =1.8-2.9). These values lie within the reported range (Raich and Schlesinger 1992; Lenton and Huntingford 2003; Hashimoto 2005).

Tropical forest in Thailand In contrast, our results showed that the seasonality of the CO2 production rate differed with depth: the CO2 production at the shallowest depth was high in the rainy season and low in the dry season, while production at deeper depths showed the opposite seasonality (Fig. 9). As a result, the contribution of the shallowest depth dropped during the dry season (Fig. 10). This may be explained as follows. The soil water changed significantly in the Kog-Ma watershed (Fig. 8), which appeared to control soil respiration (Fig. 3, Fig. 11). In general, the soil CO2 production rate showed a parabolic response, decreasing with both excessively wet and dry conditions, and peaking at an intermediate degree of saturation (e.g., Howard and Howard 1993; Bowden et al. 1998). Our observations appeared to show a similar tendency at a depth of 0.4 m (Fig. 11 (b)). At 0.1 m, the CO2 production rate increased with the degree of saturation (Fig. 11 (a)). One possible explanation is that the shallower soil has a higher porosity and saturated soil water content; consequently, the shallowest soil level was probably rarely saturated. The CO2 production rate at 0.1 m was plotted against the degree of saturation at the same depth. However, perhaps the production should have been plotted against the degree of saturation at a shallower depth (e.g. 0.05 m), because the very large production rate at 0.1 m, which was almost ten times greater than at deeper depths, was caused by the fact that the CO2 flux between 0 and 0.1 m was much larger than that for 0.1-0.4 m depths. The large difference between the two fluxes suggests that the main source of the CO2 production rate occurs at shallower depths than 0.1 m, where the soil is very porous and is in the lower range of the degree of saturation. At the same time, the CO2 production rate at deeper depths decreased with increasing degree of saturation (Fig. 11 (b)(c)), probably because the degree of saturation at those depths was comparatively high year round, even during the dry season, due to the low soil water content needed for saturation. In other words, the deeper soil was excessively wet in the rainy season, inhibiting soil CO2 production, while soil CO2 production increased in the dry season because of the adequate soil water content. Thus, the different seasonality at different depths could be explained as follows. The seasonality of the degree of saturation was similar at all depths, i.e., high in the rainy season and low in the dry season, but the CO2 production rate decreased when the degree of saturation was too low or too high, and the degree of saturation varied with depth. As a result, the seasonality of the CO2 production rate at shallow depths differed from that at deeper levels. In the rainy season, the water content near the surface was adequate for soil CO2 production, but that at greater depth was inadequate; at this time, the contribution from the near-surface layers was relatively large. In the dry season, the water content in the near-surface

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layers was insufficient, but it was sufficient at greater depths. Therefore, the contribution from the shallow layers decreased considerably in the dry season, while that at greater depths rose. The contribution of each depth changed with the seasons. Similar results were reported in a recent study of Amazonian forest by Davidson et al. (2004), who found seasonal variation in the CO2 production at different depths. Also, Fierer et al. (2005) found that the subsurface contribution (soil below 40 cm in depth) to whole CO2 production in soil was likely to be relatively less during the wet season (December to May) at a grassland hillslope in coastal southern California. We recognize the need for more observations to clarify the details of what happens in the shallowest layer. At 0.1 m, we observed no decreases in the overall soil CO2 production rate with increasing degree of saturation (Fig. 11 (a)). This is due to the model used to estimate the soil surface CO2 flux. Since decreases in the soil surface CO2 flux were not observed (Fig. 3), and one of the reasons for this was the absence of measurements in very high soil water conditions, the soil surface CO2 flux or the flux between 0 and 0.1 m was modeled using a linear model of the soil water content. As a result, it is not surprising that the CO2 production at the shallowest depth did not show a negative trend at a high soil saturation levels. While the soil at the shallowest layer may have large porosity, as discussed above, and rarely experiences a high water content, the CO2 production rate may in fact not show any negative trends. Observations during very wet conditions are needed to clarify this. In addition, no observations were made during very dry periods. It is not yet clear whether the soil surface CO2 flux actually drops to such a level (e.g., <0.1 mgCO2m-2s-1, Fig. 3), which leads to the considerable drop in the contribution of shallowest layer and the increase in the deeper layers. Our analyses showed that the contribution of shallowest depth dropped to below 50 % in the dry season, and that of deep layer increased up to 50 %. These extrapolations for very dry and very wet conditions are somewhat uncertain, and it is not clear whether they reflect actual conditions. Observations in both very wet and dry conditions and more accurate modeling of the soil CO2 efflux will allow us to evaluate the CO2 production rate in the shallowest layer more accurately. In addition, the seasonality of A0 layer, litterfall and fine root biomass may strongly affect the seasonality of soil CO2 production (e.g., Davidson et al. 2004; Goulden et al. 2004) , especially in the shallowest layer, as with soil water. For example, the carbon cycle in tropic regions is very rapid (Trumbore 1993), in part because of the high rate of decomposition. This suggests that seasonality in A0 and litterfall affects the seasonality of soil respiration in tropical regions (e.g., Chambers et al. 2004; Goulden et al. 2004), especially through CO2 production at shallow depths. In fact, the correlation between the soil CO2 efflux and soil water was not very strong, as seen with soil temperature in the temperate forest. As discussed above, despite the positive trend in the shallowest layer, strong negative trends were observed in this study (Fig. 11). Although we believe that these trends likely occur for the reasons outlined above, we should also note that these trends could be artifacts; further studies should examine whether they really occur. We evaluated CO2 production by measuring the CO2 concentration and estimating the gas diffusivity, which is a frequently used method (e.g., Hirsch et al. 2002; Davidson et al. 2004). However, few studies have been conducted at a site with a wide range of soil water content, where a change in soil water dramatically changes not only the soil CO2 concentration, but also the gas diffusivity.

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Near-surface levels were the main source of soil CO2 production in both watersheds, which agreed with other studies (de Jong and Schappert 1972; Campbell and Frascarelli 1981). Jobbágy and Jackson (2000) showed that shallow soil contains more organic matter. Moreover, plant roots are another source of CO2 at shallower depths (Jackson et al. 1996). For example, 80-90 % of roots are in the top 0.3 m of soil in tundra, boreal forest, and temperate grasslands, while the value is 50 % for deserts and temperate coniferous forest (Jackson et al. 1996). Roots also supply organic matter to the soil.

Evaluating the gas diffusion coefficient and measuring soil CO2 concentration This method, which evaluates the soil CO2 flux at each depth using Fick's first law, has the advantages that the soil remains undisturbed, and direct observation of the CO2 production rate and its seasonality is possible. As is pointed out by Hirsch et al. (2002), the key to the method is the accurate estimation of the gas diffusion coefficient. We used the empirical TPM model to estimate the gas diffusion coefficients for the Fukuroyamasawa watershed. To better estimate the gas diffusion coefficients, the air- or water-filled porosity and maximum porosity should be measured more accurately. Moreover, using empirical models may not be sufficient; the relationship between the air-filled porosity and the gas diffusion coefficient varies between soils and between sites, so that measuring the relationship at each site should improve the accuracy of this method. In addition, the estimation of gas diffusivity for the Kog-Ma watershed may have been insufficient. Although the gas diffusivity was measured near the observation site using a large sample (ca. 0.2 m in diameter), which generally provides data more typical of a site, the number of measurements was very small. Measuring the soil CO2 concentration is also a key requirement. Although many methods of measuring soil CO2 concentration have been proposed (e.g., Clegg et al. 1978; Nakayama and Kimball 1988; Fang and Moncrieff 1998; Hamada and Tanaka 2001; Hashimoto 2002), measuring the soil CO2 concentration is cumbersome, and most studies have focused only on the soil CO2 efflux, rather than the soil CO2 concentration. However, profiling of the soil CO2 concentration provides much information about CO2 production in soil and improves the prediction of soil organic carbon response. Recently, continuous monitoring of the soil CO2 concentration using small sensors buried at each depth has been reported (Hirano et al. 2003; Tang et al. 2003). These sensor developments will reveal more detailed information on processes within the soil.

Conclusions The vertical distribution of the soil CO2 production rate was investigated over several years in a temperate coniferous forest and a tropical montane forest. Our study suggested that the shallowest layers were the most important in both forests. Our results also suggested that there were significant differences between the two forests. In the temperate forest, the seasonality of soil CO2 production was similar at different depths, and the near-surface production was the highest year round. Soil temperature appeared to control the soil CO2 production rate. Conversely, our results implied that the seasonality of soil CO2 production differed with depth in

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the tropical montane forest; the CO2 production rate near the surface dropped in the dry season, while that at deeper depths increased, resulting in a decreased contribution of the shallowest depth in the dry season. These opposite seasonal trends may have been due to different levels of saturation, as the soil water level appeared to control CO2 production. These differences between the two forest sites indicate the importance of considering the vertical distribution of soil processes. Such considerations will improve our understanding of soil CO2 production and allow us to predict the changes in soil carbon content with climate change.

Acknowledgments

We are grateful to the all members of the Fukuroyamasawa project and the Kog-Ma project. Part of this research was supported by Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists. Part of this work was supported by Grants-in-Aid for Scientific Research (#06404012, #11460063, #14360081) from the Ministry of Education, Culture, Sports, Science, and Technology, of Japan. Part of this research was conducted as one of the GAME-Tropics activities funded by the Japanese Ministry of Education, Culture, Sports, Science, and Technology, under Grants-in-Aid for Scientific Research (#07041106 and #10041219, Leader: Prof. K. Musiake, The University of Tokyo) with cooperation from the National Research Council of Thailand. We sincerely thank two anonymous reviewers for their helpful comments on earlier drafts of this manuscript.

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Figure Legend Fig. 1: Seasonality of air temperature and rainfall in the Fukuroyamasawa watershed, Japan (left column; average air temperature between 1998 and 2001, average rainfall between 1996 and 2001), and the Kog-Ma watershed, Thailand (Chunkao et al. 1981; right column; average between 1966 and 1978). Fig. 2: Diagram of the partitioning model used to evaluate the CO2 production rate at each depth in the Fukuroyamasawa and Kog-Ma watersheds. Fi is the soil CO2 flux, fs(T) and fs(θ) are the modeled soil CO2 effluxes (see Fig. 3), Di is the gas diffusion coefficient, dCi/dzi is the gradient of the soil CO2 gas concentration, and αi is the CO2 production rate. Although a gas flux is represented by an arrow pointing upwards, a downward flux is also possible. Fig. 3: Relationships between soil respiration (the soil surface CO2 flux) and soil temperature and soil water content. These relationships were used to estimate the flux at 0-0.1 m (see Fig. 2) Fig. 4: Seasonal variation in (a) soil respiration, (b) temperature, (c) water content, and (d) CO2 concentration in the Fukuroyamasawa watershed. In this case, a warm (cold) period was defined as the period during which the temperature at the shallowest depth was above (below) average. For comparison, the range of the y-axis of soil respiration (a) is the same as that in Fig. 9 Fig. 5: Seasonal variation in the CO2 production rate at each depth in the Fukuroyamasawa watershed. Note that the y-axis scales differ among the plots. Fig. 6: Seasonal variation in the contribution of each depth to the total CO2 production in the Fukuroyamasawa watershed. Fig. 7: Relationship between CO2 production rate and soil temperature at each depth. Note that the y-axis scales differ among plots. An exponential function, y=A ekT, was used to fit the data, where y is the CO2 production rate, A and k are fitting parameters (Q10=e10 k). Negative values were not used. Note that the CO2 production at 0.65 m- was plotted against the temperature at 1.0 m depth (see the Discussion). The low scattering at the shallowest depth was the result of modeling the soil CO2 efflux with an exponential function (see the Discussion). Fig. 8: Seasonal variation in (a) soil respiration (bars indicate standard deviations), (b) temperature, (c) water content, and (d) CO2 concentration in the Kog-Ma watershed. Fig. 9: Seasonal variation in the CO2 production rate at each depth in the Kog-Ma watershed. Note that the y-axis scales differ among plots. Fig. 10: Seasonal variation in the contribution of each depth to the total CO2 production in the Kog-Ma watershed. Fig. 11: Relationship between the CO2 production rate and degree of saturation. The degree of saturation is the ratio of the soil water content to the saturated soil water content. The CO2 production rate at 0.1 m, 0.4 m, and 0.5 m- was plotted against the degree of saturation at 0.1, 0.4, and 0.5 m depths, respectively. Note that the y-axis scales differ among plots. The low scatter at the shallowest depth was the result of modeling the soil CO2 efflux with a linear function (see the Discussion).