CH4 Uptake and CO2 Emission

The measured soil CH4 uptake indicated that the soil of our study forest oxidized atmospheric CH4 (Fig. 3). Methane uptake showed a seasonal variation with maximum uptake rates of 200-300 mgCH4m-2h-1 between July and September and minimum rates of 20-50 mgCH4m-2h-1 during the snowpack season. The seasonal pattern was similar over the 3 years. The methane uptake rate was linearly related to soil temperature at both 0 cm (R2 - 0.782; P< 0.001) and 5 cm (R2 - 0.630; P< 0.001) depth, but not to soil water content (R2 - 0.222; P>0.05) (Fig. 4). The apparent Q10 (10-20°C) (temperature coefficient) for CH4 uptake was 1.7 for soil temperatures at both depths.

The CO2 emission rate increased from April (when the snowpack melted) to August (midsummer), and then decreased (winter) (Fig. 3). The maximum CO2 emission rate was 603mgCO2m-2h-1 on 30 July 2002, and the minimum was 21mgCO2m-2h-1 on 21 February 2002. During the snowpack season, the CO2 emission rate was less than 50mgCO2m-2h-1. Seasonal variation in the CO2 emission rate could be expressed as a simple exponential function of soil temperature at 0 cm (R2 - 0.794;

c 30

o 20

o CO

E CM 400

E CM 400

Figure 3: Seasonal variations in soil temperature ((a) open circles and broken line for 0 cm depth and closed circles and solid line for 5 cm depth), volumetric soil water content (b), CH4 uptake rate (c), and CO2 emission rate (d) during the study period. Each point represents the mean + SD (vertical bars) of the data from the nine chambers used for flux measurements. Soil temperature and water content were measured at these nine points at the same time as the fluxes were measured.

Figure 3: Seasonal variations in soil temperature ((a) open circles and broken line for 0 cm depth and closed circles and solid line for 5 cm depth), volumetric soil water content (b), CH4 uptake rate (c), and CO2 emission rate (d) during the study period. Each point represents the mean + SD (vertical bars) of the data from the nine chambers used for flux measurements. Soil temperature and water content were measured at these nine points at the same time as the fluxes were measured.

-10 0 10 20 3 (d) Soil temperature 0 cm (°C)

c 800

O 600

c 800

O 600

O 600

O 600

Figure 4: Mean CH4 uptake and CO2 emission rates in relation to soil temperature and volumetric soil water content. CH4 uptake versus soil temperature at 0 cm (a) and 5 cm (b) depth and soil water content (c); CO2 emission versus soil temperature at 0 cm (d) and 5 cm (e) depth and soil water content (f).

P< 0.001) and 5cm (R2 - 0.880; P< 0.001) depth (Fig. 4). There was no relationship between the CO2 emission rate and volumetric soil water content (R2 - 0.123; P>0.05). The Q10 values for CO2 emission were 3.4 for soil temperatures at 0 cm depth and 4.4 for soil temperatures at 5 cm depth.

The relationship between the CH4 uptake rate and the CO2 emission rate was linear and positive (R2 - 0.509; P< 0.05) for CO2 emission rates <150mgCO2m-2h-1 (Fig. 5), obtained from late autumn to early spring. However, during the rest of the year, when the CO2 emission rate was > 150 mg CO2 m-2 h-1, there was no significant relationship between the two rates (R2 - 0.005; P> 0.05).

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