Effects of Slashand Burn on Soil CO2 Efflux

Soil CO2 effluxes at the first slash-and-burn site (SA01) were significantly higher than in the 80-year-old CP site in July and August before and after the burning, ranging between 944.6 and 1,196.5 mg CO2m 2h—1 at the SA01 site and 800.9-904.0 mg CO2m—2h—1 at the CP site (Fig. 5a). A marked increase in CO2 efflux was detected after the burning at the SA 01 site. Soil CO2 efflux values at the SA01 site were similar to those of the CP site from September to November (within the range 231.8-759.6 mg CO2m~2h~1). Soil temperature at —5 cm fluctuated more at the SA01 site

(b) Days after burning

Figure 4: Total carbon (a) and nitrogen (b) contents (% of dry weight) in the surface soil (0-5 cm) at the CP site (°) and at the SA01 site (•). Vertical bars represent + one SE of the mean (n — 10). The SA01 site was burned on 16 August 2001. Superscript letters indicate the results of one-way ANOVA; means with the same letters are not significantly different by Scheffe's test at each site (p — 0.05).

(b) Days after burning

Figure 4: Total carbon (a) and nitrogen (b) contents (% of dry weight) in the surface soil (0-5 cm) at the CP site (°) and at the SA01 site (•). Vertical bars represent + one SE of the mean (n — 10). The SA01 site was burned on 16 August 2001. Superscript letters indicate the results of one-way ANOVA; means with the same letters are not significantly different by Scheffe's test at each site (p — 0.05).

(ca. 10°C difference between daily maximum and minimum) than at the CP site (ca. 2°C difference; Fig. 5b), and mean soil temperature at -5 cm between 13:00 and 15:00 was higher at the SA01 site than at the CP site (data not shown). The break in continuity of soil temperature data at the SA01 site was due to removal of the data logger before the burning. SWC at each CO2 efflux measuring event showed no significant differences (t-test, p<0.05) between the SA01 and CP sites (ranged between 17.4 and 31.6% at the SA01 site, and 21.1-34.5% at the CP site; Fig. 5c). Fig. 6 shows that the soil CO2 efflux was significantly correlated with soil temperatures at —5 cm over the measuring period. This indicated that soil temperature was the principal control on seasonal variation in soil CO2 efflux at both the o u

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\ -•- SA01 site \ \ -O- CP site

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Figure 5: Seasonal changes in (a) soil CO2 efflux, (b) soil temperature, and (c) soil water content (SWC) for 2001 at the SA01 (•) and CP (o) sites. Soil temperature at -5 cm was obtained from data recorded continuously every 30min outside of the CO2 efflux chambers. SWC over the 0-12 cm depth range was measured inside of the chamber for each efflux measurement. Vertical bars represent + one SE of the mean (n — 25). The SA01 site was burned on 16 August 2001.

(Month)

Figure 5: Seasonal changes in (a) soil CO2 efflux, (b) soil temperature, and (c) soil water content (SWC) for 2001 at the SA01 (•) and CP (o) sites. Soil temperature at -5 cm was obtained from data recorded continuously every 30min outside of the CO2 efflux chambers. SWC over the 0-12 cm depth range was measured inside of the chamber for each efflux measurement. Vertical bars represent + one SE of the mean (n — 25). The SA01 site was burned on 16 August 2001.

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0 5 10 15 20 25 30 35 Soil temperature at -5 cm (°C)

Figure 6: Relationship between soil CO2 efflux and soil temperature at the SA01 site (a) and the CP site (b) in 2001. Soil temperatures (—5 cm) were measured inside of the chamber just after each efflux measurement. Vertical bars represent 7 one SE of the mean (n — 25). The SA01 site was burned on 16 August 2001. The regression lines are significant (n — 9, p< 0.001 for the SA01 site and n — 10, p< 0.001 for the CP site).

SA01 and CP sites. The Qi0 of temperature sensitivity of CO2 efflux was higher at the SA01 site (2.330) than at the CP site (1.976).

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