Introduction
Tropical forests significantly impact global climate by regulating greenhouse gas and storing 45-50% of global terrestrial carbon stocks (IPCC, 2007). Soil CO2 emission (soil respiration) plays an important role in global carbon cycling with contributing more than 50% of the total ecosystem respiration (Janssens et al., 2001; Xu et al., 2001; Chambers et al., 2004). Soil CO2 emission from tropical forests could strongly influence future concentrations of atmospheric carbon dioxide. However, soil respiration considerably varied with space and time (Boonriam et al., 2021a). Spatial variability in soil respiration from tropical forests remains difficult to assess and it may lead to inaccurate soil respiration estimates at ecosystem level. Consequently, understanding of above and belowground soil activities on carbon cycle in tropical forest is necessary.
Soil respiration comes from CO2 production of all living organisms in the soil, including plant roots, soil microbes, and animals (Lavelle et al., 2001; Luo and Zhou, 2006. Soil microorganisms and roots are thought to dominate most soil respiration. However, soil respiration rate has been shown to fluctuate unexpectedly (10-90%) at large scales (Hanson et al., 2000). The known environmental factors: soil, water content, and temperature are unable to completely explain this variation. According to Ohashi et al. (2007; 2017), certain areas displayed extremely higher rates of soil respiration (hot spots) and soil macrofauna were focused as a predominant driver in tropical forests. Events were attributed to the undisclosed activities of soil animals, particularly social insects such as termites. Because it is widely known that termites are superabundant soil insects in seasonal tropical forests (Yamada et al., 2003, 2005; Inoue et al., 2006).
Termites are major structural components of soil partition up to 95% of the total soil insect biomass (Jones and Eggleton 2000) and population density in the tropics could reaches up to 1000 termites per m2 (Eggleton et al., 1996). Termites are widely distributed along an altitudinal gradient from lowland to mountain (Kayani et al., 1979; Akhtar et al. 1992). Termites are significantly important to drive carbon cycle by litter decomposition as much as half of the primary litter production (e.g. Matsumoto and Abe, 1979; Bignell and Eggleton, 2000; Coleman et al., 2004). Termites can be divided into two major feeding groups, wood/litter feeders (fungus and non-fungus growers) and soil feeders (non-fungus growers) (Wood, 1976; Collins, 1989; Bignell et al., 1997; Bignell and Eggleton, 2000). Fungus growing termites cultivate symbiotic fungi on fungus gardens (fungus combs) consisting of plant litter materials built with partially digested termite faeces (Korb, 2003). Termite CO2 emissions were estimated to contribute up to 2% of global terrestrial emissions (Sugimoto et al., 2000). Several studies considered epigeal termite mounds as high CO2 emission point sources. As the epigeal termite mounds in tropical savanna emitted higher CO2 than surrounding soils with emphasis on fungus growing termite (Macrotermitinae) (Konate et al., 2003; Brümmer et al., 2009; Risch et al., 2012), except in seasonal tropical forest (Boonriam et al., 2021b). Besides, De Gerenyu et al. (2015) reported that termite mounds contributed up to 10% of the total soil respiration in a tropical monsoon forest in Vietnam. Thai-tropical forests also contain various epigeal termite mounds, especially non-fungus growing termites (Termitidae) due to the number of species and nests were higher than fungus growing termites (Yamada et al., 2003). According to Korb (2003), epigeal termite mounds have a different complex architecture to maintain a constant temperature and humidity. While the seasonal tropical forests have sometimes a fluctuation of the climate. Consequently, determining epigeal mound CO2 emission in terms of non-fungus growing termites is one of the best approaches for evaluating soil biological activities in relation to carbon and energy flow in terrestrial ecosystems.
This study aimed to 1) compare CO2 emission from epigeal mounds of different non-fungus growing termites in a seasonal tropical forest, 2) compare CO2 emission from the mounds and their surrounding soils, and 3) determine the relationship between the surrounding soil CO2 emissions and soil environmental factors in the dry and wet seasons.