Responses of Soil CO2 Efflux and Carbon Dynamics to Fire

Fire-induced increases in microbial activities after burning result in an increase in post-fire soil CO2 efflux. Some studies have found that there is a pronounced increase in soil CO2 efflux after burning (Singh et al., 1991; Wuthrich et al., 2002). For example, Wuthrich et al. (2002) examined the effects of fire on soil CO2 efflux from a sweet chestnut forest in southern Switzerland and demonstrated that the greater post-fire soil CO2 efflux might have resulted from increased microbial activity and CO2 release from below the topsoil, and that the enhanced soil CO2 efflux after a fire could last for 6 months. They also suggested that large amounts of easily available organic carbon from the remains of plants, animals, and microorganisms in the soil killed by above-ground fire, are rapidly decomposed by microorganisms and significantly enhancing post-fire CO2 efflux.

The higher Q10 value of soil CO2 efflux at the slash-and-burn (SA01) site compared to the unburned control CP site (Fig. 6) might be explained partly by the enhancing post-fire CO2 efflux at SA01 site. It has been demonstrated that the Q10 function intergrates all processes that influence seasonal soil CO2 efflux, not only the temperature response of soil CO2 efflux, but also seasonal changes in respirationary activities induced by changes in root phenology, microbial biomass, and other factors (Mo et al., 2005). Fire-induced increasing in microbial activities resulted in the enhancing post-fire soil CO2 effluxes and therefore may induce a higher Q10 value for the SA01 site compared to the unburned control site.

This enhanced soil CO2 efflux after burning lasted at least one week in the present study (Fig. 7a), and possibly longer considering that root respiration might greatly decrease at the slash-and-burn site (SA02) because old roots were killed by fire and new roots of the turnip crop were still under developing. On the other hand, root respiration may contribute a large proportion to total soil CO2 efflux in the unburned control CF site across the whole growing season. By direct measurements of root respiration, it is estimated that root respiration at the slash-and-burn (SA02) site accounted for 43% of total soil CO2 efflux for the pre-fire period (July 21 to August 10) and decreased to 14% for the post-fire period (August 25 to October 4); whileas soil respiration at the unburned control CF site contributed 43% to the total soil CO2 efflux for the pre-fire period (July 21 to August 10), and increased to 63% just after burning (August 11 to August 24), and contributed to 66% for the post-fire period (August 25 to October 4) (Sako, unpublished data). Therefore, our results suggested that a significant amount of carbon was released at the slash-and-burn agriculture sites, not only by combustion of litter during the fire, but also by decomposition of SOM following the fire and during the growth of the turnip crop.

However, a decrease in SOM of the topsoil was not detected in this study (Fig. 4). This means that carbon and nitrogen fluxes caused by burning at the slash-and-burn agriculture (SA 01) site was small compared to the total stocks of SOM accumulated in the mature plantation pre-fire. In an experiment at San Carlos, the total stocks of nutrients in the soil did not appear to be as seriously depleted by slash-and-burn as other treatments, such as pasture establishment using bulldozers (Jordan, 1989). In this case, NPP can increase dramatically after abandonment in fallow, while crop yields do indeed decline rapidly in traditional slash-and-burn agricultural areas (Uhl, 1987).

In the current study site, pH and available P content of the topsoil remained at post-fire levels until the end of the turnip crop, while soil inorganic N and exchangeable cations were increased by burning and then returned to pre-fire levels. These soil conditions might result in higher carbon sequestration during the fallow in the slash-and-burn agricultureal sites as a result of the higher NPP, as suggested by Uhl (1987). Therefore, when estimating the total carbon budget of slash-and-burn agricultural ecosystems, the potential for this feedback increase in carbon sequestration should be taken into account in addition to the temporary emissions of carbon dioxide from litter burning and subsequent fire-induced increment in CO2 efflux. Long-term research on carbon dynamics beyond one annual cycle is needed on slash-and-burn systems to better understand the increase in carbon dioxide emissions from soil after burning and the feedback increase in carbon sequestration during the fallow.

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